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The True Reality Of How Big The Universe Is · Boring Science

• 0:00
  Speaker 1: The universe is ninety three billion light years across, and
• 0:04
  that sentence means almost nothing to the part of your
• 0:07
  brain that actually processes scale. A light year is nearly
• 0:12
  six trillion miles. Multiply that by ninety three billion. The
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  number you get is so large it stops functioning as
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  a number and becomes something closer to a sound meaningless,
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  a placeholder for a thing the human mind was never
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  built to hold. This is the problem with the universe.
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  The math is not in dispute. The measurements exist, They
• 0:39
  have been checked, They have been confirmed by independent teams
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  using different instruments on different continents. What is missing is
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  not data. What is missing is any honest way to
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  feel what the data describes. We evolved to track distances
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  measured in footsteps, in days of walking in the width
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  of a river we needed to cross. Nothing in that
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  history prepared us for a number with twenty four zeros
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  behind it. Nothing prepared us for light that has been
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  traveling since before our planet existed, still crossing empty space,
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  still arriving. By the end of this you will not
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  just know how large the universe is. You will feel it.
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  That is a different thing entirely, and it is harder.
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  We begin at the only place any of this can begin,
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  which is here on the ground with you. If you
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  find comfort in these gentle explorations of space and science,
• 1:47
  I'd be grateful if you'd consider subscribing and giving this
• 1:51
  video alike. Start with the ground under your feet, not
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  as a metaphor, literally, the ground is solid, measurable, something
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  your body understands without any calculation at all. This is
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  where the exercise has to begin, because the only honest
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  way to build toward numbers the human mind cannot hold
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  is to start with numbers it can. Earth has a
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  circumference of twenty four thousand, nine hundred one miles at
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  the equator. A commercial jet flying at cruising speed pointed
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  along that line without stopping would need roughly forty three
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  hours to complete the loop. That is nearly two full
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  days of continuous flight just to circle the surface. Once
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  the planet is large enough that you could travel your
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  entire life and never see most of it, deserts, the
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  size of continents, ocean floors that have never been directly
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  observed by a human eye, mountain ranges that stretch for
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  thousands of miles across the interior of land, masses most
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  people will never visit. The deepest point on Earth is
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  Challenger Deep, at the bottom of the Mariana Trench in
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  the Western Pacific. It sits about seven miles below the
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  surface of the ocean. Seven miles sound significant until you
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  place it against the twenty four thousand, nine hundred one
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  miles around the planet. At that scale. The deepest wound
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  the ocean has cut into the Earth is barely a scratch.
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  Mount Everest rises five point five miles above sea level.
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  From space, it registers as an almost invisible roughness on
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  a sphere that looks from any real distance perfectly smooth.
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  The tallest mountain on Earth is a rounding error on
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  a planet that all already becomes a rounding error the
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  moment you back up far enough. The total surface area
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  of Earth is roughly one hundred ninety seven million square miles.
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  About seventy one percent of that is water. The land
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  masses that feel so enormous when you are crossing them
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  by train or by road are together smaller than the
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  ocean basins that surround them. Every country, every border, every
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  road that any vehicle has ever traveled is packed into
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  the remaining twenty nine percent and yet the whole thing,
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  all one hundred ninety seven million square miles of it,
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  is a single object orbiting an ordinary star in an
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  ordinary galaxy. That is the setup. Here is where it breaks.
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  In nineteen ninety NASA turned the Voyager one spacecraft around.
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  At that point, it was a approximately three point seven
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  billion miles from Earth, well beyond the outer planets, moving
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  outward toward open space. The camera took a photograph. In
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  that image, Earth appears as a point of light so
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  small it barely registers. Carl Sagan, who had pushed for
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  the photograph to be taken, described it as a pale
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  blue dot, a mote of dust suspended in a sunbeam.
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  The description is precise. Earth does not appear as a sphere.
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  It does not appear as a land mass or a
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  body of water. It appears as a fraction of a pixel,
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  a faint smear of reflected sunlight against the black of space,
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  nearly lost against the scattered light artifacts of the camera itself.
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  Everything discussed in the previous paragraphs, the twenty four thousand,
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  nine hundred one mile circumference, the seven mile ocean trench,
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  the five point five mile mountain, the one hundred ninety
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  seven million square miles of surface is contained in that
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  fraction of a pixel. Every ocean, every desert, every person
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  who has ever lived, every city they built, every road
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  they drove, every war they fought over the land beneath
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  their feet, All of it is inside a point of
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  light smaller than the width of a camera artifact. This
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  is the method. The rest of this will follow. Each
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  step outward will make the previous reference point shrink into
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  something negligible. Earth is large by every measure available to
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  a human being walking around on it. From three point
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  seven billion miles, it disappears, and three point seven billion
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  miles is nothing. It is barely past the doorstep of ave,
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  our own solar system. The distances that come next will
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  make three point seven billion miles look the same way
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  everest looks against the circumference of the planet, technically real,
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  completely invisible at scale. The purpose of starting here with
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  the ground and the circumference and the mountain and the
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  ocean trench is to give this process a foothold, something
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  concrete to compress, because the compression is about to become extreme,
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  and it helps to remember what you started with before
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  it all begins to disappear. From three point seven billion
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  miles away. The entire planet, every mountain, every ocean, every city,
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  and everyone who ever lived in one fits inside a
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  single pixel of a low resolution photograph. The Moon is
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  the only other world a human being has ever stood on,
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  and it is so close by cosmic standards that astronomers
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  barely consider it a distance at all. The average distance
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  from Earth to the Moon is two hundred thirty nine
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  thousand miles. That number does not sound small, but to
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  understand how scale works in this video, you need to
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  feel it from both directions at once. From the surface
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  of the Earth, two hundred thirty nine thousand miles is
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  a genuinely staggering distance. All seven continents could be arranged
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  side by side in that gap and still leave room.
