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Inside the Eridanus Supervoid_ The Coldest Spot in Space · Boring Science

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  Speaker 1: In 2004, NASA's WMAP satellite finished painting the most detailed
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  Speaker 1: portrait of the early universe anyone had ever seen. The
• 0:10
  Speaker 1: first thing astronomers noticed when they looked at it was
• 0:13
  Speaker 1: that something was wrong. The satellite had spent more than
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  Speaker 1: a year sweeping the entire sky. Its mission was precise.
• 0:21
  Speaker 1: Scientists wanted a map, a complete full sky map of
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  Speaker 1: the faintest light in the universe, not the light from stars.
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  Speaker 2: Not from galaxies. Something older.
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  Speaker 1: Something that had been travelling toward us since before any
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  Speaker 1: of those things existed. The cosmic microwave background. The afterglow
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  Speaker 1: of the Big Bang itself. A faint wash of microwave
• 0:45
  Speaker 1: radiation arriving from every direction. Carrying information about what the
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  Speaker 1: universe looked like 380,000 years after it was born. WMAPE
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  Speaker 1: was built to read that information with a level of
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  Speaker 1: detail no instrument had ever achieved before. What came back
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  Speaker 1: from the satellite exceeded expectations. The map was extraordinary. Tens
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  Speaker 1: of millions of pixels, each one a temperature mere. Shermaned
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  Speaker 1: from a different patch of sky, averaged across the whole thing,
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  Speaker 1: the temperature was 2.725 Kelvin, just barely above absolute zero.
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  Speaker 1: And the tiny variations scattered across that map, hot patches,
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  Speaker 1: cold patches, each one a fraction of a degree from
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  Speaker 1: the average, matched the predictions of standard cosmology almost perfectly.
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  Speaker 2: Almost.
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  Speaker 1: There was a region, a broad, irregular patch of sky
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  Speaker 1: sitting in the direction of the constellation Eridanus. in the
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  Speaker 1: southern part of the celestial sphere. In that region, the
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  Speaker 1: temperature was lower. Not dramatically lower in the way you
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  Speaker 1: might imagine. Not something that leapt off the screen in
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  Speaker 1: the first glance. The difference was 70 microkelvin, 70 millionths
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  Speaker 1: of a degree below the average temperature of the surrounding sky.
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  Speaker 1: Written out as a number, it barely registers. 70 millionths
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  Speaker 1: You could be forgiven for wondering why anyone cared. Here
• 2:21
  Speaker 1: is why anyone cared. The CMB is almost perfectly smooth.
• 2:26
  Speaker 1: The variations that do exist across the entire map average
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  Speaker 1: around 18 microKelvin. Small fluctuations above, small fluctuations below. These
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  Speaker 1: tiny ripples are not noise or imperfection. They are the
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  Speaker 1: seeds of every structure in the universe. The slight over-densities
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  Speaker 1: that eventually became galaxy clusters. The under-densities that eventually became
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  Speaker 1: the vast empty regions between them. The whole story of
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  Speaker 1: cosmic structure is written in those tiny temperature differences.
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  Speaker 2: And they are small. They are supposed to be small.
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  Speaker 1: The universe, at the moment that ancient light was released,
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  Speaker 1: was remarkably uniform. So when something appears in the map
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  Speaker 1: that is 70 microkelvin below average, nearly four times the
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  Speaker 1: typical variation, it does not bless.
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  Speaker 2: End in. It stands out.
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  Speaker 1: Hard. The first question scientists asked was simple.
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  Speaker 2: Was it real?
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  Speaker 1: Instruments have artifacts. Satellites have noise. Data has errors. Every
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  Speaker 1: major discovery in astronomy demands the same first round of scrutiny.
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  Speaker 1: Could this be a mistake? Teams went back through the
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  Speaker 1: WMAP data. They checked calibration. They ran the analysis again
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  Speaker 1: with different techniques. They looked for any systematic error in
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  Speaker 1: the satellite's measurements that might produce a cold patch in
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  Speaker 1: that part of the sky. Nothing. The cold region held.
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  Speaker 1: It was not a glitch in the detector. It was
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  Speaker 1: not a processing artifact. It was not a foreground source,
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  Speaker 1: some intervening object between us and the CMB that might
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  Speaker 1: have contaminated the signal. Every standard explanation for instrumental or
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  Speaker 1: observational error came up empty. Whatever this feature was, it
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  Speaker 1: was in the data, and it was not going away.
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  Speaker 1: Then came the second question. How big was it? This
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  Speaker 1: was where things became genuinely uncomfortable. The cold patch covered
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  Speaker 1: a region of sky roughly five degrees across. Five degrees
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  Speaker 1: sounds abstract. Hold your hand at arm's length and spread
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  Speaker 1: your fingers as wide as they go. That span across
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  Speaker 1: the sky is about ten degrees.
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  Speaker 2: The cold spot covered.
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  Speaker 1: Roughly half that. Enormous, by the standards of CMB anomalies.
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  Speaker 1: The area it occupied on the sky corresponded, at cosmological distances,
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  Speaker 1: to a physical region of staggering actual scale. Whatever was
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  Speaker 1: behind that patch of sky, whatever was causing this temperature dip,
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  Speaker 1: if something was causing it, it was not small. This
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  Speaker 1: was not a local effect from some nearby structure. The
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  Speaker 1: numbers pointed towards something vast, something sitting at a distance
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  Speaker 1: measured in hundreds of millions or billions of light years.
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  Speaker 1: Cosmologists ran probability calculations. They asked, In a universe governed
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  Speaker 1: by the standard model of cosmology, with its well-tested predictions
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  Speaker 1: for how temperature fluctuations are distributed across the CMB, how
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  Speaker 1: likely was a feature like this to appear by chance?
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  Speaker 1: The answers varied depending on assumptions, but none of them
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  Speaker 1: were comfortable. Some estimates placed the odds at 1 in
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  Speaker 1: several hundred, others.
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  Speaker 2: Were more alarming.
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  Speaker 1: Whatever precise number you accepted, the cold spots sat far
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  Speaker 1: out in the tail of any reasonable distribution. It was
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  Speaker 1: not the kind of feature you expected to find once
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  Speaker 1: in the observable universe. The years following that first map
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  Speaker 1: brought more scrutiny, not less. Additionally, studies confirmed the anomaly.
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  Speaker 1: A second satellite, the European Space Agency's Planck mission, would
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  Speaker 1: eventually map the CMB with even greater resolution.
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  Speaker 2: The cold spot would still.
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  Speaker 1: Be there, refined and confirmed, its edges better characterized, its
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  Speaker 1: depth better measured. Every tool that looked at it saw
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  Speaker 1: the same thing. A cold region, roughly circular, sitting in
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  Speaker 1: the direction of Eridanus.
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  Speaker 2: No obvious cause.
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  Speaker 1: No clean explanation from the standard playbook of cosmological physics.
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  Speaker 1: The cold spot was real, it was enormous, and nobody
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  Speaker 1: had any idea what made it. To understand why astronomers
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  Speaker 1: lost sleep over a cold patch in a microwave map,
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  Speaker 1: you have to understand what the map actually shows. Roughly 13.8
• 7:02
  Speaker 1: billion years ago, the universe began in a state of
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  Speaker 1: almost unimaginable heat and density. Everything that now exists, every galaxy,
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  Speaker 1: every star, every atom in every planet, was compressed into
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  Speaker 1: a space smaller than a single proton. In that first
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  Speaker 1: fraction of a second, the universe was so hot that
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  Speaker 1: light could not travel. Photons would collide almost immediately with
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  Speaker 1: the dense fog of plasma surrounding them scatter and go
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  Speaker 1: nowhere The universe was opaque not dark exactly more like
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  Speaker 1: a wall in every direction This continued for a very
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  Speaker 1: long time three hundred eighty thousand years to be precise
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  Speaker 1: Then something changed the universe had been expanding since the
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  Speaker 1: moment of its birth and that expansion meant cooling When
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  Speaker 1: the temperature finally dropped to roughly 3000 Kelvin, about the
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  Speaker 1: surface temperature of a dim red star, something critical happened.
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  Speaker 1: Protons and electrons, which had been careening through the plasma
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  Speaker 1: as separate particles, were able to combine for the first
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  Speaker 1: time into stable hydrogen atoms. The fog cleared. Light was
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  Speaker 1: suddenly free to travel, and it has been traveling ever since.
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  Speaker 1: That light is still out there. Right now, from every
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  Speaker 1: direction in the sky, ancient photons released at that moment 13.8
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  Speaker 1: billion years ago are streaming toward Earth and through your
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  Speaker 1: walls and across your body without stopping. We call it
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  Speaker 1: the Cosmic Microwave Background, the CMB. It is the oldest
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  Speaker 1: light in the universe, a relic of the moment the
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  Speaker 1: cosmos first became transparent. stretched over billions of years of
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  Speaker 1: expansion into the microwave range.
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  Speaker 2: Of the spectrum.
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  Speaker 1: It arrives from every direction equally, which tells you something profound.
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  Speaker 1: The moment it was released surrounded us.
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  Speaker 2: On all sides.
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  Speaker 1: We are, in a meaningful sense, sitting inside it. When
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  Speaker 1: astronomers say the CMB has a temperature of 2.725 Kelvin,
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  Speaker 1: they mean the photons arriving at our detectors carry energy consistent.
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  Speaker 2: With that temperature.
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  Speaker 1: 2.725 Kelvin is 270 degrees below the freezing point of water.
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  Speaker 1: It is almost absolute zero. The universe in its old
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  Speaker 1: age runs cold. What makes this number remarkable is not
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  Speaker 1: how small it is, but how consistent it is. Point
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  Speaker 1: a microwave detector at any patch of sky, and the
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  Speaker 1: reading comes back to 2.725 Kelvin.
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  Speaker 2: Point it somewhere else. Same answer.
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  Speaker 1: The CMB is the most uniform natural signal ever measured
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  Speaker 1: by any instrument in the history of science. One part
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  Speaker 1: in 100,000. That is the scale of the variations that exist.
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  Speaker 1: You would need to measure the temperature to four decimal
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  Speaker 1: places before you would notice anything different from one region
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  Speaker 1: to the next. Those differences, though, are everything. The tiny
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  Speaker 1: temperature variations embedded in the CMB Fluctuations measured in millionths
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  Speaker 1: of a degree, in units called microkelvins, are the fossils
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  Speaker 1: of the earliest structure in the universe. Before light was released,
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  Speaker 1: the plasma filling the cosmos was not perfectly smooth. There
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  Speaker 1: were regions slightly denser than average, regions slightly less dense.
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  Speaker 1: Those density differences created pressure waves, rippling through the plasma
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  Speaker 1: the way sound waves move through air. When the light
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  Speaker 1: broke free, It carried the imprint of those waves with it.
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  Speaker 1: Denser regions were slightly hotter. Less dense regions were slightly cooler.
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  Speaker 1: The temperature map of the CMB is therefore a direct
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  Speaker 1: photograph of the state of the universe 380,000 years after
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  Speaker 1: its birth. A baby photo, taken before a single star
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  Speaker 1: had formed. Those early over densities are where everything began.
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  Speaker 1: Gravity pulled more matter toward the dense spots. over hundreds
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  Speaker 1: of millions of years that matter collapsed into the first
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  Speaker 1: stars stars clustered into the first galaxies galaxies assembled into
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  Speaker 1: clusters and filaments and the universe began to look the
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  Speaker 1: way it looks today every structure you see in the
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  Speaker 1: night sky traces back in a direct causal chain to
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  Speaker 1: those tiny temperature variations in the CMB this is not
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  Speaker 1: a metaphor the positions of galaxies billions of light years
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  Speaker 1: away were determined by fluctuations in a 380,000 year old plasma.
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  Speaker 1: The CMB map is the blueprint from which everything was built.
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  Speaker 1: This is why cosmologists care so much about getting that
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  Speaker 1: map right. It is why WMAP and Planck exist. It
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  Speaker 1: is why researchers have spent decades refining instruments capable of
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  Speaker 1: measuring temperature differences a thousand times smaller than anything a
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  Speaker 1: laboratory thermometer could detect. Every feature in that map carries
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  Speaker 1: information about the conditions of the early universe, what it
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  Speaker 1: was made of, how fast it was expanding, what forces
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  Speaker 1: were acting on it. The map is a diagnostic tool
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  Speaker 1: of extraordinary precision, and the standard cosmological model predicts its
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  Speaker 1: features with matching precision. When observation and prediction agree, you
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  Speaker 1: gain confidence the model is correct. When they diverge, something
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  Speaker 1: is wrong somewhere. which brings you back to the cold spot.
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  Speaker 1: In a map where the typical temperature variation from one
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  Speaker 1: region to another runs around 18 microkelvin, a feature sitting
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  Speaker 1: at 70 microkelvin below average is not a rounding error.
• 13:04
  Speaker 1: It is not instrument noise. It is not something the
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  Speaker 1: beam pattern of the satellite smeared into existence. It is
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  Speaker 1: a genuine, persistent, enormous deviation from what the standard picture
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  Speaker 1: says should be there. The CMB is supposed to look
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  Speaker 1: roughly the same everywhere, shaped by those primordial fluctuations into
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  Speaker 1: a pattern the model can predict and reproduce. A cold
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  Speaker 1: spot that large, and that cold, had no place in
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  Speaker 1: the picture. The numbers, when you actually sit with them,
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  Speaker 1: are the part that made cosmologists uneasy. Start with the temperature.
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  Speaker 1: The cosmic microwave background is not perfectly uniform. It has variations,
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  Speaker 1: tiny wrinkles baked in from the earliest moments of existence.
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  Speaker 1: Those variations average around 18 microkelvin above or below the mean,
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  Speaker 1: 18 millionths of a degree. That is already almost nothing
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  Speaker 1: by any human standard of measurement. The cold spot clocks
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  Speaker 1: in at 70 microkelvin below average, nearly four times the
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  Speaker 1: typical fluctuation. When you are working in a regime where
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  Speaker 1: 18 micro kelvin is the normal range of variation, 70
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  Speaker 1: micro kelvin is not a mild outlier. It is not
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  Speaker 1: a rounding error. It is not the kind of number
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  Speaker 1: that appears in your data and gets quietly set aside.
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  Speaker 1: It is the kind of number that stops a room.
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  Speaker 1: Now consider the size. The cold spot spans roughly 5
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  Speaker 1: degrees across the sky. That sounds modest. Five degrees is
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  Speaker 1: about ten times the width of a full moon as
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  Speaker 1: seen from Earth, but the CMB is not a nearby surface.
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  Speaker 1: The light making up that map left its source when
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  Speaker 1: the universe was only 380,000 years old, and it has
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  Speaker 1: been traveling toward us ever since across a distance of
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  Speaker 1: roughly 46 billion light years. At that range, five degrees
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  Speaker 1: of arc translates into a physical region of staggering scale.
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  Speaker 1: The cold spot, projected back to where it originated, covers
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  Speaker 1: a patch of the early universe several hundred million light
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  Speaker 1: years across. This is not a blemish. This is a
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  Speaker 1: continent-sized wound on the map of everything. Hold those two
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  Speaker 1: facts together. Colder than it should be by nearly four
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  Speaker 1: times the typical variation, and enormous. Not a pinpoint. Not
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  Speaker 1: an artifact of a single bad detector reading. A vast, coherent,
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  Speaker 1: anomalously cold region sitting in the same part of the
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  Speaker 1: sky every time anyone looks. The Planck satellite looked. WMAPIC
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  Speaker 1: looked before it. Both found the same thing in the
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  Speaker 1: same place. The cold spot does not go away. Here
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  Speaker 1: is where the statistics enter and where things get genuinely
• 16:03
  Speaker 1: uncomfortable for anyone doing the math. The Lambda CDM model,
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  Speaker 1: the standard cosmological framework makes specific predictions about what the CMB.
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  Speaker 2: Should look like.
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  Speaker 1: It predicts the distribution of temperature fluctuations across the sky
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  Speaker 1: with extraordinary precision. Most of the sky matches those predictions beautifully.
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  Speaker 1: Then there is Eridanus. Researchers ran the numbers on how
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  Speaker 1: likely it would be within a standard Lambda CDM universe
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  Speaker 1: to find a feature this cold and this large appearing
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  Speaker 1: by chance. The answers depend on the statistical method and
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  Speaker 1: how you define the question, which is itself part of
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  Speaker 1: the problem. But the range of answers is telling. Some
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  Speaker 1: analyses put the probability of this feature occurring randomly at
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  Speaker 1: around 1 in 50. Others, using tighter definitions or different
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  Speaker 1: statistical frameworks, calculated odds closer to 1 in several thousand.
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  Speaker 1: 1 in 50 is uncomfortable. One in several thousand is
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  Speaker 1: the kind of number that, in a different field, would
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  Speaker 1: end a debate. The fact that the range spans such
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  Speaker 1: a wide interval is not evidence that the cold spot
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  Speaker 1: is unimportant. It is evidence that the statistics are genuinely hard,
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  Speaker 1: and that serious people working carefully with the same data
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  Speaker 1: are arriving at very different conclusions about how alarmed to be.
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  Speaker 1: What everyone agrees on is that the cold spot sits
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  Speaker 1: at the extreme tail of the distribution. Whatever probability you
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  Speaker 1: assign it, it does not live near the center of
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  Speaker 1: what the standard model expects. It lives out at the edge,
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  Speaker 1: in territory the model visits rarely if at all. There
• 17:48
  Speaker 1: is a specific way that astronomers talk about statistical outliers.
• 17:53
  Speaker 1: They use sigma, the measure of how many standard deviations
• 17:56
  Speaker 1: a measurement sits from the expected mean. A one sigma
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  Speaker 1: outlier is common. 2 sigma, you start paying attention. 3
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  Speaker 1: sigma is enough to publish a paper. 5 sigma is
• 18:09
  Speaker 1: the threshold particle physicists use to declare a discovery. The
• 18:13
  Speaker 1: cold spot sits somewhere in the range of 3 to
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  Speaker 1: 4 sigma depending on whose analysis you trust. That is
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  Speaker 1: not a flicker. That is a signal. Some researchers argued
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  Speaker 1: early on that the cold spot might be a selection effect.
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  Speaker 1: When you examine a full sky map and go looking
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  Speaker 1: for the single coldest region, you are by definition going
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  Speaker 1: to find the coldest region. The act of selecting the
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  Speaker 1: most extreme feature and then asking how extreme it is
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  Speaker 1: can inflate the apparent significance of any anomaly. This is
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  Speaker 1: known as the look elsewhere effect, and it is a
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  Speaker 1: legitimate methodological concern. Cosmologists took it seriously.
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  Speaker 2: They corrected for it.
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  Speaker 1: The cold spot remained anomalous after the correction, not as
• 19:00
  Speaker 1: dramatically in some analyses, but it survived. It kept surviving.
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  Speaker 1: Then there is the question of its shape. A random
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  Speaker 1: thermal fluctuation tends to look like noise, irregular, without clean
• 19:15
  Speaker 1: internal structure. The cold spot has a measurable profile. It
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  Speaker 1: has a colder core surrounded by a slightly warmer ring.
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  Speaker 1: which in turn sits inside the broader cold region. That
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  Speaker 1: kind of internal organization is unusual. It suggests the feature
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  Speaker 1: has physical coherence rather than being a random collection of
• 19:37
  Speaker 1: unrelated temperature dips that happen to cluster together. A truly
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  Speaker 1: random fluctuation does not generally come with architecture. By the mid-2000s,
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  Speaker 1: the cold spot had survived every attempt to make it disappear.
• 19:52
  Speaker 1: It survived instrument checks. It survived reanalysis with better data.
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  Speaker 1: It survived statistical corrections designed to reduce its apparent significance.
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  Speaker 1: It kept standing there in the Eridanus region of the sky,
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  Speaker 1: colder than it should be. larger than the models could
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  Speaker 1: comfortably produce, shaped in a way that random chance could
• 20:14
  Speaker 1: not easily explain. Whatever was causing the cold spot, it
• 20:19
  Speaker 1: was not going to be something small. Matter in the
• 20:22
  Speaker 1: universe does not spread evenly and never has. From the
• 20:25
  Speaker 1: very first moments after the Big Bang, tiny fluctuations in
• 20:30
  Speaker 1: density were already written into the.
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  Speaker 2: Fabric of space.
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  Speaker 1: Some regions had slightly more matter than others. Gravity did
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  Speaker 1: the rest. Over hundreds of millions of years, matter began flowing.
• 20:43
  Speaker 1: It drained away from under-dense regions and piled up in
• 20:46
  Speaker 1: denser ones. The process accelerated. Dense regions became denser. Empty
• 20:52
  Speaker 1: regions became emptier. What you are left with, after 13.8
• 20:57
  Speaker 1: billion years of this, is not a smooth universe at all.