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  The Moon sits so far out that light traveling at
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  one hundred eighty six thousand miles per second, needs one
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  point three seconds to cover the distance. When Apollo astronaut
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  spoke with mission control, there was a two point six
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  second round trip delay. On every transmission, a question and
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  its answer were separated by silence. That was not technical failure,
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  it was distance. The Apollo missions took three days to
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  make that crossing three days of drifting through empty space
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  with nothing outside the windows and the Earth shrinking behind them.
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  And yet even that journey, which required the most complex
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  machine humanity had ever built, and remains one of the
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  most dangerous things any human being has ever attempted, covers
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  a distance that you could theoretically drive. If you pointed
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  a car directly at the Moon and drove at highway
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  speed sixty five miles per hour without stopping for fuel
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  or sleep or any reason at all, you would arrive
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  in roughly five to six months, not years. Months. Parents
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  drive farther than that in a lifetime of commutes. The
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  Moon by car is six months away. That is close
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  enough that a human life span, if it were a
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  straight road, would reach it several times over. Now hold
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  that sense of distance in mind, because here is what
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  happens when you step back even slightly. Spacecraft sent to
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  Mars have occasionally turned their cameras back toward Earth during
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  the journey. In those photographs, Earth and the Moon appear
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  together in the frame, two bright dots hanging in darkness,
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  with a small gap between them. That gap two hundred
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  thirty nine thousand miles wide, the distance that took the
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  Apollo astronauts three days at roughly twenty five thousand miles
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  per hour to cross barely registers. The two worlds look
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  like a close pair, almost touching almost one thing. From
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  a distance of tens of millions of miles, Earth and
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  the Moon are a single object. This is the first compression,
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  and it is important to stay with it for a
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  moment before moving on, because this is the pattern that
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  will repeat every time the scale increases. A distance that
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  felt enormous at one level of zoom disappears entirely at
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  the next. The Moon felt far from Earth. From Mars,
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  it does not feel far at all. What happens to
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  your sense of distance when this process continues when you
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  step back not to Mars, but to the outer edge
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  of the Solar System, then out to the nearest star,
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  then to the edge of the galaxy, then farther still
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  is the subject of what is coming. Each step makes
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  the previous one collapse. The Apollo program ended in nineteen
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  seventy two. Since then, no human being has traveled farther
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  from Earth than roughly four hundred miles the altitude of
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  low orbit. The Moon, the only other world we have
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  ever touched, has sat there at two hundred thirty nine
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  thousand miles unvisited for more than fifty years. In the
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  context of the distances this video is building toward, two
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  hundred thirty nine thousand miles is nothing. It is less
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  than nothing. It is the kind of distance that will
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  eventually need to be described as essentially zero to keep
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  the comparisons from breaking down entirely. But right now, at
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  this level of zoom, with Earth still filling most of
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  the frame, the Moon represents the outer edge of everywhere
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  a human has ever physically been, And from Mars, the
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  Earth and Moon look like a single point of light,
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  and the two hundred thirty nine thousand miles between them
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  vanishes entirely. The Sun is eighty three million miles away,
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  and that distance alone is already beyond anything a human
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  being can intuitively feel. The Moon sits two hundred thirty
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  nine thousand miles out, and we establish that driving there
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  at highway speed takes around six months. The Sun is
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  roughly three hundred ninety times farther than that stack. Those
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  six month drives end to end, and you are looking
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  at something close to two centuries of continuous travel just
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  to reach the thing that lights up your morning. The
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  number ninety three million does not register the way two
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  hundred thirty nine thousand does not register. At some point
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  the mind stops converting distance into something felt and simply
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  accepts the figure as large. Here is the version that
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  lands a little harder. Light moves at one hundred eighty
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  six thousand miles per second. At that speed, it crosses
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  the distance from Earth to the Moon in one point
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  three seconds. That same light, moving at that same incomprehensible speed,
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  takes eight minutes and twenty seconds to cross from the
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  surface of the Sun to your eyes. If the Sun
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  were somehow switched off right now, you would not know.
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  You would sit in sunlight for another eight minutes and
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  twenty seconds, watching a star that had already gone dark,
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  with no way of knowing that anything had changed. The
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  delay is built into the physics. There is no getting
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  around it. The Sun itself is a different kind of problem.
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  It accounts for ninety nine percent of the total mass
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  throughout our solar system, which already tells you something about
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  the arrangement of things out here. Its diameter is about
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  eight hundred sixty five thousand miles. If you lined up
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  Earth side by side across its face. You would need
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  one hundred nine of them to reach from one edge
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  to the other. That is, not one hundred nine stacked
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  on top of each other, but one hundred nine placed
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  flat edge to edge, covering its width. To fill the interior.
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  The number climbs to roughly one million earths. The volume
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  involved is almost impossible to picture. A commercial passenger jet
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  flying NonStop would need about six months just to complete
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  a single loop around the Sun's equator. The same jet
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  circling Earth takes roughly forty three hours. The Sun requires
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  about one thousand, three hundred times longer. Despite this, the
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  Sun is not a particularly large star among the stars
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  in the Milky Way. It sits comfortably in the middle,
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  large enough to sustain a solar system, not large enough
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  to qualify as anything astronomers would call exceptional. Red giants
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  in the galaxy dwarf it by factors of hundreds. Hypergiants
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  dwarf it by factors of thousands. The Sun is the
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  brightest object in our sky purely because it is close.
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  From a distance of even a few light years, it
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  fades to an unremarkable point of light, indistinguishable from thousands
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  of other stars visible on a clear night. The planet's
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  orbit within this enormous gravitational territory. Mercury, the innermost planet,
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  swings as close as twenty nine million miles to the
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  Sun and as far as forty three million miles because
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  its orbit is more elliptical than most Venus circles, at
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  around sixty seven million miles. Earth, as established, holds at
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  ninety three million. Each of these numbers feels substantial until
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  you push outward. Mars orbits at about one hundred forty
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  two million miles. Jupiter sits at roughly four hundred eighty
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  four million miles. Saturn is more than eight hundred eighty
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  six million miles out. By the time you reach Neptune,
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  the outermost major planet, the distance is nearly two point
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  eight billion miles. These are the figures for planets still
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  firmly within our Solar System. Objects we have sent spacecraft
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  to visit worlds we have photographed in detail. They are
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  measured in billions. The nearest star is measured in trillions.