• 21:02
  Speaker 1: It is something that looks almost biological when you map
• 21:05
  Speaker 1: it at large enough scales. A vast three-dimensional web of
• 21:09
  Speaker 1: filaments and walls and clusters, threaded through with enormous hollows
• 21:14
  Speaker 1: where almost nothing lives. Cosmologists call this the cosmic web.
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  Speaker 1: The name is apt. The bright nodes where filaments intersect
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  Speaker 1: are galaxy clusters, the most massive gravitationally bound structures in
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  Speaker 1: the universe. The filaments themselves are long chains of galaxies
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  Speaker 1: in gas, some stretching for hundreds of millions of light years.
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  Speaker 1: connecting cluster to cluster like strands under tension. Between the
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  Speaker 1: filaments are sheets, flattened walls of galaxies thinner than the
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  Speaker 1: structures they bound. And between all of it, filling the
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  Speaker 1: spaces left behind when matter abandoned them, are the voids.
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  Speaker 1: Not holes in the ordinary sense. Not walls of emptiness
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  Speaker 1: with sharp edges. Voids are softer than that. They are
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  Speaker 1: regions where the density of matter falls well below the
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  Speaker 1: cosmic average. sometimes dramatically so, and where galaxy formation simply
• 22:10
  Speaker 1: never got a strong foothold. The surprising thing, when you
• 22:14
  Speaker 1: work out the numbers, is how much of the universe
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  Speaker 1: these voids actually account for. Depending on how you define
• 22:21
  Speaker 1: the boundaries, voids fill something close to 70% of the
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  Speaker 1: total volume of the observable universe.
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  Speaker 2: 70%.
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  Speaker 1: Most of the space that exists is space where almost
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  Speaker 1: nothing is. The galaxies, the stars, the clusters, all the
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  Speaker 1: light we have ever detected. That is confined to roughly
• 22:41
  Speaker 1: a third of the universe's volume, pressed into the web's
• 22:45
  Speaker 1: filaments and walls like foam on the surface of something
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  Speaker 1: enormous and dark. The voids are not the exception. They
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  Speaker 1: are the rule. This was not obvious until 1981, when
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  Speaker 1: a team of four astronomers, Robert Kirshner, Augustus Omler, Paul
• 23:01
  Speaker 1: Schechter and Steven Schechtman published a survey of galaxy positions
• 23:06
  Speaker 1: in the direction of the constellation Boötes. What they found
• 23:10
  Speaker 1: stopped the field. Where they expected to see a more
• 23:13
  Speaker 1: or less continuous distribution of galaxies thinning gradually with distance,
• 23:18
  Speaker 1: there was instead a nearly empty region roughly 250 million
• 23:22
  Speaker 1: light years across. Almost no galaxies, just space. The Boötes
• 23:28
  Speaker 1: Void as it came to be called, became one of
• 23:31
  Speaker 1: the most studied features in large-scale structure research.
• 23:35
  Speaker 2: It was the first.
• 23:36
  Speaker 1: Clear demonstration that the universe's emptiness had structure, that voids
• 23:41
  Speaker 1: were not random noise, but genuine features carved out by
• 23:46
  Speaker 1: the same gravitational processes that built the web itself. 250
• 23:51
  Speaker 1: million light years is not a small number. If you
• 23:54
  Speaker 1: could travel at the speed of light, Crossing the Boötes
• 23:57
  Speaker 1: void would take longer than the Earth has existed.
• 24:00
  Speaker 2: Yet it sits in the.
• 24:01
  Speaker 1: Universe the way a single bubble sits in a larger foam.
• 24:05
  Speaker 1: The cosmic web contains many such voids, most of them
• 24:08
  Speaker 1: discovered after the techniques for finding them improved. Redshift surveys,
• 24:13
  Speaker 1: which measure the distance to galaxies by stretching of their light,
• 24:17
  Speaker 1: made it possible to build three-dimensional maps of the galaxy
• 24:20
  Speaker 1: distribution across billions of light years. Those maps kept showing
• 24:24
  Speaker 1: the same pattern. Filaments, clusters, walls, and everywhere between them,
• 24:31
  Speaker 1: the voids. Most voids are unremarkable in isolation. A few
• 24:36
  Speaker 1: hundred million light years across, underdense but not empty, containing
• 24:41
  Speaker 1: scattered galaxies at lower than average density. Then there are
• 24:45
  Speaker 1: the ones at the extreme end of the distribution, the
• 24:48
  Speaker 1: ones astronomers call supervoids. A supervoid is not simply a
• 24:52
  Speaker 1: large void. It is a structure where the underdensity is deep,
• 24:57
  Speaker 1: where the volume is vast enough to be measured not
• 24:59
  Speaker 1: in hundreds of millions of light years, but in billions,
• 25:03
  Speaker 1: and where the matter deficit is so severe that the
• 25:07
  Speaker 1: region stands apart from the rest of the cosmic web,
• 25:10
  Speaker 1: the way the Boötes void stood apart from everything astronomers
• 25:13
  Speaker 1: had seen before it. Supervoids are rare. The conditions needed
• 25:18
  Speaker 1: to produce them sustained outflow of matter over billions of years,
• 25:23
  Speaker 1: in a region that started off unusually underdense, make them
• 25:28
  Speaker 1: statistical outliers even in a universe full of voids. Understanding
• 25:33
  Speaker 1: why supervoids matter for the cold spot problem requires one
• 25:37
  Speaker 1: more piece of physics, which will get its own full
• 25:40
  Speaker 1: treatment in the next section. But the outline is this.
• 25:45
  Speaker 1: The cosmic microwave background photons we detect today did not
• 25:49
  Speaker 1: travel through empty, featureless space to reach us. They traveled
• 25:54
  Speaker 1: through the cosmic web. They passed through clusters and filaments
• 25:58
  Speaker 1: and walls. They also passed through voids. And when a
• 26:02
  Speaker 1: photon travels through a void, through a region where the
• 26:06
  Speaker 1: gravitational environment is weaker than average, a kind of gravitational
• 26:11
  Speaker 1: hill rather than a valley, something happens.
• 26:14
  Speaker 2: To its energy.
• 26:16
  Speaker 1: The expansion of the universe plays a role. The interaction
• 26:20
  Speaker 1: is subtle and has a name. the integrated Sachs-Wolfe effect.
• 26:25
  Speaker 1: A photon that passes through a deep enough void in
• 26:29
  Speaker 1: an accelerating universe does not emerge from the other side
• 26:33
  Speaker 1: with the same energy it carried going in. The consequence
• 26:37
  Speaker 1: is measurable. In principle, a sufficiently large and deep void
• 26:43
  Speaker 1: sitting along the line of sight between us and the
• 26:46
  Speaker 1: CMB would leave a cold imprint on the map. A
• 26:50
  Speaker 1: patch of sky in that direction would read slightly colder
• 26:54
  Speaker 1: than its surroundings. Not because the void itself is cold,
• 26:58
  Speaker 1: but because the photons that passed through it lost something
• 27:02
  Speaker 1: they could not recover. The map would show a depression
• 27:05
  Speaker 1: in temperature, faint but real, centered on the direction of
• 27:10
  Speaker 1: the void. The cold spot in Eridanus is 70 microkelvin
• 27:15
  Speaker 1: colder than average. It spans roughly 5 degrees across the sky.
• 27:21
  Speaker 1: It has no explanation in the standard model that fully
• 27:25
  Speaker 1: accounts for its depth or size. If you could find
• 27:28
  Speaker 1: a super void sitting in exactly the right place along
• 27:31
  Speaker 1: the line of sight to the cold spot, you would
• 27:34
  Speaker 1: have the beginning of an answer. There is a mechanism
• 27:37
  Speaker 1: in physics, subtle, quiet, operating across billions of light years,
• 27:43
  Speaker 1: that could connect an empty region of space to a
• 27:46
  Speaker 1: cold patch in the oldest light in the universe. To
• 27:50
  Speaker 1: understand it, you need to hold two things in your
• 27:53
  Speaker 1: mind at once. The first is that light is not
• 27:56
  Speaker 1: immune to gravity. A photon falling into a gravitational well
• 28:01
  Speaker 1: gains energy. It blue ships slightly, arriving at the far
• 28:05
  Speaker 1: side of that well a little hotter than it started.
• 28:09
  Speaker 1: The reverse is also true. A photon climbing out of
• 28:12
  Speaker 1: a gravitational well loses energy.
• 28:15
  Speaker 2: It redshifts.
• 28:16
  Speaker 1: It arrives a little colder. This is not a theoretical curiosity.
• 28:21
  Speaker 1: It is a measured, confirmed prediction of general relativity, and
• 28:25
  Speaker 1: it operates on every photon crossing every structure in the universe.
• 28:29
  Speaker 1: The second thing is that voids are not neutral terrain.
• 28:33
  Speaker 1: A void is not simply empty space. It is a
• 28:37
  Speaker 1: region where matter is absent, and that absence has gravitational consequences.
• 28:42
  Speaker 1: If a dense cluster of galaxies is a valley, a
• 28:45
  Speaker 1: deep well that photons fall into and climb out of,
• 28:49
  Speaker 1: then a void is a hill. A photon approaching a
• 28:52
  Speaker 1: void has to climb that gravitational hill before it can
• 28:56
  Speaker 1: descend the other side. It loses energy on the way up.
• 29:00
  Speaker 1: It should recover that energy on the way down. In
• 29:03
  Speaker 1: a static universe, the climb and the descent would be
• 29:06
  Speaker 1: perfectly symmetrical, and the photon would arrive at our detectors
• 29:11
  Speaker 1: with the same energy it started with. The two effects
• 29:14
  Speaker 1: would cancel exactly. But we do not live in a
• 29:17
  Speaker 1: static universe. The universe is expanding. Dark energy is accelerating
• 29:23
  Speaker 1: that expansion, and this changes everything about how a photon
• 29:27
  Speaker 1: experiences a void. Picture that gravitational hill. A photon begins
• 29:33
  Speaker 1: climbing it, losing energy as it goes. While the photon
• 29:37
  Speaker 1: is crossing the void, the universe continues to expand. The
• 29:42
  Speaker 1: void itself stretches. By the time the photon reaches the
• 29:47
  Speaker 1: far side and begins descending, the hill is not quite
• 29:50
  Speaker 1: the same hill it started climbing. The descent is shallower
• 29:54
  Speaker 1: than the ascent. The photon cannot recover everything it lost
• 29:58
  Speaker 1: on the way up. because the potential well it is
• 30:01
  Speaker 1: descending into has weakened during the crossing. The photon emerges
• 30:05
  Speaker 1: from the void carrying slightly less energy than it carried
• 30:09
  Speaker 1: going in.
• 30:10
  Speaker 2: It is colder.
• 30:11
  Speaker 1: This is the integrated Sachs-Wolfe effect. The name honors Rainer
• 30:16
  Speaker 1: Sachs and Arthur Wolfe, who worked out the mathematics in 1967,
• 30:21
  Speaker 1: well before it could be tested. The word integrated matters.
• 30:26
  Speaker 1: It is not a single event at a boundary. The
• 30:29
  Speaker 1: effect accumulates across the entire crossing, adding up over hundreds
• 30:34
  Speaker 1: of millions of light years as the photon passes through
• 30:37
  Speaker 1: the gravitational landscape of large-scale structure. Every void the photon traverses,
• 30:44
  Speaker 1: every under-dense region along its path, contributes a small deficit
• 30:49
  Speaker 1: of energy. Colder by a tiny amount here. Colder by
• 30:54
  Speaker 1: a tiny amount there. Think of it this way. Imagine
• 30:58
  Speaker 1: rolling a ball across a landscape of hills and valleys.
• 31:02
  Speaker 1: On flat ground, a hill and a matching valley cancel out.
• 31:06
  Speaker 1: You end up at the same height you started. But
• 31:09
  Speaker 1: now imagine that while the ball is crossing the hill,
• 31:12
  Speaker 1: the ground behind it slowly rises. When the ball reaches
• 31:16
  Speaker 1: the valley on the other side, the valley is shallower
• 31:19
  Speaker 1: than it.
• 31:19
  Speaker 2: Used to be.
• 31:20
  Speaker 1: The ball loses more height than it gains back. It
• 31:24
  Speaker 1: arrives lower than it began. A photon crossing a void
• 31:27
  Speaker 1: in an expanding universe is that ball. It arrives at
• 31:31
  Speaker 1: our telescopes carrying less energy than it would if the
• 31:35
  Speaker 1: void had not been there. The effect is genuinely small.
• 31:39
  Speaker 1: For a typical void, the temperature deficit imprinted on the
• 31:42
  Speaker 1: CMB is measured in microkelvin, millionths.
• 31:46
  Speaker 2: Of a degree.
• 31:48
  Speaker 1: You would never detect it from a single photon or
• 31:51
  Speaker 1: even from a small patch of sky. What makes the
• 31:54
  Speaker 1: effect observable?
• 31:55
  Speaker 2: Is scale.
• 31:57
  Speaker 1: When you look at a very large void, a supervoid,
• 32:00
  Speaker 1: you are looking at a structure that photons spend an
• 32:03
  Speaker 1: enormous amount of time crossing. The crossing might take hundreds
• 32:07
  Speaker 1: of millions of years. Every moment of that crossing, the
• 32:11
  Speaker 1: expansion of the universe is quietly working against the photon's
• 32:15
  Speaker 1: ability to recover what it lost. The energy deficit accumulates
• 32:20
  Speaker 1: The resulting cold signature, averaged across the angular extent of
• 32:24
  Speaker 1: the supervoid on the sky, becomes something measurable. This is
• 32:29
  Speaker 1: how a cold spot could appear in the CMB in
• 32:32
  Speaker 1: the direction of a supervoid. The ancient light from the
• 32:35
  Speaker 1: surface of last scattering, traveling toward us for nearly 14
• 32:40
  Speaker 1: billion years, passed through the matter landscape of the universe
• 32:44
  Speaker 1: on its way here. Wherever it crossed a dense structure,
• 32:48
  Speaker 1: it blued slightly. Wherever it crossed a void, it reddened slightly.
• 32:53
  Speaker 1: Those tiny temperature shifts are recorded in the microwave map
• 32:57
  Speaker 1: as warm and cold patches overlaid on top of the
• 33:00
  Speaker 1: primordial fluctuations from the Big Bang itself. The ISW effect
• 33:06
  Speaker 1: is a layer of cosmic history written on top of
• 33:09
  Speaker 1: the oldest signal.
• 33:10
  Speaker 2: In the universe. Were the ISW effects strong enough?
• 33:14
  Speaker 1: A sufficiently enormous void could leave a temperature depression measurable
• 33:19
  Speaker 1: on the scale of what WMAP detected in Eridanus. That
• 33:24
  Speaker 1: is the connection. That is what makes a supervoid a
• 33:28
  Speaker 1: candidate explanation for a cold patch in the CMB. It
• 33:32
  Speaker 1: is not exotic physics. It is not speculation. It follows
• 33:37
  Speaker 1: directly from general relativity, from the confirmed expansion of the universe.
• 33:43
  Speaker 1: and from the confirmed acceleration driven by dark energy. The
• 33:47
  Speaker 1: only question is whether the void in question is large
• 33:50
  Speaker 1: enough and empty enough to produce the observed effect at
• 33:54
  Speaker 1: the required magnitude. You need the void to be real.
• 33:58
  Speaker 1: You need it to be deep. You need it to
• 34:01
  Speaker 1: span a volume large enough that photons spend a meaningful
• 34:04
  Speaker 1: fraction of their journey crossing it. And you need it
• 34:07
  Speaker 1: to sit at the right distance. close enough that the
• 34:10
  Speaker 1: expanding universe has had time to work on its gravitational potential,
• 34:15
  Speaker 1: but far enough that the CMB photons were genuinely passing
• 34:19
  Speaker 1: through it long ago. A big enough void in the
• 34:22
  Speaker 1: right place would leave exactly the kind of cold mark
• 34:26
  Speaker 1: that was sitting in the Eridanus region of the map.
• 34:29
  Speaker 1: In August of 2007, astronomers announced they had found something
• 34:33
  Speaker 1: in the direction of the cold spot that made the
• 34:36
  Speaker 1: whole problem considerably stranger. Not stranger in a way that
• 34:40
  Speaker 1: pointed toward a solution, stranger in a way that raised
• 34:43
  Speaker 1: the stakes, because what they found sitting behind the cold
• 34:47
  Speaker 1: spot was not a galaxy, not a cluster, not a filament.
• 34:52
  Speaker 1: It was an absence, a region of space so much
• 34:55
  Speaker 1: emptier than anything around it that the word void barely
• 34:59
  Speaker 1: seemed adequate. The search had been methodical. After the cold
• 35:03
  Speaker 1: spot turned up in the WMAP data and survived every
• 35:07
  Speaker 1: attempt to dismiss it as noise or instrument error, a
• 35:10
  Speaker 1: small group of researchers decided to take the ISW hypothesis
• 35:15
  Speaker 1: seriously and look for a physical structure that could be
• 35:18
  Speaker 1: producing the signal.
• 35:20
  Speaker 2: The logic was straightforward.
• 35:22
  Speaker 1: If a supervoid sitting along the line of sight to
• 35:25
  Speaker 1: the cold spot was responsible for cooling those CMB photons,
• 35:29
  Speaker 1: then the supervoid had to be there.
• 35:32
  Speaker 2: It was a testable prediction. You could look. So they looked.
• 35:37
  Speaker 1: What that looking required was a galaxy survey, a systematic
• 35:42
  Speaker 1: measurement of how galaxies were distributed in that patch of
• 35:45
  Speaker 1: sky at a range of distances. Galaxies are not evenly
• 35:50
  Speaker 1: spread through space. They cluster where matter clusters. Where matter
• 35:55
  Speaker 1: thins out, galaxies thin out with it. If a supervoid
• 35:59
  Speaker 1: was sitting in the direction of Eridanus, the galaxy counts
• 36:03
  Speaker 1: in that direction should drop, and not just slightly. A
• 36:07
  Speaker 1: structure large enough to cool CMB photons by the amount
• 36:11
  Speaker 1: observed would have to be enormous, which meant the galaxy deficit.
• 36:16
  Speaker 2: Should be hard to miss.
• 36:18
  Speaker 1: Researchers pulled data from existing surveys, ran redshift measurements to
• 36:22
  Speaker 1: establish distances, and began building a three-dimensional map of the
• 36:27
  Speaker 1: matter distribution along that line of sight. Redshift measurements work
• 36:32
  Speaker 1: because the universe is expanding. Light from distant galaxies gets
• 36:36
  Speaker 1: stretched as it travels toward us, shifting toward longer wavelengths.
• 36:41
  Speaker 1: The more distant the galaxy, the greater the shift. By
• 36:45
  Speaker 1: measuring how much a galaxy's light has shifted, you can
• 36:49
  Speaker 1: calculate how far away it is with considerable precision. Stack
• 36:53
  Speaker 1: enough of those measurements together and you get a map,
• 36:57
  Speaker 1: not just of where galaxies appear on the sky, but
• 37:00
  Speaker 1: of where they sit in three-dimensional space, at what distances,
• 37:05
  Speaker 1: in what concentrations. That is the kind of map that
• 37:08
  Speaker 1: reveals voids. The map that emerged from the data pointed
• 37:12
  Speaker 1: at something extraordinary. Out at a distance of roughly 1.9
• 37:17
  Speaker 1: billion light years, in the exact direction of the cold spot,
• 37:21
  Speaker 1: the galaxy density fell off a cliff. The deficit was
• 37:25
  Speaker 1: not subtle. Galaxies that should have been there simply were not.
• 37:29
  Speaker 1: The region was not completely empty. A super void is
• 37:33
  Speaker 1: not a perfect vacuum, but the number of galaxies present
• 37:37
  Speaker 1: was roughly 30% below what you would expect based on
• 37:40
  Speaker 1: the surrounding cosmic average. That is not a statistical fluctuation.
• 37:46
  Speaker 1: That is a structural feature. As researchers mapped the extent
• 37:50
  Speaker 1: of the underdensity, the scale became harder to hold in
• 37:54
  Speaker 1: your head.
• 37:55
  Speaker 2: The void stretched.
• 37:56
  Speaker 1: Across roughly 1.8 billion light years. Its shape was not spherical.
• 38:02
  Speaker 1: It was elongated, extended along one axis more than others.
• 38:07
  Speaker 1: Some researchers described it as cigar-shaped. This asymmetry was itself informative.
• 38:14
  Speaker 1: Matter does not drain away from a void uniformly in
• 38:17
  Speaker 1: all directions.
• 38:19
  Speaker 2: The shape of a void.
• 38:20
  Speaker 1: Reflects the geometry of the large-scale structure surrounding it. The
• 38:24
  Speaker 1: directions in which matter was preferentially drawn off into neighboring
• 38:28
  Speaker 1: filaments and walls over billions of years of cosmic evolution.
• 38:33
  Speaker 1: The Aradana supervoid had been carved out gradually by the
• 38:37
  Speaker 1: slow gravitational tug of everything around it pulling matter away
• 38:41
  Speaker 1: from a region that started out slightly U. Underdents and
• 38:45
  Speaker 1: became vastly more so over cosmic time. To call this
• 38:49
  Speaker 1: one of the largest structures ever identified in the observable
• 38:53
  Speaker 1: universe is not hyperbole. It is simply the accurate description.
• 38:58
  Speaker 1: The observable universe stretches roughly 93 billion light years across.
• 39:04
  Speaker 1: A single void spanning 1.8 billion light years of that
• 39:08
  Speaker 1: represents a feature of genuine cosmic significance. For comparison, the
• 39:14
  Speaker 1: Milky Way galaxy is roughly 100,000 light years across. The
• 39:19
  Speaker 1: Eridanus supervoid is about 18,000 Milky Ways wide. That number
• 39:24
  Speaker 1: does not help very much. Nothing does, at these scales.
• 39:29
  Speaker 2: What it means.
• 39:30
  Speaker 1: Practically, is that a photon of light crossing a supervoid
• 39:33
  Speaker 1: at the speed of light would spend over a billion
• 39:36
  Speaker 1: years in transit, just getting from one side to the other.
• 39:40
  Speaker 1: The confirmation carried a specific weight that went beyond the
• 39:44
  Speaker 1: scientific result itself. For three years since the cold spot
• 39:48
  Speaker 1: appeared in the WMAP data, the dominant response from the
• 39:52
  Speaker 1: cosmological community had been skepticism and uncertainty. Was this a
• 39:58
  Speaker 1: real feature of the universe or an artifact of the
• 40:01
  Speaker 1: way the data was processed? Was it genuinely anomalous or
• 40:05
  Speaker 1: just an outlier that standard cosmology could accommodate at the
• 40:09
  Speaker 1: tails of its probability distribution? Was there any physical structure
• 40:13
  Speaker 1: in the universe that could account for it? Or was
• 40:16
  Speaker 1: the cold spot pointing at something in the early universe
• 40:19
  Speaker 1: that no structure in the modern one could explain? The
• 40:23
  Speaker 1: supervoid answered at least one of those questions. There was
• 40:26
  Speaker 1: something there. The galaxy surveys were not ambiguous on this point.
• 40:32
  Speaker 1: The matter deficit was real. It was large, and it
• 40:36
  Speaker 1: was sitting in precisely the right place. Whether it was
• 40:39
  Speaker 1: large enough and deep enough to explain the full temperature
• 40:43
  Speaker 1: depression observed in the cold spot was a separate question,
• 40:47
  Speaker 1: one that required more detailed analysis. But the starting point,
• 40:52
  Speaker 1: the question of whether a candidate physical structure existed, had
• 40:56
  Speaker 1: just gone from open to answered. Here was an under-density
• 41:00
  Speaker 1: on a scale that had not been catalogued before, sitting
• 41:04
  Speaker 1: behind the single most puzzling feature in the cosmic microwave
• 41:07
  Speaker 1: background map. in a position and at a distance where
• 41:11
  Speaker 1: the ISW effect could plausibly leave a cold imprint in
• 41:16
  Speaker 1: exactly the direction where the cold spot was observed. Coincidences
• 41:21
  Speaker 1: in science are not impossible. But a coincidence of this specificity,
• 41:26
  Speaker 1: this structure, this size, this location, was exactly the kind
• 41:31
  Speaker 1: of thing that makes researchers sit up straighter and start
• 41:35
  Speaker 1: running numbers in earnest. For the first time, the cold
• 41:39
  Speaker 1: spot had a candidate explanation, and it was sitting 1.9
• 41:43
  Speaker 1: billion light years away in the form of the largest
• 41:46
  Speaker 1: identified void.
• 41:48
  Speaker 2: In the observable universe.
• 41:50
  Speaker 1: The number 1.8 billion light years is one of those
• 41:54
  Speaker 1: figures that sounds impressive but does not automatically mean anything
• 41:58
  Speaker 1: to the brain that encounters it. So start smaller. The
• 42:02
  Speaker 1: moon sits roughly 239,000 miles from Earth. Light covers that
• 42:08
  Speaker 1: distance in about 1.3 seconds. When you look up at
• 42:12
  Speaker 1: a full moon, you are seeing light that left its
• 42:14
  Speaker 1: surface a little over a second ago. That gap, 239,000 miles,
• 42:21
  Speaker 1: is the largest distance most humans will ever intuitively feel.
• 42:25
  Speaker 1: It is where our sense of scale begins to break down,
• 42:29
  Speaker 1: push outward. The sun is 93 million miles away. Light
• 42:34
  Speaker 1: takes 8 minutes to cross that distance. If you replace
• 42:38
  Speaker 1: the moon with the sun as your mental measuring stick,
• 42:41
  Speaker 1: you have already multiplied your sense of distance by roughly 400.
• 42:45
  Speaker 1: The nearest star to our own, Proxima Centauri, sits about 4.2
• 42:51
  Speaker 1: light years away. Light leaving Proxima tonight will not reach
• 42:56
  Speaker 1: Earth until the year 2029. Four years of travel at
• 43:00
  Speaker 1: the fastest speed the universe permits.
• 43:03
  Speaker 2: Hold that.
• 43:04
  Speaker 1: A single star, the closest one, takes four years of
• 43:08
  Speaker 1: light travel to reach us. The Milky Way is 100,000
• 43:12
  Speaker 1: light years across. If you could somehow compress all of
• 43:16
  Speaker 1: human history into the time it takes light to cross
• 43:19
  Speaker 1: our own galaxy, every civilization, every war, every generation of
• 43:25
  Speaker 1: recorded thought, you would not make it past the first
• 43:29
  Speaker 1: fraction of a second. The local group, the cluster of
• 43:33
  Speaker 1: galaxies our Milky Way belongs to, including Andromeda, spans roughly
• 43:38
  Speaker 1: 10 million light years. That is 100 Milky Ways laid
• 43:42
  Speaker 1: side by side. Now you are ready for 1.8 billion
• 43:47
  Speaker 1: light years. The Eridanus Supervoid is 1.8 billion light years across.
• 43:52
  Speaker 1: That is 18,000 Milky Ways. It is 180 local groups.
• 43:58
  Speaker 1: It is a span of space so extreme that the
• 44:01
  Speaker 1: light crossing it takes longer than the entire age of
• 44:04
  Speaker 1: the Earth. Longer than the Sun has existed. Our solar
• 44:08
  Speaker 1: system formed about 4.6 billion years ago. Light that entered
• 44:13
  Speaker 1: one edge of the Eridanus supervoid when our solar system
• 44:16
  Speaker 1: was young and newly formed would still have hundreds of
• 44:19
  Speaker 1: millions of years left to travel before reaching the other side.
• 44:23
  Speaker 1: The entire observable universe, everything we can in principle detect
• 44:28
  Speaker 1: with our instruments, spans roughly 93 billion light years in diameter.
• 44:33
  Speaker 1: The Eridana supervoid represents approximately 2% of that total width.
• 44:38
  Speaker 1: 2% sounds like a small number. Apply it to a
• 44:43
  Speaker 1: structure the size of the observable universe and it stops
• 44:46
  Speaker 1: sounding small very quickly. What makes the supervoid more than
• 44:50
  Speaker 1: just an enormous empty zone is what is missing inside it.
• 44:55
  Speaker 1: Matter in the universe averages out to a certain background density.
• 45:00
  Speaker 1: The Eridanus supervoid runs about 30% below that average. Not 3%.
• 45:07
  Speaker 1: Not a mild underdensity that might blur into statistical noise.
• 45:11
  Speaker 2: 30%.
• 45:13
  Speaker 1: And that 30% figure applies to the overall structure. Toward
• 45:18
  Speaker 1: the center of the void, the deficit deepens. The middle
• 45:22
  Speaker 1: is emptier still. A region where galaxies are not merely scarce,
• 45:27
  Speaker 1: but genuinely profoundly rare. matter has been draining out of
• 45:31
  Speaker 1: this space for billions of years flowing outward toward the
• 45:35
  Speaker 1: denser filaments and walls of the cosmic web that surround
• 45:39
  Speaker 1: it and what is left behind is something close to
• 45:43
  Speaker 1: a gravitational desert the shape of the supervoid adds another
• 45:47
  Speaker 1: layer of strangeness it is not a rough sphere the
• 45:51
  Speaker 1: way you might imagine a bubble or a gap it
• 45:54
  Speaker 1: is elongated stretched something like a cigar extended across space.
• 46:00
  Speaker 1: That shape is a record of how the void formed.
• 46:03
  Speaker 1: Matter did not flow away from this region equally in
• 46:06
  Speaker 1: all directions. It drained preferentially along certain axes, leaving behind
• 46:12
  Speaker 1: an asymmetric absence. The elongated structure tells you that the
• 46:16
  Speaker 1: gravitational landscape around this void was itself uneven, pulling matter
• 46:22
  Speaker 1: harder in some directions than others. The shape is a fossil,
• 46:26
  Speaker 1: Frozen evidence of the forces that carved this region out
• 46:30
  Speaker 1: of the cosmic web. For context, consider the Boötes void.
• 46:34
  Speaker 1: Discovered in 1981, it was the first large void astronomers confirmed,
• 46:40
  Speaker 1: and for a long time it served as the standard
• 46:43
  Speaker 1: example of how empty the universe could get in places.
• 46:46
  Speaker 1: The Boötes void is roughly 250 million light years across.
• 46:51
  Speaker 1: When it was found, that size was stunning. Astronomers struggled
• 46:56
  Speaker 1: to explain how such a large empty region could form
• 46:59
  Speaker 1: within standard models of structure formation. For years it occupied
• 47:04
  Speaker 1: a special place in discussions of cosmic structure, a genuinely
• 47:08
  Speaker 1: exceptional feature. The Eridanus supervoid is roughly seven times wider
• 47:13
  Speaker 1: than the Boötes void.
• 47:15
  Speaker 2: Seven times.
• 47:17
  Speaker 1: The structure that once defined the upper boundary of what
• 47:20
  Speaker 1: seemed possible for a cosmic void is dwarfed so completely
• 47:24
  Speaker 1: by the Eridanus supervoid that the comparison almost becomes difficult
• 47:29
  Speaker 1: to make. If you place the Boötis void inside the
• 47:32
  Speaker 1: Eridanus supervoid, you would have room for several more like
• 47:36
  Speaker 1: it before you approach the edges. There is a specific
• 47:39
  Speaker 1: kind of unease that comes with processing this properly. The
• 47:44
  Speaker 1: word void suggests something simple, an absence, a gap, a nothing.
• 47:51
  Speaker 1: But what the Eridanus supervoid actually represents is the cumulative
• 47:55
  Speaker 1: result of billions of years of gravitational dynamics operating across
• 48:00
  Speaker 1: a region of space larger than most of what we
• 48:03
  Speaker 1: can detect with our instruments. Galaxies formed inside it, because
• 48:07
  Speaker 1: even an under-dense region produces some structure. But they formed
• 48:12
  Speaker 1: more slowly, more sparsely, more dimly than anywhere else we
• 48:16
  Speaker 1: can identify.
• 48:18
  Speaker 2: The void is.
• 48:18
  Speaker 1: Not empty in an absolute sense. It is empty in
• 48:22
  Speaker 1: the way that a drought-stricken landscape is empty. Life still
• 48:26
  Speaker 1: holds on, but what should be there, mostly isn't. By
• 48:30
  Speaker 1: any measure that makes sense to a human being. The
• 48:33
  Speaker 1: Eridanus supervoid is not a feature of the universe so
• 48:37
  Speaker 1: much as it is an absence that the universe has
• 48:39
  Speaker 1: been building for billions of years. By 2015, the Dark
• 48:44
  Speaker 1: Energy Survey had accumulated enough data on hundreds of millions
• 48:48
  Speaker 1: of galaxies to settle the question of whether the supervoid
• 48:52
  Speaker 1: was really there. The Dark Energy Survey is not a
• 48:55
  Speaker 1: small instrument. It is one of the most ambitious mapping
• 48:58
  Speaker 1: projects in the history of observational astronomy. A collaboration involving
• 49:03
  Speaker 1: hundreds of scientists across dozens of institutions, using a 570
• 49:09
  Speaker 1: megapixel camera mounted on a telescope in the Chilean Andes
• 49:14
  Speaker 1: to image roughly one-eighth of the entire sky. Its primary
• 49:18
  Speaker 1: purpose was to understand darkness energy by tracking how the
• 49:21
  Speaker 1: large-scale structure of the universe has changed over billions of years.
• 49:26
  Speaker 1: To do that it needed to map galaxies, hundreds of
• 49:30
  Speaker 1: millions of them. It needed to know where they clustered,
• 49:33
  Speaker 1: where they thinned out and where they were simply absent.
• 49:36
  Speaker 1: That made it exactly the right tool for the Eridanus problem.
• 49:41
  Speaker 1: The 2007 discovery of the supervoid had been suggestive, convincing even,
• 49:46
  Speaker 1: to many researchers. But the original detection relied on shallower
• 49:51
  Speaker 1: surveys with incomplete coverage of the relevant region. You could
• 49:56
  Speaker 1: see the outline of something large and empty in that direction.
• 50:00
  Speaker 1: What you could not do was measure it with confidence.
• 50:03
  Speaker 1: Pin down its true dimensions, its density profile, how deep
• 50:07
  Speaker 1: the matter deficit actually ran. The question of whether the
• 50:11
  Speaker 1: supervoid was genuinely extreme enough to matter, or whether it
• 50:15
  Speaker 1: was a modest underdensity that astronomers had talked themselves into over-interpreting,
• 50:21
  Speaker 1: remained open. That discomfort lived in the literature for nearly
• 50:25
  Speaker 1: eight years. Then the DES data arrived. Researchers turned the
• 50:31
  Speaker 1: survey's accumulated observations toward the Eridanus region and began counting.
• 50:36
  Speaker 1: Not a rough census, a precise, systematic accounting of galaxy
• 50:41
  Speaker 1: density across that entire patch of sky, at multiple distances,
• 50:46
  Speaker 1: using redshift measurements to peel apart the structure in three dimensions.
• 50:50
  Speaker 1: What they found removed the ambiguity. The supervoid was there,
• 50:55
  Speaker 1: it was large, and the matter deficit at its.
• 50:58
  Speaker 2: Core was not modest.
• 50:59
  Speaker 1: The region contained roughly 30% less matter than the surrounding
• 51:04
  Speaker 1: cosmic average, with the under-density growing more pronounced toward the center.
• 51:09
  Speaker 1: The Eridana supervoid was not a smudge in noisy data,
• 51:13
  Speaker 1: it was a feature, the most prominent void in the
• 51:16
  Speaker 1: entire DES dataset. That last point deserves a moment. The
• 51:21
  Speaker 1: Dark Energy Survey observed a volume of the universe containing
• 51:25
  Speaker 1: hundreds of millions of galaxies and spanning billions of light
• 51:29
  Speaker 1: years in multiple directions. Within that entire catalogue, researchers identified
• 51:35
  Speaker 1: many voids. Underdense regions are a normal part of the
• 51:39
  Speaker 1: cosmic web. They come in different sizes and depths. When
• 51:43
  Speaker 1: researchers ranked the voids in the DES dataset by how
• 51:47
  Speaker 1: extreme they were, the Eridanus supervoid sat at the top.
• 51:52
  Speaker 1: The largest survey of its kind had found no void
• 51:55
  Speaker 1: anywhere in its field more pronounced than the one sitting
• 51:58
  Speaker 1: behind the cold spot. But galaxy counts alone, while compelling,
• 52:03
  Speaker 1: carry a limitation. Galaxies trace ordinary matter, the stars and
• 52:08
  Speaker 1: gas that emit or block light, and can therefore be
• 52:12
  Speaker 1: detected directly. They do not trace dark matter. And dark
• 52:17
  Speaker 1: matter which makes up roughly five times as much of
• 52:20
  Speaker 1: the universe's total matter budget as ordinary matter does, is
• 52:24
  Speaker 1: invisible to direct observation. A void confirmed through galaxy counts
• 52:30
  Speaker 1: alone is a void confirmed for one component of the
• 52:34
  Speaker 1: universe's matter. The DES team knew this, so they used
• 52:39
  Speaker 1: a second method entirely. Gravitational lensing is the bending of
• 52:44
  Speaker 1: light by mass. Every massive object in the universe distorts
• 52:48
  Speaker 1: the path of light travelling near it, slightly stretching or
• 52:51
  Speaker 1: shearing the images of background sources. In a region that
• 52:55
  Speaker 1: is underdense, where mass is systematically absent, this lensing effect
• 53:00
  Speaker 1: is reduced. Background galaxies behind the void appear less distorted
• 53:04
  Speaker 1: than they would if normal amounts of matter were present.
• 53:08
  Speaker 1: By measuring the shapes of galaxies behind the Eridanus region
• 53:11
  Speaker 1: with great precision and comparing them to what standard distributions
• 53:15
  Speaker 1: would predict, the DS team could directly probe the gravitational
• 53:19
  Speaker 1: environment of the supervoid. They were not counting galaxies. They
• 53:23
  Speaker 1: were weighing the space itself. The lensing signal confirmed what
• 53:28
  Speaker 1: the galaxy.
• 53:28
  Speaker 2: Counts had shown.
• 53:30
  Speaker 1: The Eridanus region was deficient, not just in ordinary matter,
• 53:35
  Speaker 1: but in dark matter as well. Both components of the
• 53:38
  Speaker 1: universe's matter budget showed the same deficit in the same place.
• 53:43
  Speaker 1: The underdensity was not a quirk of how galaxies had
• 53:46
  Speaker 1: formed in that region, some local suppression of star formation
• 53:51
  Speaker 1: that left the dark matter structure intact. The underlying mass distribution,
• 53:56
  Speaker 1: dark matter and all, was genuinely depleted. The supervoid ran
• 54:02
  Speaker 1: all the way down. This convergence matters. In observational science,
• 54:07
  Speaker 1: Two independent methods that share no systematic errors and rely
• 54:12
  Speaker 1: on completely different physical phenomena can still occasionally agree by chance.
• 54:18
  Speaker 1: That probability shrinks towards zero when they agree on a
• 54:21
  Speaker 1: structure of this scale and at this.
• 54:23
  Speaker 2: Level of precision.
• 54:25
  Speaker 1: The DES confirmation was not merely statistical reassurance. It closed
• 54:31
  Speaker 1: the door on the most credible alternative interpretations. The Eridana
• 54:35
  Speaker 1: supervoid was real in the only sense that counts in physics.
• 54:39
  Speaker 1: Multiple distinct lines of evidence, built on different assumptions, using
• 54:44
  Speaker 1: different data, pointed at the same object and measured the
• 54:48
  Speaker 1: same thing. What the 2015 result gave the field was.
• 54:52
  Speaker 2: Not a solution.
• 54:53
  Speaker 1: The cold spot still sat in the CMB, still colder
• 54:57
  Speaker 1: than anything standard models predicted with ease. The supervoid still
• 55:02
  Speaker 1: sat along the line of sight, now confirmed beyond serious doubt.
• 55:07
  Speaker 1: Whether the one caused the other, and by how much,
• 55:10
  Speaker 1: remained a harder question. But at least the question had
• 55:14
  Speaker 1: graduated from whether the supervoid existed to what it actually meant.
• 55:19
  Speaker 2: That was progress.
• 55:20
  Speaker 1: Two completely independent methods, galaxy counts and gravitational lensing, had
• 55:26
  Speaker 1: pointed at the same vast emptiness in the same direction,
• 55:29
  Speaker 1: and both agreed. The supervoid should have solved the problem,
• 55:34
  Speaker 1: By 2015, astronomers had two things they had been looking
• 55:37
  Speaker 1: for since the cold spot first appeared in the WMAP
• 55:41
  Speaker 1: Beta 11 years earlier. They had a real structure in
• 55:44
  Speaker 1: the right place, confirmed by two independent methods. And they
• 55:49
  Speaker 1: had a physical mechanism, the integrated Sachs-Wolfe effect, that could
• 55:53
  Speaker 1: connect that structure to the temperature depression in the CMB.
• 55:58
  Speaker 2: The pieces were on the table. The case looked like
• 56:01
  Speaker 2: it was closing.
• 56:02
  Speaker 1: Then someone ran the actual numbers. The Eridana supervoid, under
• 56:07
  Speaker 1: the standard Lander CDM cosmological model, should produce an ISW
• 56:12
  Speaker 1: imprint of roughly 20 microkelvin.
• 56:15
  Speaker 2: At its center. That is a meaningful signal.
• 56:19
  Speaker 1: It is not nothing. But the cold spot in the
• 56:22
  Speaker 1: WMAP and Planck data sits about 70 microkelvin below.
• 56:26
  Speaker 2: The average temperature of the CMB.