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  Proxima centauri. The closest star to our Sun is about
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  twenty five trillion miles away. The span between Earth than
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  the Sun, which already required two centuries of highway driving
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  to cover fits into the distance to Proxima Centauri roughly
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  two hundred seventy thousand times. The outer planets, billions of
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  miles out, barely register against that scale. The Sun dominates
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  everything within its gravitational reach, but its reach measured against
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  the distances between stars is a thin shell around a
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  tiny point. There are somewhere around three hundred billion stars
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  in the Milky Way. Most of them have never been
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  individually observed, named, or catalogued. They contribute their light to
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  the pale band crossing the night sky, each one lost
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  in the aggregate. Each one a Sun in its own right,
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  with its own gravitational domain, and in most cases, its
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  own planets. Ninety three million miles between Earth and the Sun,
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  which required most of this section to make feel even
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  slightly real, is the kind of distance that disappears entirely
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  when you zoom out to galactic scales. It becomes a
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  rounding error in the arithmetic of the galaxy, a small
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  step in a measurement that eventually stops fitting in any
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  unit a person can hold in their head. It is
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  one star out of roughly three hundred billion in the
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  Milky Way alone. And by the standards of what comes next,
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  the separation between Earth and the Sun is a rounding error.
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  Most people, if asked to draw the Solar system, would
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  draw something that looks vaguely like a classroom poster, with
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  all eight planets bunched reasonably close together in neat circles
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  around the Sun. That picture is wrong, not slightly wrong,
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  wrong in a way there that makes the actual Solar
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  system almost unrecognizable by comparison. The real distances between planets
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  are so large that if you drew the Solar system
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  to scale on a piece of paper, with the Sun
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  the size of a grapefruit, the paper would need to
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  stretch for hundreds of miles before you reach the outer edge.
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  There is a scale model that astronomers and educators use
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  to make this concrete. Shrink the Sun down to the
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  size of a grapefruit, and place it somewhere in New
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  York City at that scale. Mercury is a grain of
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  sand about three paces away. Venus is another grain of sand,
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  roughly nineteen paces out. Earth is a third grain of
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  sand twenty six paces from the grapefruit. Mars slightly smaller
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  sits about forty paces out. These are the inner planets,
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  they feel relatively cozy. In the model, you could walk
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  between them in seconds. The problem begins when you cross
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  the asteroid belt and head toward the outer Solar System. Jupiter,
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  the largest planet, is a marble in this model, a
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  marble one hundred thirty five paces from the grapefruit Saturn,
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  another marble, but slightly smaller, is two hundred and fifty
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  paces out. Uranus is barely visible, a small ball bearing
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  at roughly five hundred paces. Neptune, the outermost planet, is
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  a grain of sand, again more than a quarter mile
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  from the grapefruit Sun, sitting in the distance, so small
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  and far that you would have trouble finding it without
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  knowing exactly where to look. In the real Solar System,
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  Neptune is about two point eight billion miles from the Sun.
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  That number, on its own is nearly impossible to feel.
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  In the model, you can at least walk the distance
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  and understand that the planets are not neighbours. They are
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  separated by vast, empty stretches of nothing. But Neptune is
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  not the edge. Not even close Beyond Neptune lies the
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  Kuiper Belt, a ring of icy bodies and dwarf planets
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  extending from roughly thirty to fifty astronomical units from the Sun.
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  Pluto lives here, so does Eris, make, Make and Haumeya.
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  In the Grapefruit model, the outer edge of the Kuiper
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  Belt is somewhere around a third of a mile from
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  the Grapefruit, still within walking distance, but the model is
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  already starting to strain against ordinary human scale. Then there
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  is the aort Cloud. This is where the Solar System's
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  true size becomes something that language struggles to carry. The
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  Oort Cloud is is not a disc like the Kuiper Belt.
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  It is a spherical shell surrounding the entire Solar System,
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  a vast reservoir of icy objects, the source of the
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  long period comets that occasionally fall inward toward the Sun
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  after being gravitationally nudged loose from their distant orbits. The
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  inner edge of the Aort cloud begins at roughly two
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  thousand astronomical units from the Sun. The outer edge extends
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  somewhere between fifty thousand and one hundred thousand astronomical units out.
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  One astronomical unit is the distance from the Earth to
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  the Sun ninety three million miles. Multiply that by one
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  hundred thousand, and you begin to see the problem In
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  the grapefruit model. The outer Aught Cloud sits roughly halfway
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  between New York and Los Angeles. The grapefruit is on
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  the sidewalk in Manhattan. The edge of our Solar system
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  is somewhere in Kansas. Everything inside that boundary is gravitationally
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  bound to our Sun. Comets that formed in the early
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  Solar System have been orbiting out there in the cold
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  and the dark for billions of years. So far from
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  the Sun, that sunlight at that distance is barely distinguishable
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  from the light of any other star. The Sun from
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  the outer Aught Cloud is just another point of light
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  in a sky full of them. Now consider what it
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  would take to actually cross this distance. Voyager one is
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  the fastest human made object currently leaving the Solar System.
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  It has been traveling since nineteen seventy seven. As of now,
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  it is roughly fifteen billion miles from Earth, which sounds extraordinary.
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  It is No human made object has ever been farther
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  from home yet. Fifteen billion miles is about one hundred
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  sixty astronomical units. The inner edge of the aort Cloud
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  starts at two thousand astronomical units. The outer edge is
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  at one hundred thousand astronomical units. At Voyager's current speed,
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  reaching the inner Aught Cloud would take roughly three hundred years.