• 56:29
  Speaker 1: Run the arithmetic and you find that the supervoid, vast
• 56:32
  Speaker 1: as it is, can account for somewhere between 10 and
• 56:36
  Speaker 1: 20% of the observed temperature depression.
• 56:39
  Speaker 2: The rest has.
• 56:40
  Speaker 1: No explanation sitting behind it. No structure, no mechanism, no name.
• 56:47
  Speaker 1: This is the number that has been bothering cosmologists ever since.
• 56:52
  Speaker 1: It is worth pausing here to understand why this gap
• 56:55
  Speaker 1: is so uncomfortable. The Lambda CDM model is not a
• 56:59
  Speaker 1: rough sketch of how the universe works. It is one
• 57:02
  Speaker 1: of the most precisely tested frameworks in all of science.
• 57:06
  Speaker 1: It predicts the temperature fluctuations of the CMB across the
• 57:10
  Speaker 1: entire sky with extraordinary accuracy, matching observations at the level
• 57:15
  Speaker 1: of parts per 100,000 almost everywhere you look. When astronomers
• 57:19
  Speaker 1: say the Standard Model works, they mean it works the
• 57:22
  Speaker 1: way a Swiss watch works, not approximately, not most of
• 57:26
  Speaker 1: the time. Consistently, to a degree that is almost unreasonable,
• 57:32
  Speaker 1: The cold spot does.
• 57:33
  Speaker 2: Not fit that picture.
• 57:35
  Speaker 1: At 70 microkelvin below average, it sits far outside what
• 57:39
  Speaker 1: the standard model comfortably predicts. The typical variation across the
• 57:44
  Speaker 1: CMB is around 18 microkelvin. The cold spot is nearly
• 57:49
  Speaker 1: four times that size, in the wrong direction. in a
• 57:52
  Speaker 1: region that should look more or less like everything else.
• 57:55
  Speaker 1: And now, even after finding the largest identified supervoid in
• 58:00
  Speaker 1: the observable universe sitting directly behind it, researchers can explain
• 58:04
  Speaker 1: at best one fifth of what they are seeing. The
• 58:07
  Speaker 1: supervoid shifts the problem from completely mysterious to mostly mysterious.
• 58:13
  Speaker 1: That is progress technically. It does not feel like much.
• 58:17
  Speaker 1: The question became urgent precisely because the universe should follow
• 58:21
  Speaker 1: these tested.
• 58:21
  Speaker 2: Rules at every scale.
• 58:23
  Speaker 1: The standard model had proven itself across countless independent measurements
• 58:28
  Speaker 1: and predictions. A failure of this magnitude suggested something fundamentally
• 58:33
  Speaker 1: unresolved in the current understanding, even if only at the
• 58:36
  Speaker 1: margins of what the model claimed to explain perfectly. Andras Kovacs,
• 58:41
  Speaker 1: one of the lead researchers on the 2015 study, and
• 58:45
  Speaker 1: subsequent work connecting the supervoid to the cold spot, described
• 58:50
  Speaker 1: the situation with the kind of careful precision that scientists
• 58:53
  Speaker 1: use when they want to be honest about what they
• 58:56
  Speaker 1: do not know. Having the coincidence of two individually rare structures,
• 59:01
  Speaker 1: he said, one in the CMB and one in the
• 59:04
  Speaker 1: distribution of matter, is not by itself enough to prove
• 59:08
  Speaker 1: causality to a scientific standard. Two rare things appearing in
• 59:12
  Speaker 1: the same direction in the sky is striking. It compels attention.
• 59:17
  Speaker 1: It invites explanation. But striking is not the same as proven.
• 59:22
  Speaker 1: The supervoid and the cold spot may be related. the
• 59:26
  Speaker 1: physics of the ISW effect says they almost certainly interact.
• 59:31
  Speaker 1: What the math will not support is the claim that
• 59:33
  Speaker 1: the supervoid is the whole answer, because the whole answer
• 59:37
  Speaker 1: requires something five to ten times stronger than the supervoid
• 59:41
  Speaker 1: can deliver. This distinction mattered profoundly to researchers evaluating competing explanations.
• 59:48
  Speaker 1: A complete accounting demanded not merely a supervoid, but one
• 59:52
  Speaker 1: so powerful and so positioned geometrically that it could explain
• 59:56
  Speaker 1: the entire anomaly through physics everyone already understood and accepted.
• 1:00:01
  Speaker 1: This creates a specific kind of scientific frustration. When an
• 1:00:06
  Speaker 1: anomaly has no explanation at all, you can at least
• 1:00:09
  Speaker 1: hold onto the hope that the right structure, the right mechanism,
• 1:00:13
  Speaker 1: the right observation is still.
• 1:00:15
  Speaker 2: Out there waiting.
• 1:00:16
  Speaker 1: When an anomaly is partially explained, the remaining gap becomes
• 1:00:20
  Speaker 1: harder to think about clearly. Researchers know the supervoid is real.
• 1:00:25
  Speaker 1: They know the ISW effect operates. They know those two
• 1:00:29
  Speaker 1: facts together account for some fraction of what they see.
• 1:00:33
  Speaker 1: And they are left staring at the rest of the
• 1:00:34
  Speaker 1: gap with very few obvious places to look. Some researchers
• 1:00:39
  Speaker 1: have proposed that the supervoid might be deeper or more
• 1:00:43
  Speaker 1: extended than current surveys have fully resolved. And that a
• 1:00:46
  Speaker 1: more complete three-dimensional map of the matter distribution in that
• 1:00:50
  Speaker 1: region might close the gap somewhat. If the underdensity is
• 1:00:54
  Speaker 1: steeper toward the center than current measurements suggest, the ISW
• 1:00:59
  Speaker 1: signal could be stronger than the standard estimates.
• 1:01:02
  Speaker 2: This is possible.
• 1:01:03
  Speaker 1: It is also not a comfortable explanation to lean on,
• 1:01:07
  Speaker 1: because it amounts to saying the data we have is
• 1:01:09
  Speaker 1: incomplete in exactly the way needed to fix the problem.
• 1:01:13
  Speaker 1: That kind of reasoning tends to make careful scientists uneasy.
• 1:01:18
  Speaker 2: Others have pointed to.
• 1:01:19
  Speaker 1: The statistical difficulty of the problem itself, The cold spot
• 1:01:23
  Speaker 1: is an extreme feature. The supervoid is an extreme structure.
• 1:01:28
  Speaker 1: Both are rare within the standard model individually. The probability
• 1:01:32
  Speaker 1: of finding them both in the same direction, even if
• 1:01:35
  Speaker 1: they have no causal relationship, is not zero. Some researchers
• 1:01:40
  Speaker 1: have argued that the cold spot may simply be a
• 1:01:43
  Speaker 1: place where several independent rare fluctuations happen to overlap, producing
• 1:01:49
  Speaker 1: a temperature depression that looks too large to be explained
• 1:01:53
  Speaker 1: by any single cause because it genuinely has more than
• 1:01:56
  Speaker 1: one cause, none of which is particularly exotic. This explanation
• 1:02:01
  Speaker 1: is technically allowable. It is also, to put it plainly, unsatisfying.
• 1:02:07
  Speaker 1: It explains the anomaly by multiplying the number of things
• 1:02:10
  Speaker 1: that need explaining. What nobody has produced is a mechanism
• 1:02:15
  Speaker 1: that bridges the remaining gap cleanly, one that uses known
• 1:02:19
  Speaker 1: physics fits the confirmed properties of the supervoid and accounts
• 1:02:24
  Speaker 1: for the full 70 microkelvin without requiring new assumptions layered
• 1:02:28
  Speaker 1: on top of old ones. The cold spot arrived in
• 1:02:32
  Speaker 1: the data in 2004. The supervoid was confirmed in 2015.
• 1:02:38
  Speaker 1: The ISW calculation was done carefully by multiple groups using
• 1:02:42
  Speaker 1: different methods arriving at the same uncomfortable fraction. A decade
• 1:02:47
  Speaker 1: of work after the confirmation produced a well-characterized partial explanation
• 1:02:53
  Speaker 1: and a gap that has not narrowed. The void explained
• 1:02:56
  Speaker 1: some of the cold spot, but not nearly enough, and
• 1:02:59
  Speaker 1: whatever was making up the difference had no name in
• 1:03:02
  Speaker 1: the standard picture. The Lambda CDM model is the most
• 1:03:06
  Speaker 1: successful theory in cosmology, and that is precisely what makes
• 1:03:10
  Speaker 1: the cold spot uncomfortable. Lambda CDM is the name cosmologists
• 1:03:15
  Speaker 1: give to the standard framework describing how the universe is
• 1:03:18
  Speaker 1: built and how it behaves. The Lambda stands for dark energy,
• 1:03:22
  Speaker 1: the mysterious force driving the accelerating expansion of space itself.
• 1:03:27
  Speaker 1: CDM stands for cold dark matter. the invisible scaffolding around
• 1:03:33
  Speaker 1: which ordinary matter clumps and organizes. Add in the small
• 1:03:38
  Speaker 1: fraction of ordinary atoms, electrons, and protons that make up
• 1:03:42
  Speaker 1: everything you can see or touch, and you have the
• 1:03:46
  Speaker 1: three ingredients the model uses to construct a universe. What
• 1:03:51
  Speaker 1: makes this framework remarkable is not its elegance, though physicists
• 1:03:56
  Speaker 1: find it elegant. What makes it remarkable is how precisely
• 1:04:01
  Speaker 1: it works. Lambda CDM predicts the large-scale distribution of galaxies.
• 1:04:08
  Speaker 1: It predicts the age of the universe, its geometry, its
• 1:04:12
  Speaker 1: expansion rate. It predicts the temperature fluctuations in the cosmic
• 1:04:17
  Speaker 1: microwave background, those tiny millionths of a degree ripples across
• 1:04:22
  Speaker 1: the oldest light in existence. With an accuracy that borders
• 1:04:26
  Speaker 1: on unsettling, point a satellite.
• 1:04:28
  Speaker 2: At the sky.
• 1:04:30
  Speaker 1: Measure the CMB, and then run the Lambda CDM equations.
• 1:04:34
  Speaker 1: The match, across nearly the entire sky, is extraordinary. This
• 1:04:40
  Speaker 1: is not a model that has survived on theoretical elegance alone.
• 1:04:44
  Speaker 1: It has survived because the data keeps confirming it. Decades
• 1:04:48
  Speaker 1: of observations from COBE, WMAP, Planck, and dozens of ground-based
• 1:04:54
  Speaker 1: experiments have tested Lambda CDM against the real universe and
• 1:04:58
  Speaker 1: found it holding. Cosmologists do not trust it out of habit.
• 1:05:03
  Speaker 1: They trust it because it has earned that trust, measurement
• 1:05:06
  Speaker 1: by measurement across an enormous range of scales and phenomena.
• 1:05:10
  Speaker 1: Which is exactly why the cold spot creates such discomfort.
• 1:05:15
  Speaker 1: Not because the model is obviously wrong, but because the
• 1:05:18
  Speaker 1: cold spot sits in a part of the statistical distribution
• 1:05:22
  Speaker 1: where Lambda CDM says features like it should be extraordinarily rare.
• 1:05:27
  Speaker 1: The model does not forbid the cold spot. That is
• 1:05:30
  Speaker 1: a crucial distinction. What it says is that a temperature
• 1:05:34
  Speaker 1: depression this large, this cold, and this spatially coherent should
• 1:05:39
  Speaker 1: occur by chance only very rarely in a universe like ours.
• 1:05:44
  Speaker 1: And when a successful model says something should be rare,
• 1:05:47
  Speaker 1: and then you find it sitting in your data anyway,
• 1:05:50
  Speaker 1: you have a problem that demands careful handling. The way
• 1:05:54
  Speaker 1: physicists approach this kind of problem is through statistical analysis.
• 1:05:59
  Speaker 1: You ask a precise question. Given the universe Lambda CDM predicts,
• 1:06:04
  Speaker 1: how often would we expect to see a feature at
• 1:06:07
  Speaker 1: least as extreme as the cold spot appearing in a
• 1:06:10
  Speaker 1: random sky map? To answer that, researchers run simulations, thousands
• 1:06:15
  Speaker 1: of them, sometimes tens of thousands. Each simulation generates a
• 1:06:20
  Speaker 1: CMB sky consistent with Lambda CDM's predictions. and researchers scan
• 1:06:26
  Speaker 1: each one for cold spots comparable to the Eridanus anomaly.
• 1:06:30
  Speaker 1: The fraction of simulations where a comparable feature appears gives
• 1:06:34
  Speaker 1: you a probability estimate. Simple in principle, enormously sensitive to
• 1:06:39
  Speaker 1: the details in practice. Here is where the debate becomes
• 1:06:43
  Speaker 1: uncomfortable in its own right. Different research groups have run
• 1:06:47
  Speaker 1: this kind of analysis and arrived at genuinely different answers.
• 1:06:52
  Speaker 1: Some find that the cold spot is a 1 in
• 1:06:54
  Speaker 1: 50 event. Unusual, worth noting, but not evidence of physics
• 1:06:59
  Speaker 1: beyond the standard model. Others run the same fundamental test
• 1:07:03
  Speaker 1: with different assumptions and find the odds closer to 1
• 1:07:07
  Speaker 1: in several hundred. A handful of analyses push that number
• 1:07:11
  Speaker 1: further still, into the range of 1 in a few thousand.
• 1:07:16
  Speaker 1: The probability estimates span nearly two orders of magnitude depending
• 1:07:20
  Speaker 1: on who is doing the calculation and what choices they
• 1:07:23
  Speaker 1: make along the way.
• 1:07:24
  Speaker 2: What drives that spread? Several things.
• 1:07:28
  Speaker 1: The first is a problem statisticians.
• 1:07:31
  Speaker 2: Call the look elsewhere effect.
• 1:07:33
  Speaker 1: When you examine an entire CMB sky and search for
• 1:07:37
  Speaker 1: anomalous cold spots anywhere in it, you are essentially running
• 1:07:41
  Speaker 1: thousands of independent tests simultaneously. Finding one extreme feature somewhere
• 1:07:46
  Speaker 1: in the data is less surprising than finding it at
• 1:07:49
  Speaker 1: a specific predetermined location. How you correct for searching the
• 1:07:54
  Speaker 1: whole sky at once significantly changes your final probability estimate.
• 1:07:59
  Speaker 1: Researchers disagree, sometimes sharply, about the right correction to apply.
• 1:08:04
  Speaker 1: The second complication is the choice of what exactly you
• 1:08:08
  Speaker 1: are measuring. A cold spot can be characterized by its
• 1:08:12
  Speaker 1: minimum temperature. its average temperature across its area, its total
• 1:08:17
  Speaker 1: angular size, the sharpness of its edges, or combinations of
• 1:08:22
  Speaker 1: all of these. Different statistics capture different aspects of the feature,
• 1:08:27
  Speaker 1: and different groups have used different statistics, sometimes producing meaningfully
• 1:08:32
  Speaker 1: different assessments of how anomalous the cold spot is. The
• 1:08:36
  Speaker 1: third problem is deeper and harder to resolve. Landa CDN
• 1:08:41
  Speaker 1: itself has three parameters. numbers that are measured from the
• 1:08:44
  Speaker 1: data and then fed into the model. When you use
• 1:08:47
  Speaker 1: the same dataset to both calibrate the model and test
• 1:08:51
  Speaker 1: whether an anomaly is consistent with it, the statistics become
• 1:08:55
  Speaker 1: entangled in ways that are genuinely difficult to untangle. Some
• 1:08:59
  Speaker 1: critics of the cold spot's claimed significance argue that the
• 1:09:03
  Speaker 1: model has already been adjusted implicitly to accommodate the universe
• 1:09:07
  Speaker 1: we observe, and that tests of rare features using that
• 1:09:11
  Speaker 1: same calibrated model understate their true improbability. This is not
• 1:09:15
  Speaker 1: a fringe concern. It is a legitimate methodological worry. that
• 1:09:20
  Speaker 1: careful cosmologists take seriously. What this means in practice is
• 1:09:24
  Speaker 1: that no single probability number attached to the cold spot
• 1:09:28
  Speaker 1: should be taken at face value without understanding the assumptions
• 1:09:32
  Speaker 1: behind it. When a headline says the cold spot has
• 1:09:35
  Speaker 1: a 1 in 50 chance of occurring randomly, that is
• 1:09:38
  Speaker 1: one answer to one specific question asked in one specific way.
• 1:09:42
  Speaker 1: It is not the whole picture. The honest assessment is
• 1:09:46
  Speaker 1: that the cold spot is unusual enough that many independent researchers,
• 1:09:50
  Speaker 1: using different methods, keep flagging it as something worth explaining,
• 1:09:55
  Speaker 1: while simultaneously being just ambiguous enough statistically that no one
• 1:09:59
  Speaker 1: can declare with finality that the standard model is broken.
• 1:10:03
  Speaker 1: That ambiguity has persisted for 20 years. The data have
• 1:10:08
  Speaker 1: gotten better, the simulations have become more sophisticated, and the
• 1:10:12
  Speaker 1: debate has not resolved. Researchers who entered this field expecting
• 1:10:17
  Speaker 1: that better measurements would settle the question have instead watched
• 1:10:20
  Speaker 1: the disagreement sharpen around methodological choices rather than close around
• 1:10:25
  Speaker 1: a clean answer. The cold spot's statistical status is, itself,
• 1:10:31
  Speaker 1: an unsolved problem nested inside the larger unsolved problem of
• 1:10:36
  Speaker 1: what physically produced it. What lambda-CDM would need to fully
• 1:10:40
  Speaker 1: accommodate the cold spot is some combination of rare fluctuation
• 1:10:45
  Speaker 1: and extreme void producing an ISW signal strong enough to
• 1:10:49
  Speaker 1: account for the observed temperature depression.
• 1:10:52
  Speaker 2: The model can stretch.
• 1:10:53
  Speaker 1: To include both of those things happening together in the
• 1:10:56
  Speaker 1: same small patch of sky. But asking how often that
• 1:11:00
  Speaker 1: specific combination of circumstances occurs in a lambda CDM universe returns, again,
• 1:11:07
  Speaker 1: to a probability estimate that makes cosmologists shift in their
• 1:11:13
  Speaker 1: The supervoid is there. The cold spot is there. They
• 1:11:17
  Speaker 1: are aligned in direction. And yet the moth connecting them,
• 1:11:21
  Speaker 1: under the standard model, stubbornly refuses to fully close. A
• 1:11:25
  Speaker 1: model that explains almost everything in the universe with extraordinary
• 1:11:29
  Speaker 1: precision should not produce a feature like the coal. D-spot
• 1:11:34
  Speaker 1: without any good reason, but so far it has not
• 1:11:37
  Speaker 1: been able to avoid doing exactly that. Over the two
• 1:11:41
  Speaker 1: decades since the cold spot turned up in the WMAP data,
• 1:11:45
  Speaker 1: it has not suffered from a shortage of explanations. Cosmologists
• 1:11:50
  Speaker 1: are not a group that tolerates an anomaly sitting in
• 1:11:52
  Speaker 1: the open without taking a run at it. Since 2004,
• 1:11:57
  Speaker 1: research groups on several continents have proposed mechanisms, built models,
• 1:12:02
  Speaker 1: run simulations, published papers trying to account for what sits
• 1:12:07
  Speaker 1: in that patch of Eridana's sky. Some of these explanations
• 1:12:11
  Speaker 1: are technically sophisticated, some are elegant, and every single one
• 1:12:16
  Speaker 1: of them has eventually come up short. One of the
• 1:12:19
  Speaker 1: earliest serious candidates was something called a cosmic texture. To
• 1:12:24
  Speaker 1: understand what that means, you have to go back to
• 1:12:27
  Speaker 1: the first fractions of a second after the Big Bang,
• 1:12:30
  Speaker 1: when the universe was cooling rapidly and fundamental forces were separating.
• 1:12:35
  Speaker 2: Out from each other.
• 1:12:37
  Speaker 1: During that cooling, the quantum fields that govern particle physics
• 1:12:41
  Speaker 1: pass through phase transitions. In certain theoretical frameworks, those transitions
• 1:12:47
  Speaker 1: could leave behind stable defects in the structure.
• 1:12:50
  Speaker 2: Of spacetime itself.
• 1:12:52
  Speaker 1: Think of what happens when a material cools unevenly. Floors
• 1:12:56
  Speaker 1: form at the boundaries between regions that cooled at different rates.
• 1:13:01
  Speaker 1: Textures are the cosmic equivalent of those floors. They are
• 1:13:04
  Speaker 1: knots or twists in quantum fields, carrying energy, moving through
• 1:13:09
  Speaker 1: the universe, leaving marks on anything they pass through. When
• 1:13:15
  Speaker 1: a texture unwinds, it releases energy in a pattern that
• 1:13:18
  Speaker 1: would cool photons passing through that region in a specific,
• 1:13:22
  Speaker 1: predictable way. A 2007 paper proposed that a single cosmic
• 1:13:27
  Speaker 1: texture located in the direction of Eridanus could account for
• 1:13:31
  Speaker 1: the cold spot's temperature profile almost exactly. The shape of
• 1:13:35
  Speaker 1: the cold spot on the sky, the depth of the
• 1:13:38
  Speaker 1: temperature dip, the spatial distribution of the cooling. All of
• 1:13:44
  Speaker 1: it fit the texture model with uncomfortable precision. For a while,
• 1:13:48
  Speaker 1: textures were a serious contender. Then the problem started. Textures
• 1:13:55
  Speaker 1: of the size required to produce a cold spot this
• 1:13:58
  Speaker 1: large should not be alone. They form throughout the universe
• 1:14:02
  Speaker 1: during phase transitions. If one exists in Eridanus, the CMB
• 1:14:08
  Speaker 1: should show signatures of others scattered across the sky. Planck's
• 1:14:12
  Speaker 1: high resolution data failed to find the broader population of
• 1:14:16
  Speaker 1: texture signatures the model predicted. A single texture without a
• 1:14:21
  Speaker 1: family of companions is not a prediction. It is an
• 1:14:24
  Speaker 1: exception made for one anomaly. Physics rarely works that way.
• 1:14:30
  Speaker 1: Textures quietly retreated from the front of the debate. What
• 1:14:33
  Speaker 1: if the simplest answer is the right one? The cold
• 1:14:37
  Speaker 1: spot could be nothing more than a statistical fluke. Standard
• 1:14:41
  Speaker 1: cosmology generates a random distribution of temperature fluctuations across the CMB.
• 1:14:47
  Speaker 1: In any random distribution, extreme values appear somewhere. The question
• 1:14:52
  Speaker 1: is not whether cold regions exist, but whether a region
• 1:14:56
  Speaker 1: this cold and this large was likely to show up
• 1:14:59
  Speaker 1: in one particular sky. Researchers have run millions of simulated
• 1:15:04
  Speaker 1: universes based on lambda CDM parameters, generated artificial CMB maps
• 1:15:10
  Speaker 1: from those simulations, counted how often a feature as extreme
• 1:15:14
  Speaker 1: as the cold spot appears. The results vary depending on
• 1:15:18
  Speaker 1: how you define extreme and how you measure the statistics.
• 1:15:22
  Speaker 1: Some analyses put the probability somewhere around 1 in 50,
• 1:15:27
  Speaker 1: others using different filters or diff. Errant measures of deviation
• 1:15:32
  Speaker 1: Push that number down to 1 in several hundred or
• 1:15:35
  Speaker 1: even 1 in several thousand. That range is itself a problem.
• 1:15:40
  Speaker 1: When your statistical significance depends heavily on which test you choose,
• 1:15:45
  Speaker 1: you are not working with a clean result.
• 1:15:48
  Speaker 2: Even at 1 in 50.
• 1:15:49
  Speaker 1: Though, the fluke explanation has a fundamental weakness that goes
• 1:15:54
  Speaker 1: beyond the numbers. It is technically valid. It is not satisfying.
• 1:16:00
  Speaker 1: Saying that the cold spot is an outlier within the
• 1:16:02
  Speaker 1: model explains nothing about why it is there. It is
• 1:16:06
  Speaker 1: the scientific equivalent of shrugging. When a feature survives 20
• 1:16:11
  Speaker 1: years of scrutiny and continues to resist physical explanation, calling
• 1:16:16
  Speaker 1: it bad luck starts to feel less like an answer
• 1:16:20
  Speaker 1: and more like giving up. Other proposals have circulated through
• 1:16:23
  Speaker 1: the literature over the years, most of them less prominent
• 1:16:27
  Speaker 1: but worth acknowledging. Some researchers proposed that foreground contamination from
• 1:16:32
  Speaker 1: our own galaxy was distorting the signal in that direction.
• 1:16:36
  Speaker 1: That dust or gas between us and the CMB was
• 1:16:39
  Speaker 1: artificially suppressing the measured temperature. Careful multi-frequency analysis using Planck
• 1:16:44
  Speaker 1: data was able to isolate the CMB signal from galactic
• 1:16:48
  Speaker 1: foregrounds with high confidence. The cold spot remained. It was
• 1:16:53
  Speaker 1: not an artifact of our galaxy leaking into the measurement.
• 1:16:57
  Speaker 1: Others looked at whether secondary anisotopies, effects introduced by large-scale
• 1:17:02
  Speaker 1: structure on photons after the CNB was released, could conspire
• 1:17:06
  Speaker 1: to produce such a deep temperature minimum. The numbers never
• 1:17:10
  Speaker 1: worked cleanly. Some pap urs proposed that the cold spot
• 1:17:15
  Speaker 1: reflected unusual primordial conditions. A region of the early universe
• 1:17:22
  Speaker 1: where the density fluctuations seeded by inflation were simply different
• 1:17:26
  Speaker 1: from the average, a rare draw from the inflationary probability distribution.
• 1:17:34
  Speaker 1: This is a version of the statistical fluke argument dressed
• 1:17:37
  Speaker 1: in different language. It carries the same weakness. It identifies
• 1:17:43
  Speaker 1: where the anomaly sits in the distribution without explaining what
• 1:17:47
  Speaker 1: produced it. There were proposals involving modified initial conditions for inflation,
• 1:17:53
  Speaker 1: unusual topological properties of the universe at the largest scales,
• 1:17:58
  Speaker 1: even suggestions that the universe might be finite rather than
• 1:18:01
  Speaker 1: infinite in a way that would generate unexpected correlations at
• 1:18:05
  Speaker 1: large angular scales. Each of these ideas addressed one feature
• 1:18:10
  Speaker 1: of the cold spot while struggling with others.
• 1:18:13
  Speaker 2: None of them.
• 1:18:13
  Speaker 1: Produced a comprehensive explanation that accounted for the temperature depth
• 1:18:18
  Speaker 1: The spatial scale, the shape, the absence of similar features
• 1:18:22
  Speaker 1: elsewhere all at once. The cold spot is remarkably good
• 1:18:27
  Speaker 1: at surviving contact with explanations. Each theory had its moment.
• 1:18:32
  Speaker 1: Each eventually broke down under closer inspection, leaving the cold
• 1:18:36
  Speaker 1: spot exactly where it started, unexplained. There is one explanation
• 1:18:41
  Speaker 1: for the cold spot that has never quite gone away.
• 1:18:44
  Speaker 1: no matter how many times mainstream cosmologists try to set
• 1:18:48
  Speaker 1: it aside. Every other theory eventually faded. Textures were ruled out.
• 1:18:55
  Speaker 1: Statistical flukes felt unsatisfying. The supervoid explained a fraction of
• 1:19:01
  Speaker 1: the temperature depression and stopped there. And into that remaining gap,
• 1:19:06
  Speaker 1: quietly and persistently, One idea kept returning. What if the
• 1:19:12
  Speaker 1: cold spot is a scar left by another universe? This
• 1:19:16
  Speaker 1: is not a fringe idea in the sense of being ignored.
• 1:19:19
  Speaker 1: It has appeared in peer-reviewed papers. It has been taken
• 1:19:23
  Speaker 1: seriously enough to model, to test against the data, to
• 1:19:27
  Speaker 1: argue about in conference rooms. The hypothesis goes by several names,
• 1:19:32
  Speaker 1: but the core claim is the same. Our universe is
• 1:19:35
  Speaker 1: not the only one.
• 1:19:36
  Speaker 2: It never was.
• 1:19:38
  Speaker 1: And at some point in the distant past, something brushed
• 1:19:41
  Speaker 1: against us. To understand how this works, you have to
• 1:19:45
  Speaker 1: understand what inflationary cosmology actually predicts. In the first fraction
• 1:19:50
  Speaker 1: of a second after the Big Bang, the universe underwent
• 1:19:53
  Speaker 1: a period of explosive expansion called inflation. Space stretched faster
• 1:19:59
  Speaker 1: than light can travel, smoothing out irregularities and seeding the
• 1:20:04
  Speaker 1: density fluctuations that would eventually become galaxies. The theory of
• 1:20:09
  Speaker 1: inflation is extraordinarily successful. It explains the flatness of the universe,
• 1:20:15
  Speaker 1: the uniformity of the CMB, and the pattern of temperature
• 1:20:18
  Speaker 1: fluctuations that satellites like WMAP and Planck have mapped in
• 1:20:23
  Speaker 1: such precise detail. But inflation has a consequence that many
• 1:20:27
  Speaker 1: physicists find uncomfortable. Once it starts, it does not stop
• 1:20:32
  Speaker 1: everywhere at once. Different regions of space drop out of
• 1:20:36
  Speaker 1: the inflationary, state at different times. Regions that stop inflating
• 1:20:41
  Speaker 1: become universes like ours. Regions that keep inflating keep producing
• 1:20:46
  Speaker 1: new pockets, new bubbles of space that each expand into
• 1:20:50
  Speaker 1: their own universe. This process is called eternal inflation. And
• 1:20:55
  Speaker 1: in this picture, our universe is one bubble in a vast,
• 1:20:59
  Speaker 1: possibly infinite sea of other bubbles.
• 1:21:02
  Speaker 2: Each one is.
• 1:21:03
  Speaker 1: Causally isolated from the others. Each one expands on its own.
• 1:21:08
  Speaker 1: Most of them can never communicate with us in any way. Most,
• 1:21:12
  Speaker 1: but not necessarily all. In some models, two bubble universes
• 1:21:17
  Speaker 1: can come into contact, not collide in the conventional sense
• 1:21:21
  Speaker 1: of two objects slamming together, but touch at their boundaries
• 1:21:25
  Speaker 1: as their expanding edges meet. If that happened, the collision
• 1:21:29
  Speaker 1: would send a shockwave through.
• 1:21:30
  Speaker 2: The interior of each bubble.
• 1:21:32
  Speaker 1: In our universe, that shockwave would propagate outward from the
• 1:21:36
  Speaker 1: point of contact, affecting the temperature and density of matter
• 1:21:41
  Speaker 1: and radiation in a ring-shaped region on the sky. From
• 1:21:45
  Speaker 1: our vantage point, billions of years later, we would see
• 1:21:49
  Speaker 1: the imprint as a disc-shaped anomaly in the CMB, cold
• 1:21:53
  Speaker 1: or hot, depending on the geometry, circular in shape, positioned
• 1:21:58
  Speaker 1: wherever in the sky our bubble happened to make contact
• 1:22:01
  Speaker 1: with its neighbor. Now look at the cold spot. It
• 1:22:05
  Speaker 1: is roughly circular, It is anomalously cold. It sits in
• 1:22:09
  Speaker 1: a well-defined region of sky, with an internal structure showing
• 1:22:14
  Speaker 1: a colder core surrounded by a warmer ring. Some researchers
• 1:22:18
  Speaker 1: noted that this profile is at least qualitatively consistent with
• 1:22:22
  Speaker 1: what a bubble collision signature might look like. That observation
• 1:22:26
  Speaker 1: was enough to keep the hypothesis alive, even as the
• 1:22:30
  Speaker 1: majority of the field stayed skeptical. The skepticism is substantial,
• 1:22:35
  Speaker 1: and it is not unreasonable. The most immediate problem is testability.
• 1:22:40
  Speaker 1: A multiverse by construction lies beyond the boundary of our
• 1:22:44
  Speaker 1: observable universe. We cannot send a probe there. We cannot
• 1:22:48
  Speaker 1: observe the other bubbles directly. We cannot run an experiment
• 1:22:52
  Speaker 1: that distinguishes a universe with bubble neighbors from one without them.
• 1:22:57
  Speaker 1: Any theory that explains a single anomaly while being structurally
• 1:23:00
  Speaker 1: immune to broader testing sits in uncomfortable scientific territory. Extraordinary
• 1:23:06
  Speaker 1: claims require extraordinary evidence, and the cold spot alone does
• 1:23:10
  Speaker 1: not come close to meeting that threshold. There is also
• 1:23:14
  Speaker 1: the question of what, specifically, the prediction is. Different models
• 1:23:19
  Speaker 1: of bubble collisions produce different signatures. Some predict a hot ring,
• 1:23:24
  Speaker 1: some predict a cold disk, some predict asymmetric features. The
• 1:23:29
  Speaker 1: parameter space is large enough that critics argue the hypothesis
• 1:23:32
  Speaker 1: is not making a precise prediction, so much as finding
• 1:23:36
  Speaker 1: a way to fit whatever anomaly exists. A theory that
• 1:23:39
  Speaker 1: can explain anything explains nothing. When researchers ran rigorous statistical
• 1:23:45
  Speaker 1: comparisons between the cold spot's actual profile and the expected
• 1:23:49
  Speaker 1: profile from bubble collision models, the match was imperfect, suggestive
• 1:23:54
  Speaker 1: in some framings, unconvincing in others. The strongest version of
• 1:23:59
  Speaker 1: the skeptical position is this. The cold spot has a
• 1:24:03
  Speaker 1: mundane candidate explanation in the form of the Eridana's supervoid.
• 1:24:08
  Speaker 1: The supervoid is real, confirmed by independent surveys and physically
• 1:24:14
  Speaker 1: connected to the cold spot through a well-understood mechanism. The
• 1:24:18
  Speaker 1: fact that the supervoid only explains 10-20% of the temperature
• 1:24:22
  Speaker 1: depression is a serious problem, but that problem points toward
• 1:24:27
  Speaker 1: incomplete physics or incomplete data. not necessarily toward a second universe.
• 1:24:33
  Speaker 1: Invoking the multiverse to fill the remaining gap is to
• 1:24:37
  Speaker 1: most cosmologists like explaining a crack in the sidewalk by
• 1:24:41
  Speaker 1: proposing that tectonic plates exist without first checking whether someone
• 1:24:46
  Speaker 1: dropped something heavy. And yet, the hypothesis has not been
• 1:24:52
  Speaker 1: ruled out. No measurement has returned a result that definitively
• 1:24:56
  Speaker 1: excludes bubble collision models. The cold spot's circular structure, its
• 1:25:01
  Speaker 1: temperature profile, its position. None of these features are incompatible
• 1:25:07
  Speaker 1: with an inflationary collision signature. They are also not incompatible
• 1:25:11
  Speaker 1: with a supervoid plus a statistical fluctuation. But that is
• 1:25:16
  Speaker 1: different from disproving the multiverse interpretation. The absence of disproof
• 1:25:21
  Speaker 1: is not evidence. But in a field where the most
• 1:25:24
  Speaker 1: exotic option remains technically consistent with every observation, technically consistent
• 1:25:31
  Speaker 1: is enough to keep people writing papers. What does it
• 1:25:34
  Speaker 1: say about the state of the problem that this explanation
• 1:25:36
  Speaker 1: keeps resurfacing? It says that two decades of work by
• 1:25:40
  Speaker 1: some of the most careful observers and theorists in cosmology
• 1:25:44
  Speaker 1: have not produced a clean answer. The supervoid is real,
• 1:25:48
  Speaker 1: the ISW effect is real, the math has been done,
• 1:25:52
  Speaker 1: and still the bulk of the cold spot's temperature depression
• 1:25:56
  Speaker 1: sits there, unaccounted for, waiting. When nothing in the standard
• 1:26:00
  Speaker 1: toolkit closes the gap, people reach for tools that are
• 1:26:04
  Speaker 1: further out. The multiverse hypothesis is the furthest tool on
• 1:26:09
  Speaker 1: the shelf, the one that requires the most assumptions and
• 1:26:12
  Speaker 1: offers the least direct testability. But it is on the
• 1:26:16
  Speaker 1: shelf for a reason. The universe presented us with something
• 1:26:19
  Speaker 1: that keeps resisting ordinary explanations, and physicists do not like
• 1:26:24
  Speaker 1: loose ends. The multiverse hypothesis is still alive. Not because
• 1:26:29
  Speaker 1: the evidence demands it, but because nothing else has fully
• 1:26:32
  Speaker 1: closed the case. Set aside the cold spot for a moment,
• 1:26:36
  Speaker 1: and the Eridanus supervoid has a second reason to command
• 1:26:40
  Speaker 1: scientific attention. The mystery of the cold spot brought astronomers
• 1:26:44
  Speaker 1: to this region of sky. But what they found there
• 1:26:47
  Speaker 1: turns out to be useful for something far larger than
• 1:26:51
  Speaker 1: one anomaly in one microwave map. The supervoid is a laboratory.
• 1:26:57
  Speaker 1: And what it is testing is the most poorly understood
• 1:27:00
  Speaker 1: force in the universe. To understand why, you have to
• 1:27:04
  Speaker 1: understand what dark energy actually is. Or rather, what we
• 1:27:09
  Speaker 1: know it does, since nobody has explained what it actually is.
• 1:27:14
  Speaker 1: In 1998, two independent teams of astronomers measuring the distances
• 1:27:20
  Speaker 1: to distant supernovae made the same discovery at nearly the
• 1:27:24
  Speaker 1: same time. the universe was not just expanding it was
• 1:27:29
  Speaker 1: accelerating something was pushing space itself outward at an increasing
• 1:27:35
  Speaker 1: rate working against gravity on the largest scales and that
• 1:27:40
  Speaker 1: something have no name in any existing theory they called
• 1:27:44
  Speaker 1: it dark energy it now accounts for roughly 68 percent
• 1:27:49
  Speaker 1: of everything in the universe by energy content and we
• 1:27:52
  Speaker 1: have no idea what it is what we do know
• 1:27:56
  Speaker 1: is that dark energy dominates on the largest scales. Where
• 1:28:00
  Speaker 1: gravity pulls matter together into clusters and filaments, dark energy
• 1:28:04
  Speaker 1: operates in the spaces between, driving regions of low density
• 1:28:09
  Speaker 1: to expand faster than regions with more matter. This distinction matters.
• 1:28:14
  Speaker 1: In dense regions, galaxy clusters, filaments, the bright nodes of
• 1:28:20
  Speaker 1: the cosmic web, gravity is running the show. But in voids,
• 1:28:24
  Speaker 1: the story is different. Voids are regions where gravity lost,
• 1:28:29
  Speaker 1: matter drained away into the surrounding structure billions of years
• 1:28:33
  Speaker 1: ago and what was left behind is a region dominated
• 1:28:37
  Speaker 1: not by gravitational collapse but by the expansion of space itself.
• 1:28:43
  Speaker 1: Dark energy is in charge in a void and that
• 1:28:46
  Speaker 1: makes voids the ideal place to measure it.
• 1:28:49
  Speaker 2: This is the key insight. A super void.
• 1:28:51
  Speaker 1: Is not just an absence of matter, it is a
• 1:28:54
  Speaker 1: region where the physics of expansion is essentially running unopposed.
• 1:28:59
  Speaker 2: The rate at which a void grows over.
• 1:29:00
  Speaker 1: Time, the depth of the underdensity it develops, the precise
• 1:29:05
  Speaker 1: shape of its density profile from center to edge, all
• 1:29:09
  Speaker 1: of these are determined by the balance between gravity trying
• 1:29:12
  Speaker 1: to pull matter back in and dark energy pushing the
• 1:29:15
  Speaker 1: expansion outward. Change the properties of dark energy and voids
• 1:29:20
  Speaker 1: change with it. Change the equation of state of dark energy,
• 1:29:24
  Speaker 1: the ratio that describes how its pressure relates to its density,
• 1:29:28
  Speaker 1: and the void population across the universe shifts in measurable ways,
• 1:29:33
  Speaker 1: supervoids are, in a very real sense, tuning forks for
• 1:29:37
  Speaker 1: dark energy. The integrated Sacks-Wolfe effect comes back in here.
• 1:29:42
  Speaker 1: The strength of the ISW signal produced by a supervoid
• 1:29:46
  Speaker 1: depends directly on how dark energy behaves. In a universe
• 1:29:50
  Speaker 1: without dark energy, photons passing through a void would lose
• 1:29:54
  Speaker 1: energy climbing out of the gravitational hill, But the hill
• 1:29:58
  Speaker 1: itself would not change during the crossing, and the photon
• 1:30:01
  Speaker 1: would recover most of what it lost. Dark energy changes this.
• 1:30:06
  Speaker 1: It stretches the void while the photon is still inside it.
• 1:30:10
  Speaker 1: The hill the photon descends on the far side is
• 1:30:13
  Speaker 1: shallower than the hill it climbed on the near side.
• 1:30:16
  Speaker 1: The photon arrives slightly colder. The colder the signature, the
• 1:30:20
  Speaker 1: stronger the dark energy signal imprinted on the crossing. Measure
• 1:30:24
  Speaker 1: the ISW effect across enough supervoids and you are directly
• 1:30:28
  Speaker 1: measuring how aggressively dark energy is reshaping space. This is
• 1:30:33
  Speaker 1: not theoretical speculation. The Dark Energy Survey was built precisely
• 1:30:38
  Speaker 1: to gather the kind of galaxy data needed to find
• 1:30:41
  Speaker 1: and characterize supervoids at cosmological distances. and the Eridanus supervoid
• 1:30:47
  Speaker 1: is the most prominent structure it found. The DES measurements
• 1:30:51
  Speaker 1: confirmed not just the supervoid's existence, but its density profile.