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  Reaching the outer edge would take something in the range
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  of thirty thousand years. Human civilization as we recognize it,
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  cities and writing and agriculture, is only about ten thousand
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  years old. We would need to run that entire span
• 25:44
  of history three times over just to reach the gravitational
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  boundary of our own Solar System. We have been launching
• 25:52
  spacecraft for about sixty years. In that time, we have
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  sent probes to every planet, landed on Mars, and pushed
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  one small machine to fifteen billion miles from Earth. None
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  of those spacecraft have left the Solar System in any
• 26:10
  meaningful sense. They are still well inside the gravitational reach
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  of the Sun. The nearest star, Proxima Centaury, is two
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  hundred sixty eight thousand astronomical units away. The outer Aught Cloud,
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  already impossibly distant by any human measure, is only a
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  third of the way there. Voyager, one, our fastest departing spacecraft,
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  launched in nineteen seventy seven, would need roughly thirty thousand
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  years to reach the outer edge. Of the Solar system's
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  gravitational territory, and it has not yet gone far enough
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  to reach the nearest star, Proxima Centauri. The closest star
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  to our Sun is four point two four light years away.
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  One light year is nearly six trillion miles. Multiply that
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  by four point two four and the result is roughly
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  twenty five trillion miles. That is not the distance across
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  the galaxy. That is not the distance to some remote
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  region of space, that is the nearest neighbor. The closest
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  thing to our Sun in the entire universe is twenty
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  five trillion miles away. And that number should stop you cold,
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  because we just spent considerable time establishing that our Solar
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  system is already enormous, and it barely registers against this.
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  Go back to the grapefruit model from the previous section.
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  If the Sun is a grapefruit sitting in New York City,
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  Proxima Centauri is in Los Angeles, nearly two thousand, eight
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  hundred miles of open country between them. That is the
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  skale of interstellar space, even at its smallest. Two neighboring
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  stars closer to each other than any other pair in
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  our region of the galaxy, separated by the equivalent of
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  crossing the continental United States, and the stars in between,
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  there are none just vacuum. Voyager one has been traveling
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  since nineteen seventy seven. It is the fastest human made
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  object currently leaving the Solar System, and right now it
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  sits roughly fifteen billion miles from Earth. That sounds like
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  a lot. In twenty five trillion miles, it is almost nothing.
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  If Voyager one were pointed directly at proxima centaury, which
• 28:46
  it is not, it would need approximately seventy three thousand
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  years to arrive, not seventy three thousand days, not seventy
• 28:56
  three thousand months years. Agriculture is about ten thousand years old.
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  Written language is roughly five thousand years old. Voyager would
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  need more than a dozen of those entire spans of
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  recorded civilization just to cover the distance to the nearest star.
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  There have been serious proposals about how to actually reach
• 29:21
  proxima centaury. The most discussed concept involves an array of
• 29:26
  powerful ground based lasers accelerating a tiny spacecraft to around
• 29:32
  twenty percent the speed of light. At that speed, the
• 29:37
  trip would take roughly twenty years. The technology does not exist.
• 29:43
  The engineering required is so far beyond anything currently buildable
• 29:48
  that most scientists treat it as a multi century project.
• 29:52
  At minimum, even at ten percent the speed of light,
• 29:56
  which is itself far beyond anything in current devel the
• 30:01
  journey would take about forty two years. That is a
• 30:04
  human lifetime dedicated entirely to crossing the gap to the
• 30:08
  first star outside our own system here is something worth
• 30:13
  holding onto. Proxima Centauri is not alone. It belongs to
• 30:18
  a three star system called Alpha Centauri. Two of those stars,
• 30:24
  alpha Centauri A and Alpha Centauri B, former binary pair.
• 30:30
  They orbit each other at a distance comparable to the
• 30:33
  distance between the Sun and Uranus, close enough that both
• 30:37
  would appear as bright points in the sky from any
• 30:40
  planet orbiting either one. Proxima Centauri is the third member
• 30:47
  of the system, a dim red dwarf orbiting the binary
• 30:51
  pair at an enormous distance, taking somewhere around five hundred
• 30:56
  fifty thousand years to complete a single orbit around them.
• 31:01
  It earns the name Proxima the Nearest because of where
• 31:05
  it happens to sit in its long orbit around its companions.
• 31:10
  That red dwarf has a planet designated Proxima Centaury b
• 31:16
  it orbits within the habitable zone, the range of distances
• 31:20
  where liquid water could potentially exist on a surface. Detected
• 31:26
  in twenty sixteen. It completes an orbit every eleven days,
• 31:31
  which means it hugs its star closely. Proxima Centauri is
• 31:36
  a flaring star prone to bursts of radiation that could
• 31:40
  strip away an atmosphere. Whether Proxima Centauri B could actually
• 31:46
  support life remains deeply uncertain. What is not uncertain is
• 31:53
  that we have no way of reaching it. No technology
• 31:56
  under development, no concept, even in the speculative planning stages,
• 32:02
  would deliver any human being or any meaningful instrument to
• 32:07
  that planet in less than thousands of years. It is
• 32:11
  the closest planet outside our Solar System we have confirmed
• 32:15
  in the habitable zone, and it is effectively unreachable by
• 32:20
  every measure that matters. This is the shape of interstellar distance.
• 32:26
  The nearest star in the best case scenario using concepts
• 32:31
  that do not yet exist as functional technology, takes twenty
• 32:35
  to forty two years to reach by a spacecraft no
• 32:39
  larger than a postage stamp. A crude mission is a
• 32:43
  problem belonging to a civilization far more advanced than ours,
• 32:48
  and Proxima Centauri is the closest one. The galaxy contains
• 32:53
  between one hundred and four hundred billion stars. The vast
• 32:57
  majority of them are separated from their neighbours by distances
• 33:02
  comparable two or larger than the twenty five trillion miles
• 33:06
  between our Sun and Proxima Centauri. Interstellar space is not
• 33:12
  a sea you cross. It is a silence measured in
• 33:16
  trillions of miles repeated one hundred billion times across a
• 33:21
  single galaxy. Proxima Centauri is the nearest star, and everything
• 33:27
  from here outward is going to be measured in units
• 33:31
  that make the distance to Proxima Centauri look like the
• 33:35
  distance from your front door to your mailbox. The Milky
• 33:40
  Way is one hundred thousand light years across, which means
• 33:44
  light moving at one hundred eighty six thousand miles per
• 33:48
  second takes one hundred thousand years to travel from one
• 33:53
  edge to the other. Sit with that for a moment.