• 1:30:56
  Speaker 1: How the underdensity deepens toward the center, how it falls
• 1:31:00
  Speaker 1: off at the edges, how its elongated shape relates to
• 1:31:04
  Speaker 1: the matter flows that emptied it. These are not decorative details.
• 1:31:09
  Speaker 1: Each one constrains the cosmological model. Each one puts a
• 1:31:14
  Speaker 1: slightly tighter boundary on what dark energy can and cannot
• 1:31:17
  Speaker 1: be doing on these scales. The constraints that supervoids provide
• 1:31:22
  Speaker 1: are complementary to the constraints from other dark energy probes.
• 1:31:26
  Speaker 1: Type Ia supernovae measure the expansion history of the universe
• 1:31:30
  Speaker 1: by tracking how the brightness of standard candles changes with distance.
• 1:31:35
  Speaker 1: Baryon acoustic oscillations measure the scale of the cosmic web.
• 1:31:40
  Speaker 1: by looking for a characteristic spacing in galaxy distributions. The
• 1:31:45
  Speaker 1: CMB itself constrains the early universe's composition. Supervoids add something different.
• 1:31:52
  Speaker 1: They probe dark energy's influence on structure growth. Not just
• 1:31:57
  Speaker 1: how fast the universe is expanding, but how that expansion
• 1:32:01
  Speaker 1: is shaping the largest underdense regions over cosmic time. Different
• 1:32:06
  Speaker 1: probes can give the same answer for some dark energy
• 1:32:09
  Speaker 1: models while disagreeing sharply for others. Supervoids help break those degeneracies.
• 1:32:16
  Speaker 1: There is a specific number that cosmologists track when they
• 1:32:20
  Speaker 1: try to characterize dark energy. It is called W, the
• 1:32:25
  Speaker 1: equation of state parameter. In the simplest model, the cosmological constant,
• 1:32:32
  Speaker 1: the lambda in lambda CDM, W equals negative one exactly, permanently, everywhere.
• 1:32:41
  Speaker 1: In more complicated models, W could vary with time or
• 1:32:45
  Speaker 1: differ from negative one in ways that would change the
• 1:32:48
  Speaker 1: history of cosmic acceleration. The growth rate of supervoids is
• 1:32:53
  Speaker 1: sensitive to W. A universe with a different value of
• 1:32:57
  Speaker 1: W would produce supervoids with different typical sizes. different depth profiles,
• 1:33:04
  Speaker 1: and different ISW signatures. The Aridana supervoid alone cannot pin
• 1:33:10
  Speaker 1: down W with the precision needed to distinguish between models.
• 1:33:14
  Speaker 1: But a population of supervoids measured across the full DES survey,
• 1:33:19
  Speaker 1: combined with data from galaxy lensing and CMB observations, can
• 1:33:25
  Speaker 1: push those constraints considerably further.
• 1:33:28
  Speaker 2: That work is ongoing.
• 1:33:30
  Speaker 1: What sits underneath all of this is the possibility that
• 1:33:33
  Speaker 1: the cold spot, even if it turns out to be
• 1:33:36
  Speaker 1: explainable within the standard model, has pointed astronomers toward a
• 1:33:40
  Speaker 1: tool they would have spent decades looking for. The supervoid
• 1:33:45
  Speaker 1: revealed by the search for the cold spot's cause is
• 1:33:48
  Speaker 1: now one of the most valuable dark energy laboratories available.
• 1:33:52
  Speaker 1: That is an unlikely outcome for what began as a
• 1:33:55
  Speaker 1: strange smudge on a microwave map. Whatever answer eventually emerges
• 1:34:00
  Speaker 1: from the supervoid data, it will have something to say
• 1:34:03
  Speaker 1: about the force that is tearing the universe apart. Once
• 1:34:07
  Speaker 1: the cold spot had a name, astronomers started looking for others.
• 1:34:12
  Speaker 1: That impulse is not unusual. When any anomaly survives early scrutiny,
• 1:34:18
  Speaker 1: the natural next move is to ask whether it is
• 1:34:21
  Speaker 1: truly singular or whether the universe has been producing similar
• 1:34:24
  Speaker 1: features all along. and nobody was looking carefully enough to notice.
• 1:34:30
  Speaker 1: The cold spot had survived scrutiny. It had survived multiple
• 1:34:34
  Speaker 1: instrument checks, multiple statistical analyses, multiple attempts to attribute it
• 1:34:40
  Speaker 1: to foreground contamination or noise.
• 1:34:43
  Speaker 2: So in the years after the.
• 1:34:44
  Speaker 1: WMAP discovery, groups of researchers turned their attention outward, combing
• 1:34:49
  Speaker 1: through the data for other cold regions that might share
• 1:34:52
  Speaker 1: the same character. In 2009, A team of astronomers in
• 1:34:57
  Speaker 1: Australia conducted one of the most comprehensive surveys of the
• 1:35:01
  Speaker 1: southern sky undertaken at that point. They mapped roughly 41%
• 1:35:04
  Speaker 1: of the southern hemisphere, cataloguing hundreds of candidate voids distributed
• 1:35:10
  Speaker 1: across enormous stretches of sky. The sheer number of candidates was,
• 1:35:15
  Speaker 1: at first, striking. Hundreds of potential under-densities, each one a
• 1:35:20
  Speaker 1: place where galaxies were thinner on the ground than the
• 1:35:22
  Speaker 1: cosmic average, What it suggested was something cosmologists had suspected,
• 1:35:27
  Speaker 1: but not fully confirmed, that voids were not rare curiosities
• 1:35:31
  Speaker 1: scattered through an otherwise uniform universe.
• 1:35:34
  Speaker 2: They were everywhere.
• 1:35:36
  Speaker 1: The cosmic web, it turned out, was made as much
• 1:35:38
  Speaker 1: of emptiness as of structure. That emptiness was distributed across
• 1:35:43
  Speaker 1: the sky in patterns that had never been fully mapped.
• 1:35:46
  Speaker 1: But quantity was not the same as equivalence. Most of
• 1:35:49
  Speaker 1: the candidate voids found in that survey were small, shallow,
• 1:35:54
  Speaker 1: Unremarkable by the standards of the Eridanus structure, they were
• 1:35:57
  Speaker 1: the ordinary furniture of a universe organized around filaments and walls.
• 1:36:02
  Speaker 1: A handful were genuinely large. None matched the Eridanus supervoid
• 1:36:07
  Speaker 1: in scale or depth. The survey expanded the inventory of
• 1:36:11
  Speaker 1: known underdense regions, but it did not immediately produce a
• 1:36:15
  Speaker 1: second Eridanus. Then came a more unsettling development. Researchers working
• 1:36:21
  Speaker 1: through the WMAP data began reporting that the original cold
• 1:36:25
  Speaker 1: spot might not be the only thermal anomaly worth tracking.
• 1:36:29
  Speaker 1: There appeared to be cold features in the northern hemisphere
• 1:36:32
  Speaker 1: of the CMB as well. Not a single sharp counterpart,
• 1:36:37
  Speaker 1: but a pattern of cooler-than-average regions distributed in ways that
• 1:36:41
  Speaker 1: did not match simple statistical expectation. Some teams described this
• 1:36:46
  Speaker 1: as a general asymmetry between the northern and southern hemispheres
• 1:36:50
  Speaker 1: of the CMB map. with the southern hemisphere running slightly
• 1:36:53
  Speaker 1: cooler on average.
• 1:36:55
  Speaker 2: Others were more.
• 1:36:56
  Speaker 1: Cautious, pointing out that the statistical tests being applied were
• 1:37:00
  Speaker 1: sensitive to the choices researchers made when designing them. What
• 1:37:04
  Speaker 1: this opened was a harder question than the one the
• 1:37:06
  Speaker 1: cold spot had originally posed. If the anomaly was not
• 1:37:10
  Speaker 1: a single point but a broader pattern, then the explanation
• 1:37:14
  Speaker 1: could not simply be a single supervoid sitting in the
• 1:37:16
  Speaker 1: right place. A supervoid could explain one cold spot, It
• 1:37:21
  Speaker 1: could not explain a henisphere. Something about the large-scale structure
• 1:37:25
  Speaker 1: of the universe, or something about the CMB itself, might
• 1:37:29
  Speaker 1: be producing cold features more systematically than the standard model predicted.
• 1:37:34
  Speaker 1: Were supervoids everywhere, draining energy from CMB photons at a
• 1:37:39
  Speaker 1: rate that standard simulations consistently underestimated? Was the integrated Sachs-Wolfe
• 1:37:44
  Speaker 1: effect stronger in reality than the models suggested? These were
• 1:37:49
  Speaker 1: not comfortable questions. This was because answering them would require
• 1:37:53
  Speaker 1: confronting the possibility that the standard cosmological picture had a
• 1:37:57
  Speaker 1: systematic flaw, not just a single outlier. The Planck satellite,
• 1:38:03
  Speaker 1: which launched in 2009 and began releasing data in the
• 1:38:06
  Speaker 1: following years, gave researchers their first genuinely high-resolution look at
• 1:38:11
  Speaker 1: the cold spot's internal structure. What Planck showed was that
• 1:38:16
  Speaker 1: the cold region was not featureless.
• 1:38:18
  Speaker 2: It had a.
• 1:38:19
  Speaker 1: Colder core surrounded by a ring of slightly warmer temperatures,
• 1:38:23
  Speaker 1: a profile that did not obviously match the smooth, gradual
• 1:38:27
  Speaker 1: imprint expected from a super void acting the ISW effect alone.
• 1:38:32
  Speaker 1: Some researchers argued that the internal structure was consistent with
• 1:38:36
  Speaker 1: a more complex void geometry. Others saw it as evidence
• 1:38:40
  Speaker 1: that something additional was going on. that the supervoid and
• 1:38:43
  Speaker 1: the CMB feature were related, but not simply caused by
• 1:38:47
  Speaker 1: one another in the straightforward way the original hypothesis had implied.
• 1:38:52
  Speaker 1: Planck also confirmed what WMAPE had shown, which mattered more
• 1:38:56
  Speaker 1: than it might sound. Instruments fail. Calibrations drift. Foreground signals
• 1:39:02
  Speaker 1: from our own galaxy can masquerade as cosmological features. Every
• 1:39:07
  Speaker 1: time a new instrument with different systematics looked at the
• 1:39:10
  Speaker 1: cold spot and saw.
• 1:39:11
  Speaker 2: The same thing.
• 1:39:12
  Speaker 1: The probability that the whole thing was an artifact dropped
• 1:39:16
  Speaker 1: closer to zero. By the time Planck's full data releases
• 1:39:19
  Speaker 1: were complete, the cold spot had been confirmed by two
• 1:39:23
  Speaker 1: independent satellites operating on different principles. The confirmation was not marginal.
• 1:39:30
  Speaker 2: The feature was there. It was cold. It was large.
• 1:39:34
  Speaker 1: And its internal structure was real. What the search for
• 1:39:38
  Speaker 1: other cold spots ultimately produced was not a clean resolution.
• 1:39:43
  Speaker 1: It produced a harder version of the original problem. The
• 1:39:46
  Speaker 1: universe appeared to contain more large under densities than the
• 1:39:50
  Speaker 1: standard model comfortably predicted. CMB cold features appeared in more
• 1:39:55
  Speaker 1: than one location, though none with the combined size and
• 1:39:59
  Speaker 1: depth of the Eridanus anomaly. The Eridanus cold spot remained
• 1:40:04
  Speaker 1: in a class of its own, but it was no
• 1:40:06
  Speaker 1: longer easy to treat it as a completely isolated freak
• 1:40:10
  Speaker 1: of cosmic statistics. It was beginning to look like the
• 1:40:13
  Speaker 1: most extreme example of a phenomenon the models were consistently
• 1:40:17
  Speaker 1: getting wrong. Whether that was a problem with the models
• 1:40:21
  Speaker 1: or simply the expected behavior of a universe large enough
• 1:40:24
  Speaker 1: to contain genuine outliers was a debate the data had
• 1:40:27
  Speaker 1: not yet settled. The question was no longer whether the
• 1:40:30
  Speaker 1: cold spot was real, but whether it was alone. When
• 1:40:34
  Speaker 1: the European Space Agency launched the Planck satellite in 2009,
• 1:40:39
  Speaker 1: The cold spot was one of the anomalies it was
• 1:40:41
  Speaker 1: specifically equipped to study. WMAP had done something remarkable. It
• 1:40:46
  Speaker 1: had given astronomers their first detailed portrait of the microwave sky.
• 1:40:51
  Speaker 1: But Planck was a different instrument entirely. Where WMAP had
• 1:40:55
  Speaker 1: angular resolution of about 13 arc minutes at best, Planck
• 1:40:59
  Speaker 1: pushed that to 5 arc minutes. That is not a
• 1:41:02
  Speaker 1: modest improvement. It is the difference between a photograph taken
• 1:41:07
  Speaker 1: through a fogged window and one taken with a precision
• 1:41:10
  Speaker 1: lens in clean air. You are not looking at the
• 1:41:13
  Speaker 1: same image. Planck also carried something WMAP could not match.
• 1:41:19
  Speaker 1: Nine frequency channels spanning a range from 30 to 857 gigahertz.
• 1:41:25
  Speaker 1: This matters because the CMB is not the only thing
• 1:41:29
  Speaker 1: emitting microwave radiation across the sky. Our own galaxy leaks
• 1:41:33
  Speaker 1: signal into every map. Dust, free electrons, synchrotron radiation from
• 1:41:39
  Speaker 1: charged particles spiraling through magnetic fields. All of it sits
• 1:41:44
  Speaker 1: between us and the ancient light we are trying to read.
• 1:41:48
  Speaker 1: Separating the genuine CMB signal from that foreground contamination requires
• 1:41:53
  Speaker 1: comparing measurements at different frequencies, since each source has a
• 1:41:58
  Speaker 1: different spectral signature. WMAP had five frequency channels, Planck had nine,
• 1:42:05
  Speaker 1: That extra coverage let researchers scrub the foreground contamination far
• 1:42:09
  Speaker 1: more cleanly than had been possible before. What was left
• 1:42:13
  Speaker 1: behind was a map of the CMB that was sharper, colder,
• 1:42:17
  Speaker 1: and more honest than anything that had come before it.
• 1:42:20
  Speaker 1: When Plank's team turned that cleaned map toward the cold spot,
• 1:42:24
  Speaker 1: what they found was not simpler.
• 1:42:26
  Speaker 2: It was stranger.
• 1:42:28
  Speaker 1: The cold region was not a uniform patch of depressed temperature.
• 1:42:32
  Speaker 2: It had structure.
• 1:42:34
  Speaker 1: At its center sat a core noticeably colder than the
• 1:42:37
  Speaker 1: surrounding area. Around that core was a ring. Warmer than
• 1:42:41
  Speaker 1: the core, but still colder than the CMB average. Around
• 1:42:45
  Speaker 1: the ring was another zone, slightly warmer relative to the
• 1:42:49
  Speaker 1: average in the surrounding sky. This was not a smooth,
• 1:42:53
  Speaker 1: featureless dip in the data. It was layered, concentric, almost architectural.
• 1:42:59
  Speaker 1: Whatever had produced it had not produced it uniformly. and
• 1:43:03
  Speaker 1: that internal structure carried information. Exactly what information was the
• 1:43:08
  Speaker 1: parts that divided researchers? Different teams looked at the same
• 1:43:12
  Speaker 1: Planck data and reached different numbers. Some groups ran statistical
• 1:43:17
  Speaker 1: tests that placed the cold spot at roughly a 1
• 1:43:20
  Speaker 1: in 50 probability of occurring by chance within a standard
• 1:43:23
  Speaker 1: cosmological model. Others applied different methods to the same dataset
• 1:43:29
  Speaker 1: and arrived at odds closer to 1 in several hundred.
• 1:43:33
  Speaker 2: Or even rarer still.
• 1:43:35
  Speaker 1: The fact that the range was this wide was not
• 1:43:37
  Speaker 1: a sign of sloppiness. It was a sign of genuine
• 1:43:40
  Speaker 1: statistical difficulty. The cold spot sits at the edge of
• 1:43:44
  Speaker 1: where the standard tools start to disagree with each other.
• 1:43:48
  Speaker 1: One group would define the anomaly using a specific filter
• 1:43:52
  Speaker 1: and a specific measure of coldness. Another group would apply
• 1:43:56
  Speaker 1: a slightly different filter, a slightly different measure, and get
• 1:44:00
  Speaker 1: a number that looked meaningfully dyed.
• 1:44:03
  Speaker 2: Ferrant. Both teams were being rigorous.
• 1:44:06
  Speaker 1: They were just asking slightly different questions, and the cold
• 1:44:10
  Speaker 1: spot was answering them differently. This kind of disagreement sounds
• 1:44:14
  Speaker 1: like uncertainty, and it is. But there is a signal
• 1:44:18
  Speaker 1: buried inside the disagreement. No matter which method was applied,
• 1:44:23
  Speaker 1: no matter which team ran the numbers, no team found
• 1:44:26
  Speaker 1: the cold spot to be statistically unremarkable. The floor of
• 1:44:30
  Speaker 1: improbability never dropped below one in several dozen, even in
• 1:44:35
  Speaker 1: the most conservative analyses. The cold spot was always sitting
• 1:44:39
  Speaker 1: at the edge of what the standard model would comfortably produce,
• 1:44:42
  Speaker 1: regardless of exactly how you drew the boundary. That persistent discomfort,
• 1:44:48
  Speaker 1: across years and teams and methods, is what the Planck
• 1:44:52
  Speaker 1: era gave the field as a kind of consensus.
• 1:44:55
  Speaker 2: Planck also refined what researchers knew about the Eridanus supervoid itself.
• 1:45:01
  Speaker 1: When the Planck CMB maps were combined with the galaxy
• 1:45:05
  Speaker 1: distribution surveys being assembled at the same time, it became
• 1:45:09
  Speaker 1: possible to study the relationship between the supervoid structure and
• 1:45:14
  Speaker 1: the cold spot's temperature profile in far more detail than before.
• 1:45:19
  Speaker 1: The colder core of the cold spot appeared to correspond
• 1:45:22
  Speaker 1: with the deepest part of the void, the region of
• 1:45:26
  Speaker 1: most extreme underdensity near the center, where the matter deficit
• 1:45:30
  Speaker 1: was most severe. The surrounding ring of slightly less extreme
• 1:45:35
  Speaker 1: temperature depression matched a region of less extreme underdensity toward
• 1:45:40
  Speaker 1: the void's outer boundary. The correspondence was not perfect, and
• 1:45:44
  Speaker 1: researchers argued about exactly how much of that alignment was
• 1:45:47
  Speaker 1: meaningful versus coincidental. but the shapes were talking to each
• 1:45:52
  Speaker 1: other in a way that older, lower resolution data had
• 1:45:55
  Speaker 1: not revealed clearly. What Planck could not do was close
• 1:45:59
  Speaker 1: the case. The temperature measurements it produced were precise enough
• 1:46:03
  Speaker 1: to sharpen the question considerably, but sharpening the question only
• 1:46:07
  Speaker 1: made the gap between the supervoid's predicted ISW effect and
• 1:46:11
  Speaker 1: the observed temperature depression look more stark. A cruder instrument
• 1:46:16
  Speaker 1: can leave room for error to hide in, Planck removed
• 1:46:19
  Speaker 1: much of that room. The discrepancy between what the supervoid
• 1:46:23
  Speaker 1: should produce and what the cold spot actually shows did
• 1:46:27
  Speaker 1: not shrink when Planck arrived. If anything, the improved data
• 1:46:31
  Speaker 1: made it harder to wave away. Planck did not solve
• 1:46:35
  Speaker 1: the cold spot, but it made absolutely certain that the
• 1:46:38
  Speaker 1: cold spot was real. After two decades of observation, analysis
• 1:46:43
  Speaker 1: and argument, the honest summary of what we know about
• 1:46:46
  Speaker 1: the cold spot is shorter than it should be. The
• 1:46:50
  Speaker 1: Eridana's supervoid is real. The cold spot is real. They
• 1:46:55
  Speaker 1: occupy the same patch of sky. Those three facts are settled.
• 1:47:00
  Speaker 1: Everything else, why the temperatures are what they are, whether
• 1:47:04
  Speaker 1: one caused the other, what fills the gap between the
• 1:47:07
  Speaker 1: physics and the observations, remains genuinely open.
• 1:47:12
  Speaker 2: Not almost solved. Not close. Open.