• 33:57
  The light leaving one edge of our galaxy right now
• 34:00
  will not reach the other side until long after the
• 34:03
  Sun has expanded and swallowed the Earth. When that light
• 34:08
  first began its journey, our species did not exist. Homo
• 34:14
  sapiens had not yet appeared on this planet. The journey
• 34:18
  across our own galaxy is not a long trip. It
• 34:22
  is longer than the entire history of human existence, multiplied
• 34:27
  by orders of magnitude that make the comparison almost pointless.
• 34:33
  Voyager one is the fastest object humanity has ever launched
• 34:37
  into deep space. It has been traveling since nineteen seventy seven,
• 34:43
  and it is now roughly fifteen billion miles from Earth.
• 34:48
  That sounds like a significant distance. To cross the Milky
• 34:53
  Way at that speed would take approximately one point seven
• 34:57
  billion years, not one point bio seven million billion. The
• 35:02
  galaxy is not a place any spacecraft we have ever
• 35:06
  built could meaningfully cross. The Solar System, which already felt
• 35:12
  impossibly large, is a speck of dust inside something that
• 35:16
  cannot be traversed in any time frame that connects to
• 35:20
  human experience. The galaxy contains somewhere between one hundred and
• 35:27
  four hundred billion stars. The range in that estimate is
• 35:32
  not imprecision borne from carelessness. Vast regions of the Milky
• 35:37
  Way are hidden behind dense clouds of gas and dust
• 35:42
  that block visible light entirely. We are trying to count
• 35:46
  the contents of a room while blindfolded in one corner
• 35:50
  of it. Radio telescopes and infrared instruments can peer through
• 35:56
  some of that obscuring material, but significant portions of the
• 36:00
  galactic interior remain effectively invisible to us. The number of
• 36:05
  stars in our own galaxy is something we genuinely do
• 36:09
  not know, and the uncertainty spans hundreds of billions. Our
• 36:15
  solar system is not at the center of all this.
• 36:18
  It sits roughly twenty six thousand light years from the
• 36:22
  galactic core, about two thirds of the way out from
• 36:26
  the center, toward the outer edge, on a structure called
• 36:30
  the Orion Arm. This is one of several spiral arms
• 36:34
  curving outward from the galaxy's dense central region. The arm
• 36:38
  itself is a region of somewhat higher concentration of stars
• 36:43
  and gas, but the boundaries are not sharp. Stars do
• 36:48
  not stop at the edge of a spiral arm, like
• 36:50
  houses at the end of a street. The density simply
• 36:54
  thins and then rises again farther out. The shape of
• 36:57
  the Milky Way is a flat disc roughly one thousand
• 37:01
  light years thick at the outer edges, at the centre,
• 37:05
  where stars are packed far more densely, and where old
• 37:08
  stellar populations dominate, the disc swells into a bulge about
• 37:13
  ten thousand light years thick. This central bulge is crowded
• 37:18
  with ancient stars, some nearly as old as the universe itself,
• 37:24
  orbiting in slower, more tangled paths than the orderly, circular
• 37:28
  orbits more typical of stars in the outer disc. The
• 37:33
  spiral arms extend outward from this bulge, like grooves in
• 37:37
  a slow turning pin wheel. Though the rotation of the
• 37:40
  galaxy is not uniform, stars at different distances from the
• 37:45
  centre orbit at different speeds and on different time scales.
• 37:51
  Our own solar system completes one full orbit around the
• 37:55
  galactic centre roughly every two hundred twenty five million year.
• 38:00
  Us Here is the thing about understanding the shape of
• 38:03
  the galaxy we live in. We cannot see it, not
• 38:07
  from outside, not from above. We are embedded inside the disk,
• 38:14
  roughly halfway between the center and the rim, and from
• 38:17
  that position the galaxy surrounds us in every direction. Looking
• 38:23
  toward the center of the Milky Way, you are looking
• 38:26
  through twenty six thousand light years of stars, gas, and dust.
• 38:33
  Looking in the opposite direction, you are looking out toward
• 38:36
  the more sparsely populated outer disk. Looking straight up or
• 38:42
  down perpendicular to the plane of the galaxy, you escape
• 38:46
  its crowded middle and see clear sky. What we experience
• 38:50
  from Earth is the combined glow of billions of stars
• 38:54
  arranged along the plane of that disc. Seen edge on
• 38:59
  on a dark night far from city lights, this appears
• 39:03
  as a faint, irregular band of light crossing the sky
• 39:07
  from horizon to horizon. That band is not a feature
• 39:12
  at the edge of the galaxy. It is the galaxy
• 39:15
  itself seen from inside the flat disc, appearing as a
• 39:20
  ribbon because we are looking along its length rather than
• 39:24
  across it. It is the same reason standing in a
• 39:28
  forest makes the trees look like a wall rather than
• 39:32
  a circle. The shape of the thing you are inside
• 39:36
  is not visible to you from that position. Astronomers have
• 39:40
  reconstructed the galaxy structure using radio telescopes, mapping the positions
• 39:45
  of hydrogen gas clouds by their characteristic emission, tracing the
• 39:50
  spiral arms, by the distribution of young stars and star
• 39:54
  forming regions. The picture that has emerged is detailed and
• 39:58
  largely consistent across different methods, but it is still a reconstruction,
• 40:04
  an inference built from data collected by instruments that cannot
• 40:09
  leave the galaxy to verify the view from outside. Every
• 40:14
  picture you have ever seen labeled as the Milky Way
• 40:18
  is actually a photograph of a completely different galaxy, one
• 40:23
  we use as a stand in because we cannot get
• 40:26
  far enough above our own to photograph it from the outside.
• 40:31
  At the center of the Milky Way, twenty six thousand
• 40:35
  light years from Earth, there is a black hole four
• 40:39
  million times the mass of our sun. It is called
• 40:44
  Sagittarius a Star, and for most of human history, nobody
• 40:50
  knew it was there. The galactic center is hidden behind
• 40:53
  so many layers of gas and dust that visible light
• 40:57
  from that region never reaches us at all. We live
• 41:01
  inside the galaxy twenty six thousand light years out from
• 41:05
  the core, and for centuries the heart of our own
• 41:09
  home was completely invisible. That changed as telescopes learned to
• 41:14
  see in wavelengths beyond visible light. Radio telescopes, infrared cameras,
• 41:22
  and X ray observatories began punching through the dust and
• 41:26
  returning images of the galactic center. What they found was
• 41:31
  not a quiet place. The region surrounding Sagittarius a Star
• 41:37
  is one of the densest most violent environments in the
• 41:41
  entire galaxy, Stars crowded together in concentrations that would be
• 41:47
  unrecognizable from the relatively sparse neighbourhood where our solar system
• 41:52
  sits gas clouds move at speeds that would be catastrophic
• 41:56
  by any ordinary standard. And at the center of all
• 41:59
  of it sits this black hole, four million solar masses
• 42:04
  compressed into a region smaller than our solar system. In
• 42:08
  twenty twenty two, the Event Horizon Telescope collaboration released something
• 42:14
  that would have seemed impossible a generation earlier, an actual
• 42:19
  image of Sagittarius a star, not an illustration, not an
• 42:24
  artist rendering, a photograph assembled from radio telescope data collected
• 42:30
  by observatories spread across the entire planet, all working together
• 42:35
  as a single Earth sized instrument. The image showed a
• 42:39
  glowing ring of superheated gas surrounding a dark central region,
• 42:44
  the shadow cast by the black hole itself. It was
• 42:48
  blurry by ordinary photographic standards. It was also one of
• 42:53
  the most extraordinary things humanity has ever seen, a picture
• 42:57
  of an object twenty six thousand and light years away,
• 43:01
  an object so massive and so compact that not even
• 43:05
  light escapes it. The stars orbiting Sagittarius A star are
• 43:11
  among the fastest moving objects ever tracked by astronomers. A
• 43:16
  group of stars near the galactic centre, designated the S stars,
• 43:21
  have been observed completing full orbits around the black hole.
• 43:26
  Some of those orbits take only a few decades. The
• 43:29
  star designated S two, completes a full orbit in roughly
• 43:34
  sixteen years. At its closest approach, it passes within about
• 43:39
  seventy five astronomical units of Sagittarius, A star, which is
• 43:44
  not far by galactic standards. Seventy five astronomical units is
• 43:49
  roughly twice the distance from the Sun to Neptune. At
• 43:53
  that point, S two is moving at approximately five thousand
• 43:57
  miles per second text Earth orbits the Sun at about
• 44:03
  eighteen miles per second. S two, at its closest approach
• 44:08
  to the black hole is moving roughly two hundred eighty
• 44:12
  times faster than that. These are not theoretical numbers. Astronomers
• 44:18
  tracked two for over two decades and watched it complete
• 44:22
  more than one full orbit, measuring its speed directly. The
• 44:26
  region as a whole radiates across the electromagnetic spectrum. X
• 44:31
  ray flares erupt from close to the black hole. Irregularly
• 44:36
  dense clusters of massive young stars sit surprisingly close to
• 44:40
  the core, which is itself a mystery, since the gravitational
• 44:44
  tides near a super massive black hole should make star
• 44:48
  formation nearly impossible. Something is generating new stars there anyway.
• 44:55
  The galactic center is, by any measure, one of the
• 44:59
  most extreme dream places in the Milky Way, and yet
• 45:03
  it is twenty six thousand light years away. That distance
• 45:08
  is so enormous that none of this is visible to
• 45:10
  the naked eye. On a clear night, looking toward the
• 45:15
  constellation Sagittarius, you are looking in the direction of the
• 45:19
  galactic center. You see stars, You see a slightly brighter
• 45:24
  concentration of the Milky Way's glow. You see nothing that
• 45:28
  suggests a four million solar mass black hole surrounded by
• 45:32
  stars racing at thousands of miles per second. The dust
• 45:36
  and distance swallow all of it. The violence at the
• 45:40
  core of our own galaxy is so remote that it
• 45:43
  registers as nothing, a slight brightening in one region of
• 45:47
  a faint band of light crossing the sky. The galaxy
• 45:52
  is not a neighbourhood, It is a continent, and our
• 45:56
  solar system is a small town, sitting quietly in the
• 45:59
  air regions twenty six thousand light years from the center,
• 46:05
  separated from the most extreme environment in the Milky Way
• 46:10
  by a distance that light takes twenty six thousand years
• 46:14
  to cross. We live in the galaxy suburbs, twenty six
• 46:18
  thousand light years from a four million solar mass black hole,
• 46:22
  and it is so far away that without telescopes, none
• 46:26
  of us would have any idea it was there. The
• 46:29
  nearest large galaxy to the Milky Way is Andromeda, and
• 46:33
  it is two point five three seven million light years away.
• 46:38
  That number needs a moment. The Milky Way is one
• 46:41
  hundred thousand light years across. The distance to Andromeda is
• 46:46
  twenty five times that. Light leaving Andromeda right now will
• 46:51
  spend two point five three seven million years crossing the
• 46:55
  Void before it reaches Earth. The light arriving in your
• 47:00
  eyes tonight left Andromeda before our species existed, before Homo
• 47:06
  sapiens had evolved, before the first stone tools. The galaxy
• 47:12
  you are looking at is not the galaxy as it is.