• 1:47:16
  Speaker 1: Start with the number that should bother you. The integrated
• 1:47:19
  Speaker 1: Sachs-Wolfe effect, acting through the supervoid, can account for somewhere
• 1:47:24
  Speaker 1: between 10 and 20% of the cold spots observed temperature depression.
• 1:47:29
  Speaker 1: That is the best case estimate under the standard cosmological model.
• 1:47:34
  Speaker 1: It means that after finding the largest identified void in
• 1:47:37
  Speaker 1: the observable universe sitting directly behind the most anomalous feature
• 1:47:41
  Speaker 1: in the CMB, after confirming that void through two independent
• 1:47:46
  Speaker 1: observational methods, After two decades of increasingly precise measurements, the
• 1:47:51
  Speaker 1: physics still leaves 80-90% of the temperature signal unexplained. The
• 1:47:57
  Speaker 1: supervoid is real. The supervoid is enormous. And the supervoid
• 1:48:02
  Speaker 1: is not enough. Andrus Kovacs, the lead researcher on one
• 1:48:05
  Speaker 1: of the most comprehensive recent studies of the region, put
• 1:48:09
  Speaker 1: it carefully. Having two individually rare structures coincide in the
• 1:48:14
  Speaker 1: same direction, he said, is not sufficient to prove causality
• 1:48:18
  Speaker 1: by the standards science demands. That is a precise and
• 1:48:21
  Speaker 1: honest statement, and it deserves to sit in your mind
• 1:48:25
  Speaker 1: for a moment. Spatial coincidence is suggestive, it is compelling.
• 1:48:31
  Speaker 2: It keeps researchers working.
• 1:48:33
  Speaker 1: But coincidence is not causality, and in a field where
• 1:48:37
  Speaker 1: the margins between a real signal and a statistical artifact
• 1:48:41
  Speaker 1: have enormous consequences, the distinction matters. The case has not
• 1:48:46
  Speaker 1: been made to the standard the physics requires. The internal
• 1:48:50
  Speaker 1: structure of the supervoid adds another layer of uncertainty. What
• 1:48:54
  Speaker 1: researchers have measured so far is a broad brush picture,
• 1:48:59
  Speaker 1: a region roughly 1.8 billion light years across, containing approximately
• 1:49:04
  Speaker 1: 30% less matter than the cosmic average. elongated along certain axes.
• 1:49:10
  Speaker 1: But a void of that scale almost certainly contains sub-voids
• 1:49:14
  Speaker 1: nested inside it, denser filaments threading.
• 1:49:17
  Speaker 2: Through its interior.
• 1:49:19
  Speaker 1: Pockets of varying underdensity that the current galaxy surveys do
• 1:49:23
  Speaker 1: not resolve clearly enough to map. The density profile, how
• 1:49:28
  Speaker 1: sharply the underdensity deepens toward the center, how it fades
• 1:49:32
  Speaker 1: at the edges, is constrained but not fully characterized. That
• 1:49:37
  Speaker 1: profile matters because the ISW temperature signal depends directly on
• 1:49:42
  Speaker 1: how the gravitational potential changes as a photon passes through.
• 1:49:46
  Speaker 1: A different density profile produces a different temperature imprint. Without
• 1:49:51
  Speaker 1: a precise three-dimensional map of the supervoid's interior, researchers are
• 1:49:56
  Speaker 1: working with an approximation, and approximations compound. The role of
• 1:50:01
  Speaker 1: dark matter inside the void is similarly incomplete. The 2015
• 1:50:07
  Speaker 1: Dark Energy Survey results confirmed a dark matter under density
• 1:50:11
  Speaker 1: in the same region as the galaxy under density. That
• 1:50:15
  Speaker 1: confirmation was significant. But knowing that dark matter is underrepresented
• 1:50:21
  Speaker 1: in that region is different from knowing by exactly how
• 1:50:24
  Speaker 1: much and in exactly what distribution. Dark matter cannot be
• 1:50:29
  Speaker 1: observed directly. Its presence is inferred through gravitational effects on
• 1:50:34
  Speaker 1: light and on the motions of galaxies. In a region
• 1:50:39
  Speaker 1: specifically defined by having very few galaxies, those inferences carry
• 1:50:44
  Speaker 1: larger uncertainties than they would anywhere else. The very emptiness
• 1:50:49
  Speaker 1: that makes the super void scientifically interesting also makes it
• 1:50:53
  Speaker 1: harder to measure with precision. Then there is the statistical question,
• 1:50:58
  Speaker 1: and this one has not settled down despite years of argument.
• 1:51:02
  Speaker 1: Is the cold spot a genuine anomaly? something that the
• 1:51:05
  Speaker 1: Lambda CDM model cannot produce with any reasonable probability? Or
• 1:51:11
  Speaker 1: is it an extreme but legitimate fluctuation that the model
• 1:51:14
  Speaker 1: technically allows? Different research groups applying different statistical frameworks to
• 1:51:20
  Speaker 1: the same Planck data have reached conclusions ranging from roughly
• 1:51:25
  Speaker 1: 1 in 50 to roughly 1 in several thousand. That
• 1:51:29
  Speaker 1: spread is not a sign that one team made an error.
• 1:51:32
  Speaker 1: It reflects genuine disagreement about how to count, what to compare,
• 1:51:36
  Speaker 1: and how to define.
• 1:51:37
  Speaker 2: The feature being tested.
• 1:51:39
  Speaker 1: The cold spot has a colder core and a surrounding ring,
• 1:51:43
  Speaker 1: which makes defining its boundaries a choice rather.
• 1:51:46
  Speaker 2: Than a measurement. Statistics run O.
• 1:51:50
  Speaker 1: N different boundary choices produce different significance values. The answer
• 1:51:55
  Speaker 1: you get depends partly on how you ask the question,
• 1:51:58
  Speaker 1: and no consensus has emerged on how to ask it correctly.
• 1:52:02
  Speaker 1: What about similar configurations elsewhere? If the cold spot is
• 1:52:07
  Speaker 1: caused by a supervoid through the ISW effect, then similar
• 1:52:11
  Speaker 1: supervoids in similar positions along other lines of sight should
• 1:52:15
  Speaker 1: produce similar cold signatures. Researchers have looked. The 2009 survey
• 1:52:21
  Speaker 1: of the southern sky found hundreds of candidate voids. Some
• 1:52:25
  Speaker 1: CMB surveys have found other cold regions, but none of
• 1:52:29
  Speaker 1: them combine the extreme temperature depression and the extreme underdensity
• 1:52:34
  Speaker 1: of the aridinous case at the same level of significance.
• 1:52:37
  Speaker 1: Whether that means the aridinous case is genuinely unique, or
• 1:52:41
  Speaker 1: whether it means the surveys have not yet found its counterparts,
• 1:52:45
  Speaker 1: is not known. A universe filled with undetected supervoids producing
• 1:52:50
  Speaker 1: undetected cold signatures would look exactly like a universe where
• 1:52:54
  Speaker 1: the cold spot is a 1T.
• 1:52:56
  Speaker 2: Ein event.
• 1:52:57
  Speaker 1: At least with the tools currently available. This is the
• 1:53:01
  Speaker 1: position the field is in. Not stuck, exactly. New data
• 1:53:05
  Speaker 1: keeps arriving. New methods keep developing. New constraints keep tightening
• 1:53:11
  Speaker 1: around the problem. But not close to resolved either. The
• 1:53:15
  Speaker 1: supervoid explains a fraction of what needs explaining. The causal
• 1:53:19
  Speaker 1: link between the void and the cold spot is strongly
• 1:53:22
  Speaker 1: suggested and not proven. The void's internal structure is partially mapped.
• 1:53:27
  Speaker 1: The dark matter distribution inside it is constrained but not characterized.
• 1:53:33
  Speaker 1: The statistical significance of the cold spot is disputed. And
• 1:53:38
  Speaker 1: whether the standard model is genuinely incomplete or simply being
• 1:53:42
  Speaker 1: tested at its extreme edge remains the central question with
• 1:53:46
  Speaker 1: no definitive answer.
• 1:53:48
  Speaker 2: The gap between.
• 1:53:50
  Speaker 1: What the supervoid can explain and what the cold spot
• 1:53:53
  Speaker 1: actually shows has not closed. If anything, better measurements have
• 1:53:58
  Speaker 1: made it look wider.
• 1:54:00
  Speaker 2: There is a.
• 1:54:00
  Speaker 1: Patch of sky in the constellation Eridanus that is, in
• 1:54:04
  Speaker 1: a specific and meaningful sense, the coldest place we can
• 1:54:09
  Speaker 1: observe in the universe. That sentence requires some unpacking, because
• 1:54:13
  Speaker 1: the coldness here is not the kind your instruments would
• 1:54:17
  Speaker 1: measure if you flew a spacecraft into that region and
• 1:54:21
  Speaker 1: checked the temperature of the gas. It is not cold
• 1:54:24
  Speaker 1: the way deep space between galaxies is cold. The supervoid
• 1:54:28
  Speaker 1: contains matter. It contains galaxies, gas clouds, the occasional cluster
• 1:54:33
  Speaker 1: of stars. Whatever matter exists inside that void has temperatures
• 1:54:38
  Speaker 1: comparable to anything else at similar scales in the universe.
• 1:54:42
  Speaker 1: The coldness is not in the void itself. It is
• 1:54:45
  Speaker 1: in the light that passed through it. What makes the
• 1:54:48
  Speaker 1: Eridanus region the coldest observable feature in the universe is
• 1:54:52
  Speaker 1: strictly this. The photons arriving from that direction carry less
• 1:54:56
  Speaker 1: energy than photons arriving from nearly any other comparable patch
• 1:55:01
  Speaker 1: of sky. The cosmic microwave background radiation streaming toward us
• 1:55:05
  Speaker 1: from the Eridanus direction has been slightly depleted. It has
• 1:55:10
  Speaker 1: climbed a gravitational hill. and the expansion of the universe
• 1:55:14
  Speaker 1: stretched that hill while the photons were crossing it, so
• 1:55:17
  Speaker 1: they could not recover what they lost on the way up.
• 1:55:21
  Speaker 1: The result is radiation that reads cold. Not physically cold
• 1:55:25
  Speaker 1: in any local sense. Cold in the most observational sense possible.
• 1:55:30
  Speaker 1: Radiation arriving at our detectors with a deficit of energy
• 1:55:34
  Speaker 1: that measures roughly 70 microkelvin below the average temperature of
• 1:55:38
  Speaker 1: the CMB. That number sounds trivial, It is not. The
• 1:55:43
  Speaker 1: average temperature of the CMB is 2.725 Kelvin, barely above
• 1:55:48
  Speaker 1: absolute zero already. The typical fluctuations that seed galaxies that
• 1:55:53
  Speaker 1: carry the entire history of structure formation in the universe
• 1:55:57
  Speaker 1: run about 18 microkelvin above or below the mean. The
• 1:56:01
  Speaker 1: cold spot sits at 70 microkelvin below. It is not
• 1:56:05
  Speaker 1: a whisper outside the normal range.
• 1:56:08
  Speaker 2: It is a shout.
• 1:56:10
  Speaker 1: So what does it actually look like inside the region
• 1:56:13
  Speaker 1: producing this? Imagine the cosmic web as it exists across
• 1:56:17
  Speaker 1: billions of light years. Filaments of dark matter and ordinary
• 1:56:22
  Speaker 1: matter stretching between nodes. Dense clusters where matter has pooled
• 1:56:27
  Speaker 1: over billions of years of gravitational attraction. And between those structures, voids.
• 1:56:33
  Speaker 1: Regions where matter drained away following the pull of denser neighbourhoods.
• 1:56:38
  Speaker 1: Leaving behind something closer to genuine emptiness than almost anywhere.
• 1:56:42
  Speaker 2: Else in the universe.
• 1:56:44
  Speaker 1: The Eridana Super Void is one of these regions scaled
• 1:56:47
  Speaker 1: up to a size that strains comprehension. Roughly 1.8 billion
• 1:56:52
  Speaker 1: light years across. Containing perhaps 30% less matter than the
• 1:56:57
  Speaker 1: cosmic average. Toward its center, even less. Galaxy formation did
• 1:57:02
  Speaker 1: not proceed efficiently here. That is not a metaphor for
• 1:57:06
  Speaker 1: something subtle. In the densest regions of the universe, galaxies
• 1:57:11
  Speaker 1: form because matter pulls on matter. Over-dense patches attract more material.
• 1:57:17
  Speaker 1: Dark matter halos grow. Gas falls in, cools, collapses, ignites
• 1:57:24
  Speaker 1: into stars. The process feeds itself. In an under-dense region,
• 1:57:30
  Speaker 1: the opposite dynamic dominates. Matter that might have collapsed was
• 1:57:34
  Speaker 1: pulled away toward denser surroundings before it could accumulate. The
• 1:57:39
  Speaker 1: few galaxies that did form inside the supervoid are sparse
• 1:57:43
  Speaker 1: and isolated, scattered across a volume so enormous that the
• 1:57:48
  Speaker 1: distances between them dwarf the distances in more typical regions
• 1:57:52
  Speaker 1: of the cosmic web. If you were inside the supervoid
• 1:57:56
  Speaker 1: and could somehow see the structure around you, the sky
• 1:57:59
  Speaker 1: would be darker. The filaments visible in denser parts of
• 1:58:03
  Speaker 1: the universe simply.
• 1:58:04
  Speaker 2: Would not be there.
• 1:58:06
  Speaker 1: The cosmic web in that region stretched thin over billions
• 1:58:09
  Speaker 1: of years. The large-scale gravitational environment around the supervoid has
• 1:58:14
  Speaker 1: been draining matter from its interior across most of cosmic history.
• 1:58:19
  Speaker 1: Dark energy accelerates this process by expanding space itself, pulling
• 1:58:24
  Speaker 1: the void's walls apart, deepening the under-density. extending the gravitational
• 1:58:29
  Speaker 1: hill that CMB photons must cross when they travel in
• 1:58:32
  Speaker 1: our direction from behind it. What you are looking at
• 1:58:36
  Speaker 1: in the Eridanus direction is not an event. It is
• 1:58:39
  Speaker 1: a slow process, running across billions of years, that has
• 1:58:43
  Speaker 1: produced the single largest and deepest departure from the average
• 1:58:47
  Speaker 1: texture of the universe that we have identified anywhere in
• 1:58:51
  Speaker 1: the observable sky. There is something worth sitting with here.
• 1:58:55
  Speaker 1: When you look at the full sky CMB map, That
• 1:58:58
  Speaker 1: oval image you have probably seen, the ancient light of
• 1:59:02
  Speaker 1: the universe displayed in false color. The cold spot is visible.
• 1:59:07
  Speaker 1: A blue region in the lower half of the map,
• 1:59:10
  Speaker 1: slightly more prominent than its surroundings, representing the oldest and
• 1:59:14
  Speaker 1: coldest signal in the portrait of the early universe. What
• 1:59:18
  Speaker 1: produced that feature was not present in the early universe.
• 1:59:22
  Speaker 1: The Super Void did not exist 380,000 years after the
• 1:59:27
  Speaker 1: Big Bang, when that light was released. It formed later,
• 1:59:32
  Speaker 1: as structure grew, as matter flowed and clustered, and left
• 1:59:36
  Speaker 1: certain regions behind. The cold imprint on that ancient map
• 1:59:40
  Speaker 1: is a late addition, a signature written onto the oldest
• 1:59:44
  Speaker 1: light in the universe by something that came afterward. That
• 1:59:48
  Speaker 1: is a strange thing to hold in your mind. The
• 1:59:51
  Speaker 1: CMB is a snapshot of the universe in its infancy.
• 1:59:55
  Speaker 1: But the cold spot is not a feature of that infancy.
• 1:59:59
  Speaker 2: It is a.
• 2:00:00
  Speaker 1: Shadow cast by something that grew up long after the
• 2:00:03
  Speaker 1: snapshot was taken. A super void that imprinted itself onto
• 2:00:08
  Speaker 1: ancient might as that light crossed through it on its
• 2:00:11
  Speaker 1: way to us. What we see as a feature of
• 2:00:14
  Speaker 1: the early universe is partly a product of something billions
• 2:00:18
  Speaker 1: of years more recent. You could point a telescope at
• 2:00:21
  Speaker 1: the center of the Eridanus supervoid and find galaxies, fewer
• 2:00:26
  Speaker 1: than elsewhere, spread across distances that reflect the thinness of
• 2:00:30
  Speaker 1: structure in that region. Nothing about those galaxies, looked at directly,
• 2:00:36
  Speaker 1: tells you they are sitting inside the coldest observable feature
• 2:00:39
  Speaker 1: in the universe. The coldness is not a property of
• 2:00:42
  Speaker 1: anything physical inside the void. It is a property of
• 2:00:46
  Speaker 1: what happened to light passing through. It is not cold
• 2:00:49
  Speaker 1: because something shield it. It is cold because something was
• 2:00:53
  Speaker 1: never there. The next generation of observational tools is not
• 2:00:57
  Speaker 1: going to leave.
• 2:00:58
  Speaker 2: The cold spot alone.
• 2:01:00
  Speaker 1: For two decades, the mystery has outpaced the instruments chasing it.
• 2:01:05
  Speaker 1: That is about to change. Several of the most ambitious
• 2:01:08
  Speaker 1: observational projects in the history of cosmology are now either
• 2:01:12
  Speaker 1: operational or approaching first light. A significant portion of their
• 2:01:17
  Speaker 1: scientific programs point directly at the question of what is
• 2:01:21
  Speaker 1: happening in the Eridanus region. The cold spot has been waiting.
• 2:01:26
  Speaker 1: The instruments have finally caught up. The Rubin Observatory, sitting
• 2:01:31
  Speaker 1: at 8,000 feet in the Chilean Andes. is preparing to
• 2:01:35
  Speaker 1: begin its legacy survey of space and time. LSST will
• 2:01:39
  Speaker 1: photograph the entire southern sky repeatedly over 10 years, building
• 2:01:44
  Speaker 1: a catalog of roughly 20 billion galaxies. 20 billion! The
• 2:01:49
  Speaker 1: Eridanus supervoid sits in the southern hemisphere. LSST will pass
• 2:01:54
  Speaker 1: over that region of sky thousands of times during its
• 2:01:57
  Speaker 1: survey lifetime. What this means in practice is a galaxy
• 2:02:02
  Speaker 1: map of the cold spot region with a depth and
• 2:02:04
  Speaker 1: precision that no previous instrument has come close to matching.
• 2:02:08
  Speaker 2: Researchers will be able.
• 2:02:10
  Speaker 1: To trace the distribution of galaxies, not just in projection
• 2:02:13
  Speaker 1: on the sky, but in three dimensions, mapping how the
• 2:02:17
  Speaker 1: density changes as you move along the line of sight
• 2:02:20
  Speaker 1: through and beyond the supervoid. The question of where exactly
• 2:02:25
  Speaker 1: the underdensity begins, where it deepens, where it fades back
• 2:02:29
  Speaker 1: into the surrounding cosmic web, will finally have a precise answer.
• 2:02:34
  Speaker 1: At the same time, the Euclid satellite is already in space.
• 2:02:39
  Speaker 1: The European Space Agency launched it in 2023. Its mission
• 2:02:44
  Speaker 1: is mapping the three-dimensional distribution of galaxies across more than
• 2:02:48
  Speaker 1: a third of the entire sky with extraordinary resolution. Euclid
• 2:02:53
  Speaker 1: is specifically designed to study dark energy and large-scale structure
• 2:02:58
  Speaker 1: That makes the Eridanus supervoid essentially a primary target. The
• 2:03:03
  Speaker 1: satellite measures galaxy shapes with enough precision to detect weak
• 2:03:07
  Speaker 1: gravitational lensing signals from structures at cosmological distances. That is
• 2:03:13
  Speaker 1: the same lensing technique that helped confirm the supervoid's dark
• 2:03:17
  Speaker 1: matter under density in 2015, only now applied with sensitivity
• 2:03:22
  Speaker 1: several times greater. If there is structure inside the supervoid
• 2:03:26
  Speaker 1: that earlier surveys missed, subvoids, filaments of residual matter, gradients
• 2:03:33
  Speaker 1: in the density profile. Euclid has a realistic chance of
• 2:03:36
  Speaker 1: finding it. The three-dimensional picture Euclid and LSST will build together,
• 2:03:42
  Speaker 1: combined with existing data from the Dark Energy Survey, will
• 2:03:46
  Speaker 1: be far more complete than anything the field currently has
• 2:03:49
  Speaker 1: to work with. The CMB side of the problem is
• 2:03:52
  Speaker 1: also moving forward. Ground-based experiments like the Simons Observatory in
• 2:03:57
  Speaker 1: Chile and the planned CMBS4 experiment will reach sensitivities in
• 2:04:02
  Speaker 1: CMB temperature and polarization measurements that would have seemed implausible
• 2:04:07
  Speaker 1: a decade ago. CMBS4 alone is designed to produce a
• 2:04:12
  Speaker 1: map of the microwave sky roughly seven times more sensitive
• 2:04:16
  Speaker 1: than Planck's final data release. What that sensitivity buys you,
• 2:04:20
  Speaker 1: in practical terms, is the ability to detect the integrated
• 2:04:24
  Speaker 1: Sachs-Wolfe signal from individual large structures rather than just stacking
• 2:04:29
  Speaker 1: statistical averages across many voids. Right now, the ISW signal
• 2:04:35
  Speaker 1: from the Eridanus supervoid cannot be cleanly isolated against the
• 2:04:39
  Speaker 1: noise in existing CMB maps. It is too faint relative
• 2:04:43
  Speaker 1: to the instrument's sensitivity. With CMB S4 data, that calculation changes.