• 47:16
  It is the galaxy as it was preserved in light
• 47:20
  that has been traveling since before humanity began. Andromeda is
• 47:25
  not a small thing It contains roughly a trillion stars,
• 47:30
  which puts it comfortably larger than the Milky Way. Our
• 47:34
  own galaxy holds somewhere between one hundred and four hundred
• 47:38
  billion stars, depending on which estimate you use. Andromeda dwarfs
• 47:44
  even the high end of that range. It is the
• 47:47
  dominant mass in our immediate corner of the universe, a
• 47:51
  spiral galaxy tilted slightly toward us, enormous, ancient, and closing
• 47:59
  the distance between us every second of every day. Here
• 48:02
  is the strange part. On a clear night, far enough
• 48:06
  from city lights, you can see Andromeda without a telescope.
• 48:11
  It appears as a faint smudge, a small blur of
• 48:15
  diffuse light just above the constellation Perseus, easy to overlook,
• 48:21
  easy to mistake for a wisp of cloud. That smudge
• 48:25
  is two point five three seven million light years away.
• 48:30
  The light forming that soft blur in your eye traveled
• 48:34
  for longer than the entire history of our species to
• 48:37
  reach you. It is the most distant object a human
• 48:41
  being can see without any instrument at all. Nothing else
• 48:45
  you will ever see with your naked eye is even close.
• 48:49
  For most of human history, nobody knew what that smudge was.
• 48:54
  Ancient astronomers noted it. Persian astronomer Abd al Rahman al
• 48:59
  Sufi described it in the year nine hundred sixty four
• 49:03
  as a small cloud. For nearly a thousand years after that,
• 49:08
  the best minds in astronomy debated whether objects like Andromeda
• 49:13
  were nearby gas clouds inside our own galaxy, or something
• 49:18
  else entirely. The debate wasn't settled until nineteen twenty five,
• 49:24
  when Edwin Hubble measured the distance to Andromeda using a
• 49:29
  type of pulsing star called a Cepheid variable. The answer
• 49:33
  came back at nearly a million light years, which was
• 49:36
  already enough to place Andromeda far outside the Milky Way.
• 49:41
  It was unmistakably a separate galaxy, an island universe. The
• 49:47
  realization reshaped everything. What Hubble also detected, and what later
• 49:53
  measurements confirmed in sharper detail, is that Andromeda is not
• 49:58
  sitting still. It is moving toward us at roughly seventy
• 50:04
  miles per second. The Andromeda galaxy is falling in our direction,
• 50:10
  pulled by the mutual gravity of two enormous collections of stars,
• 50:14
  gas and dark matter, separated by millions of light years,
• 50:19
  but locked into an inevitable approach. Seventy miles per second
• 50:24
  sounds fast. It is fast by any human standard. At
• 50:29
  that speed, you could circle Earth in roughly six minutes,
• 50:33
  But two and a half million light years is an
• 50:36
  almost incomprehensible distance. Even at that pace, the merger will
• 50:41
  not begin for approximately four point five billion years. What
• 50:46
  happens when two galaxies collide is not what the word
• 50:50
  collision implies. Stars inside galaxies are separated by such vast
• 50:56
  distances that the actual probability of ties two stars directly
• 51:01
  striking each other during a galactic merger is essentially zero.
• 51:07
  The Milky Way could pass through Andromeda, and the odds
• 51:10
  of any single star hitting another are almost laughably small.
• 51:16
  What happens instead is gravitational. The two galaxies will pull
• 51:21
  at each other, distort each other, fling stars into long
• 51:26
  tidal streams stretching hundreds of thousands of light years out
• 51:31
  into space. The elegant spiral structures of both galaxies, the arms,
• 51:37
  the disk, the flat ordered rotation, will be completely torn apart.
• 51:43
  Simulations show the process playing out over hundreds of millions
• 51:47
  of years, a long gravitational waltz repeated passes, each one
• 51:53
  pulling the two galaxies closer together and more thoroughly scrambled.
• 51:59
  The end result will be a single structure, no longer
• 52:02
  a spiral of any kind. What emerges from the merger
• 52:07
  is expected to be an elliptical galaxy, a smooth, roughly
• 52:12
  egg shaped cloud of stars without the organized rotation that
• 52:16
  currently defines both the Milky Way and Andromeda. Some astronomers
• 52:22
  have already given it a name milk Ameda. The combined
• 52:27
  system the gravitational inheritor of both galaxies, settling slowly into
• 52:33
  a new shape over time scales that dwarf everything in
• 52:36
  recorded human history. Our Sun will almost certainly still exist
• 52:42
  when this process begins. It will be old, running down
• 52:47
  toward the end of its hydrogen supply, but present the
• 52:52
  Solar System will be jostled by the changing gravitational environment,
• 52:57
  possibly nudged into a different orbit around whatever galactic center
• 53:01
  survives the merger. The night sky will look nothing like
• 53:06
  it does now. New stars will be visible that were
• 53:10
  once too far away. Old familiar constellations will have long
• 53:15
  since drifted apart. The shape of everything we think of
• 53:19
  as home will have been remade by forces that began
• 53:22
  pulling two point five three seven million light years away,
• 53:27
  long before the first human ever looked up and noticed
• 53:31
  that faint smudge of light. Andromeda and the Milky Way
• 53:35
  are headed toward each other at seventy miles per second
• 53:39
  and will begin to merge in about four point five
• 53:42
  billion years, slowly dismantling both galaxies and rebuilding them into
• 53:49
  something new. The Milky Way and Andromeda are the two
• 53:54
  heavyweights of a larger collection of gravitationally bound galaxies called
• 53:59
  the Low Group. Together they dominate it the way two
• 54:05
  large cities might dominate a region of smaller towns. But
• 54:09
  the local group itself is a real structure, a defined thing,
• 54:14
  a gravitationally bound system stretching roughly ten million light years
• 54:19
  across and containing somewhere between fifty and eighty member galaxies,
• 54:25
  depending on how you count the faintest ones at the edges.