• 2:04:50
  Speaker 1: researchers may be able to directly measure how much the
• 2:04:53
  Speaker 1: supervoid has cooled the photons passing through it. Compare that
• 2:04:57
  Speaker 1: number to what the standard model predicts. If the numbers match,
• 2:05:01
  Speaker 1: the standard picture survives. If they do not, T, he discrepancy,
• 2:05:07
  Speaker 1: will be sitting in high-quality data that no one can
• 2:05:11
  Speaker 1: argue away. There is another piece of the puzzle that
• 2:05:14
  Speaker 1: better data will sharpen.
• 2:05:16
  Speaker 2: Neutrino mass.
• 2:05:17
  Speaker 1: Neutrinos are extraordinarily light particles that fill the universe in
• 2:05:22
  Speaker 1: enormous numbers. Their mass, still only loosely constrained, affects how
• 2:05:27
  Speaker 1: structure forms on large scales. Massive neutrinos suppress the growth
• 2:05:32
  Speaker 1: of cosmic structure at certain scales. That changes the predicted
• 2:05:36
  Speaker 1: depth and evolution of voids. Current uncertainty in neutrino mass
• 2:05:41
  Speaker 1: is large enough that it introduces meaningful uncertainty into predictions
• 2:05:45
  Speaker 1: about supervoid properties. The next generation of cosmological surveys, including
• 2:05:51
  Speaker 1: Euclid and LSST, will constrain neutrino mass to a level
• 2:05:55
  Speaker 1: of precision that closes this loophole. Models of how the
• 2:05:59
  Speaker 1: Eridana supervoid formed and deepened over cosmic time will become
• 2:06:03
  Speaker 1: correspondingly sharper, either tightening the agreement with Lambda CDM or
• 2:06:09
  Speaker 1: making the tension harder to dismiss.
• 2:06:12
  Speaker 2: What researchers are.
• 2:06:13
  Speaker 1: Building toward is something that has never existed before. A complete, high-resolution,
• 2:06:19
  Speaker 1: three-dimensional picture of the matter distribution in the Eridanus direction.
• 2:06:24
  Speaker 1: A precise CMB temperature profile of the cold spot, including
• 2:06:28
  Speaker 1: its internal structure. A direct observational measurement of the ISW
• 2:06:33
  Speaker 1: signal from the supervoid, all within the same decade. Put
• 2:06:37
  Speaker 1: those three things together and you have a system of
• 2:06:40
  Speaker 1: constraints tight enough to actually test whether the standard model
• 2:06:44
  Speaker 1: can accommodate what is observed. The cold spot has survived
• 2:06:48
  Speaker 1: for 20 years in part because the data was never
• 2:06:51
  Speaker 1: quite complete enough to close the argument. That situation is ending.
• 2:06:56
  Speaker 1: The instruments being built right now will be pointed at
• 2:06:59
  Speaker 1: Eridanus within the decade. Whatever they find will not leave
• 2:07:05
  Speaker 1: the standard cosmological picture unchanged. Every so often in the
• 2:07:10
  Speaker 1: history of physics, an anomaly that could have been waved
• 2:07:13
  Speaker 1: away as a statistical fluke turned out to be the
• 2:07:16
  Speaker 1: fingerprint of something the existing theory had no room for.
• 2:07:20
  Speaker 1: The perihelion of Mercury drifted slightly too fast for Newtonian
• 2:07:25
  Speaker 1: gravity to explain. For decades it sat in the data, uncomfortable,
• 2:07:30
  Speaker 1: officially unsolved. Then general relativity arrived and the drift made
• 2:07:35
  Speaker 1: perfect sense. The ultraviolet catastrophe in classical thermodynamics seemed like
• 2:07:41
  Speaker 1: a minor technical problem until Planck resolved it with quantum
• 2:07:45
  Speaker 1: mechanics and changed everything. Anomalies like these are not just curiosities,
• 2:07:51
  Speaker 1: they are the seams in a theory, the places where
• 2:07:54
  Speaker 1: something underneath is trying to show through. The cold spot
• 2:07:58
  Speaker 1: might be one of those seams. or it might not be.
• 2:08:02
  Speaker 1: That uncertainty is itself the story. Lambda CDM is not
• 2:08:07
  Speaker 1: a narrow theory about one patch of sky. It is
• 2:08:10
  Speaker 1: the structural foundation of modern cosmology. Every model of how
• 2:08:15
  Speaker 1: galaxies formed, Every calculation of how dark matter clumped in
• 2:08:19
  Speaker 1: the early universe. Every prediction about the geometry of space
• 2:08:23
  Speaker 1: and the ultimate fate of everything in it.
• 2:08:26
  Speaker 2: All of that.
• 2:08:26
  Speaker 1: Rests on Lambda CDM. When the model works, it works
• 2:08:31
  Speaker 1: extraordinarily well. It predicted the acoustic peaks in the CMB
• 2:08:35
  Speaker 1: before satellites confirmed them. It correctly describes the large-scale distribution
• 2:08:41
  Speaker 1: of galaxies across billions of light years. It accounts for
• 2:08:45
  Speaker 1: the expansion history of the universe with remarkable precision. A
• 2:08:49
  Speaker 1: theory with that kind of track record does not get
• 2:08:52
  Speaker 1: abandoned lightly. It should not be. But track records do
• 2:08:56
  Speaker 1: not make theories immune to failure. And if the cold
• 2:09:00
  Speaker 1: spot genuinely cannot be accommodated within Lambda CDM, if better
• 2:09:04
  Speaker 1: data and sharper analysis confirm that the temperature depression is
• 2:09:08
  Speaker 1: too extreme, too large, and too structured to be a
• 2:09:12
  Speaker 1: statistical accident, Then the model has a problem. Not a
• 2:09:16
  Speaker 1: footnote problem. A foundational problem. What would that actually require?
• 2:09:24
  Speaker 1: What kinds of new physics could step in where lambda
• 2:09:26
  Speaker 1: CDM falls short? One class of proposals involves modifying how
• 2:09:31
  Speaker 1: gravity behaves on the larger scales. General relativity is the
• 2:09:36
  Speaker 1: gravitational theory embedded inside lambda CDM. and it has been
• 2:09:41
  Speaker 1: tested with extraordinary precision within the solar system and in
• 2:09:45
  Speaker 1: the strong field regime around neutron stars and black holes.
• 2:09:50
  Speaker 1: But cosmic scales are different. The behavior of gravity on
• 2:09:54
  Speaker 1: scales of hundreds of megaparsecs has not been tested with
• 2:09:57
  Speaker 1: the same rigor. Modified gravity theories, frameworks like FR gravity,
• 2:10:03
  Speaker 1: or theories with additional scalar fields can change the way
• 2:10:07
  Speaker 1: large-scale structure forms and the way voids evolve. Some of
• 2:10:12
  Speaker 1: these models predict deeper, faster-growing voids than standard lambda CDN does.
• 2:10:18
  Speaker 1: A supervoid in a modified gravity universe could in principle
• 2:10:22
  Speaker 1: produce a stronger ISW imprint on T, potentially accounting for
• 2:10:28
  Speaker 1: more of the cold spot's temperature depression than the standard
• 2:10:32
  Speaker 1: model allows. Whether the numbers actually work depends on which
• 2:10:36
  Speaker 1: modification you choose and what the observations constrain. So far,
• 2:10:41
  Speaker 1: no specific modified gravity model has cleanly solved the cold
• 2:10:45
  Speaker 1: spot problem, but the possibility remains open. A second direction
• 2:10:51
  Speaker 1: involves dark energy. Lambda CDM treats dark energy as a
• 2:10:55
  Speaker 1: cosmological constant. a fixed uniform energy density filling all of space,
• 2:11:01
  Speaker 1: characterized by a single number. Real dark energy might be
• 2:11:06
  Speaker 1: more complicated than that. If dark energy varies across space
• 2:11:10
  Speaker 1: or changes with time, it would alter the rate at
• 2:11:13
  Speaker 1: which voids grow and deepen, and it would change the
• 2:11:16
  Speaker 1: magnitude of the ISW effect. A dark energy equation of state,
• 2:11:21
  Speaker 1: different from the standard value could, depending on the direction
• 2:11:25
  Speaker 1: of the deviation, either amplify or suppress the cold spot signal.
• 2:11:31
  Speaker 1: Supervoids like Eridanus are particularly sensitive to this, because void
• 2:11:36
  Speaker 1: growth is driven almost entirely by cosmic expansion rather than
• 2:11:41
  Speaker 1: local gravitational dynamics. The Eridanus supervoid's measured properties, combined with
• 2:11:47
  Speaker 1: the strength of the cold spot, place constraints on what
• 2:11:51
  Speaker 1: dark energy can be doing. And some analyses suggest those
• 2:11:55
  Speaker 1: constraints are already pushing against the edges of what Lambda
• 2:11:59
  Speaker 1: CDM comfortably predicts. There are stranger proposals. Some researchers have
• 2:12:05
  Speaker 1: explored whether the cold spot could reflect an anisotropy in
• 2:12:09
  Speaker 1: the universe itself, a preferred direction in space, which Lambda
• 2:12:14
  Speaker 1: CDM explicitly forbids. A universe that is not perfectly isotropic
• 2:12:19
  Speaker 1: on the largest scales would violate one of the foundational
• 2:12:22
  Speaker 1: assumptions of the standard model. If the cold spot is
• 2:12:25
  Speaker 1: part of a broader pattern of large-scale anisotropy, that would
• 2:12:29
  Speaker 1: be a significantly more serious challenge than a single anomalous
• 2:12:33
  Speaker 1: temperature measurement. Evidence for this remains weak and contested, but
• 2:12:38
  Speaker 1: the possibility has not been ruled out. Here is what
• 2:12:41
  Speaker 1: makes all of this difficult.
• 2:12:43
  Speaker 2: To resolve quickly.
• 2:12:45
  Speaker 1: To distinguish between Lambda CDM and its competitors, you need
• 2:12:49
  Speaker 1: measurements precise enough to detect differences that are genuinely small.
• 2:12:53
  Speaker 2: Small in terms.
• 2:12:54
  Speaker 1: Of the equation of state of dark energy. Small in
• 2:12:58
  Speaker 1: terms of how void growth rates deviate from standard predictions.
• 2:13:02
  Speaker 1: The gap between what current instruments can measure and what
• 2:13:06
  Speaker 1: you would need to definitively settle these questions is real.
• 2:13:10
  Speaker 1: The next decade of surveys and experiments will narrow that
• 2:13:13
  Speaker 1: gap considerably, but the answer is not here yet. What
• 2:13:18
  Speaker 1: is here is the cold spot itself, sitting in the data,
• 2:13:22
  Speaker 1: 70 microkelvin below average, spanning a region of sky that
• 2:13:27
  Speaker 1: corresponds to one of the largest under-densities in the observable universe,
• 2:13:32
  Speaker 1: partially explained and partially not. If it is a fluke,
• 2:13:37
  Speaker 1: It is one of the most persistent and precisely characterized
• 2:13:41
  Speaker 1: flukes in the history of observational science. If it is
• 2:13:44
  Speaker 1: not a fluke, if the standard model genuinely has no
• 2:13:48
  Speaker 1: room for it, then something beyond Lambda CDM is operating
• 2:13:52
  Speaker 1: at cosmic scales, and the cold spot is where we
• 2:13:56
  Speaker 1: first noticed. Blind spots in successful theories are exactly where
• 2:14:01
  Speaker 1: new physics has always lived. Mercury's drifting orbit looked minor,
• 2:14:06
  Speaker 1: The ultraviolet catastrophe looked technical.
• 2:14:09
  Speaker 2: Neither one was.
• 2:14:10
  Speaker 1: The cold spot may be nothing more than a rare
• 2:14:13
  Speaker 1: fluctuation in an otherwise well-behaved universe. Or it may be
• 2:14:17
  Speaker 1: the place where the standard model of cosmology quietly runs
• 2:14:21
  Speaker 1: out of answers. On a clear night in the southern hemisphere,
• 2:14:25
  Speaker 1: the constellation Eridanus arcs low across the sky, a long
• 2:14:29
  Speaker 1: winding river of dim stars named after a mythological waterway.
• 2:14:34
  Speaker 1: It is not a dramatic constellation. No bright anchor star
• 2:14:38
  Speaker 1: draws the eye the way Sirius does. No distinctive shape
• 2:14:42
  Speaker 1: snaps into focus the way Orion does. Eridanus is faint, sprawling,
• 2:14:47
  Speaker 1: easy to overlook. Most people who have spent years watching
• 2:14:51
  Speaker 1: the southern sky have passed over it dozens of times
• 2:14:54
  Speaker 1: without stopping. But there is a specific patch within it,
• 2:14:58
  Speaker 1: unmarked and invisible. that has occupied some of the sharpest
• 2:15:02
  Speaker 1: minds in cosmology for twenty years. You cannot see it.
• 2:15:07
  Speaker 1: There is nothing there to see. That is precisely the point.
• 2:15:12
  Speaker 1: If you were hovering in a spacecraft somewhere inside the
• 2:15:15
  Speaker 1: Eridana supervoid, you would not know it. The void is
• 2:15:19
  Speaker 1: not dark in the way a shadow is dark. There
• 2:15:22
  Speaker 1: are galaxies inside it. Not many, perhaps thirty percent fewer
• 2:15:27
  Speaker 1: than you would find in a comparable volume elsewhere. But
• 2:15:31
  Speaker 1: they are there, burning the same way galaxies always burn,
• 2:15:35
  Speaker 1: producing light indistinguishable from any other galaxy in any other
• 2:15:40
  Speaker 1: corner of the universe. The void does not announce itself.
• 2:15:44
  Speaker 1: It is defined by what is absent, not by what
• 2:15:48
  Speaker 1: is present. Matter simply thinned out here billions of years ago,
• 2:15:53
  Speaker 1: drawn away by the gravitational pull of denser surrounding regions,
• 2:15:57
  Speaker 1: and the thinning never stopped. What remains is a region
• 2:16:02
  Speaker 1: of space that is almost ordinary, except in aggregate, and
• 2:16:06
  Speaker 1: at a scale no human eye could ever take in
• 2:16:09
  Speaker 1: at once. The coldness you are dealing with is not
• 2:16:12
  Speaker 1: a temperature you could feel. It is not the cold
• 2:16:15
  Speaker 1: of deep space, where gas clouds drift at a few
• 2:16:19
  Speaker 1: degrees above absolute zero. It is something stranger. The coldness
• 2:16:24
  Speaker 1: exists in light. Specifically, in the photons of the cosmic
• 2:16:29
  Speaker 1: microwave background that passed through this supervoid on their way
• 2:16:33
  Speaker 1: to our detectors. Each photon climbed the gentle gravitational slope
• 2:16:37
  Speaker 1: of the void, lost a small amount of energy doing it,
• 2:16:41
  Speaker 1: and then came down the other side into a universe
• 2:16:44
  Speaker 1: that had expanded just enough during that crossing that the
• 2:16:47
  Speaker 1: energy could not be fully recovered. 70 microkelvin below the
• 2:16:51
  Speaker 1: surrounding average. That is what the cold spot is. Not
• 2:16:55
  Speaker 1: a cold place, but cold light. Light that arrived slightly depleted,
• 2:17:01
  Speaker 1: carrying the faint record of a billion light-year crossing through
• 2:17:04
  Speaker 1: near emptiness. What does it mean to look in that
• 2:17:07
  Speaker 1: direction and know this? It changes something. The patch of
• 2:17:11
  Speaker 1: sky you are looking at is not special in any
• 2:17:14
  Speaker 1: way your eyes can register. The stars between here and
• 2:17:18
  Speaker 1: Eridanus are ordinary stars at ordinary distances, completely unrelated to
• 2:17:23
  Speaker 1: the supervoid behind them. The supervoid itself sits 1.9 billion
• 2:17:28
  Speaker 1: light years away. Nothing about it is visible.
• 2:17:32
  Speaker 2: And yet the.
• 2:17:33
  Speaker 1: Radiation coming from that direction has passed through the largest
• 2:17:36
  Speaker 1: identified void in the observable universe and arrived here carrying
• 2:17:41
  Speaker 1: a temperature signature that two decades of cosmology have not
• 2:17:45
  Speaker 1: been able to fully explain.
• 2:17:47
  Speaker 2: That is the situation.
• 2:17:49
  Speaker 1: You will look at an explanation that is still missing
• 2:17:52
  Speaker 1: most of its pieces. Here is what the science actually says,
• 2:17:56
  Speaker 1: stated plainly.
• 2:17:58
  Speaker 2: The supervoid is real.
• 2:18:00
  Speaker 1: Multiple independent surveys confirmed it. The cold spot is real.
• 2:18:04
  Speaker 1: Multiple satellites measured it. The supervoid sits in the right
• 2:18:09
  Speaker 1: location to influence the cold spot through the integrated Saks
• 2:18:12
  Speaker 1: Wolf effect. And that effect is real and physically well understood.
• 2:18:17
  Speaker 1: But the supervoid can only account for somewhere between 10
• 2:18:21
  Speaker 1: and 20% of the observed temperature depression. The rest, the
• 2:18:26
  Speaker 1: majority of what makes the cold spot anomalous, has no
• 2:18:29
  Speaker 1: accepted explanation within the standard cosmological model. That gap has
• 2:18:35
  Speaker 1: not closed. Better measurements have, if anything, made it look
• 2:18:40
  Speaker 1: more stubborn. Whatever is producing the remainder of the cold
• 2:18:44
  Speaker 1: spot is either something within our current theoretical framework that
• 2:18:47
  Speaker 1: we have not accounted for correctly, or something outside it.
• 2:18:52
  Speaker 1: Nobody knows which. Were we watching a patch of sky
• 2:18:56
  Speaker 1: that carries evidence of a collision with another universe?
• 2:18:59
  Speaker 2: Probably not. Is it a statistical accident?
• 2:19:03
  Speaker 1: An outlier that the standard model technically permits at low probability.
• 2:19:08
  Speaker 2: Possibly.
• 2:19:09
  Speaker 1: Is it evidence of physics that the Lambda CDM model
• 2:19:12
  Speaker 1: does not contain?
• 2:19:14
  Speaker 2: Maybe.
• 2:19:15
  Speaker 1: The discomfort of this situation is that all three remain
• 2:19:19
  Speaker 1: on the table, and the observations so far have not
• 2:19:22
  Speaker 1: been precise enough to eliminate any of them with confidence.
• 2:19:26
  Speaker 2: The instruments being.
• 2:19:27
  Speaker 1: Built right now, the surveys coming online over the next decade,
• 2:19:32
  Speaker 1: will map the matter distribution in that region at a
• 2:19:35
  Speaker 1: resolution we have never had before. That data will close
• 2:19:39
  Speaker 1: some of the possibilities. It may not close all of them.
• 2:19:43
  Speaker 1: Find Eridanus on a clear southern night and hold that
• 2:19:47
  Speaker 1: in your mind. The constellation itself is just a convenient label,
• 2:19:52
  Speaker 1: a name we put on a region of sky so
• 2:19:54
  Speaker 1: we can talk about it. The River of Stars is
• 2:19:57
  Speaker 1: not connected to the supervoid behind it in any physical way.
• 2:20:01
  Speaker 2: But the direction is the direction.
• 2:20:03
  Speaker 1: Somewhere along that line of sight, 1.9 billion light years out,
• 2:20:09
  Speaker 1: space grew thin. Matter drained away. The cosmic web stretched
• 2:20:13
  Speaker 1: to almost nothing in a region larger than most of
• 2:20:16
  Speaker 1: the mind can honestly picture. And the ancient light of
• 2:20:20
  Speaker 1: the early universe passed through it and came out the
• 2:20:22
  Speaker 1: other side, slightly depleted, arriving at our satellites carrying a
• 2:20:27
  Speaker 1: temperature anomaly that we still cannot explain. 20 years of
• 2:20:31
  Speaker 1: work on a single patch of sky, and the answer
• 2:20:34
  Speaker 1: is still not there. The coldest spot in the observable
• 2:20:38
  Speaker 1: universe looks like nothing at all from down here, which
• 2:20:41
  Speaker 1: is perhaps the strangest thing about it.