• 54:29
  Most of those members are dwarf galaxies, small, faint, and
• 54:35
  easy to miss. Some contain only a few million stars,
• 54:40
  others reach a few billion. By the standards of the
• 54:43
  Milky Way, with its hundreds of billions of stars, these
• 54:48
  are not much more than clouds of stellar dust caught
• 54:51
  in the gravitational pull of their larger neighbours. The two
• 54:56
  most famous dwarf members are the Large Magellanic Cloud and
• 55:01
  the Small Magellanic Cloud from the southern Hemisphere. On a clear,
• 55:05
  dark night, they look like detached pieces of the Milky
• 55:09
  Way floating nearby, two smudges of light that early navigators
• 55:14
  used as reference points at sea. They are not background objects.
• 55:19
  They are satellite galaxies orbiting our own, close enough that
• 55:23
  the Milky Way's gravity is slowly pulling them apart. The
• 55:28
  Large Magellanic Cloud sits roughly one hundred sixty thousand light
• 55:33
  years away, the Small Magellanic Cloud is around two hundred
• 55:37
  thousand light years out. Both of them are close enough
• 55:41
  that astronomers can resolve individual stars inside them, study their composition,
• 55:48
  watch supernova detonate within their borders in real time. In
• 55:53
  nineteen eighty seven, a supernova erupted in the Large Magellanic Cloud.
• 56:00
  It was visible to the naked eye for months. That
• 56:05
  was the closest supernova observed since sixteen oh four, and
• 56:09
  it happened inside one of our satellite galaxies. Beyond the
• 56:13
  Magellanic Clouds, the local group contains dozens of other dwarfs,
• 56:18
  most of them so faint they were discovered only in
• 56:21
  the last few decades. Sculptor Fornax, Carina Leo the first.
• 56:29
  Each of these is a galaxy in name, a gravitationally
• 56:32
  distinct system of stars, gas, and dark matter, but they
• 56:38
  are dim enough that finding them requires careful surveys of
• 56:42
  the sky looking for slight concentrations of faint stars against
• 56:47
  the background. Some of the smallest known members contain fewer
• 56:52
  stars than a single large globular cluster inside the Milky Way.
• 56:58
  They are galaxies in the technical sense, but barely. The
• 57:03
  local group also contains a handful of other moderately sized
• 57:06
  members beyond the Milky Way and Andromeda Triangulum, also known
• 57:12
  as M thirty three, is the third largest member. It
• 57:17
  sits about two point seven million light years away, roughly
• 57:21
  the same distance as Andromeda, and contains somewhere around forty
• 57:26
  billion stars, small compared to both dominant members, but a
• 57:32
  substantial galaxy in its own right. On the darkest nights.
• 57:37
  In ideal conditions, Triangulum is technically visible to the naked eye,
• 57:43
  making it one of the most distant objects a human
• 57:46
  being can see without any instrument at all. The entire
• 57:50
  local group is moving nothing in the universe sits still.
• 57:55
  The whole collection Milky Way and Andromeda and all their
• 57:59
  satellites and smaller companions is falling toward a much larger
• 58:04
  concentration of matter in the direction of the Virgo cluster,
• 58:08
  pulled by gravity across scales that make the local group
• 58:12
  itself look compact. From far enough away, the local group
• 58:18
  would appear as a small knot of light, two bright
• 58:22
  points surrounded by faint companions, bound together but drifting in
• 58:27
  a direction it has no saying. What is worth holding
• 58:31
  on to? Here is the compression that has already happened
• 58:35
  in the span of this video. We started with the
• 58:38
  distance from Earth to the Moon, then the Sun, then
• 58:43
  the outer edge of the Solar system proxima centaury, the
• 58:48
  width of the Milky Way, the distance to Andromeda. Now
• 58:53
  the local group ten million light years across sounds like
• 58:56
  a natural stopping point, a boundary that feels final. It
• 59:01
  is not. The local group is one small structure among many.
• 59:07
  Step back far enough and it disappears into a larger
• 59:10
  pattern that stretches across scales that make ten million light
• 59:14
  years feel like walking distance. The Virgo cluster alone contains
• 59:20
  more than a thousand galaxies and sits fifty three million
• 59:24
  light years away. The local Group, for all its sprawl,
• 59:30
  barely registers against what surrounds it. T n million light
• 59:35
  years seems like a number that should be the end
• 59:38
  of the story, and we have not even reached the
• 59:41
  threshold of what cosmologists consider the local universe. The local
• 59:46
  Group sits at the edge of a much larger structure
• 59:49
  called the Laniakia Supercluster, a gravitationally bound region of space
• 59:54
  containing more than one hundred thousand galaxies spanning five hundred
• 59:59
  twenty million light years. The word Laniakia comes from the
• 1:00:04
  Hawaiian language and means immeasurable heaven. That is not a
• 1:00:09
  poetic choice, it is an accurate one. The full extent
• 1:00:14
  of this structure was only mapped in twoenty fourteen, which
• 1:00:19
  means that for most of the history of modern astronomy,
• 1:00:23
  we had no idea what neighbourhood we actually lived in.
• 1:00:26
  Before Laniakia was properly mapped, astronomers knew its pieces, but
• 1:00:31
  not the whole. The Virgo Cluster alone sits about fifty
• 1:00:36
  three million light years from Earth and contains at least
• 1:00:40
  one thousand, three hundred galaxies packed into a relatively small
• 1:00:45
  region of space. It is one of the most massive
• 1:00:49
  gravitational concentrations near us. The local group sits near its
• 1:00:54
  outer edge, not quite a member, but close enough to
• 1:00:58
  feel its pull. What the twoenty fourteen mapping revealed was
• 1:01:04
  that the Virgo cluster is just one feature inside something
• 1:01:08
  far larger, a supercluster whose full shape had been hidden
• 1:01:13
  by the difficulty of tracing galaxy motions across hundreds of
• 1:01:18
  millions of light years. To map Laniakia, researchers tracked the
• 1:01:23
  velocities of galaxies, not just how fast they were moving
• 1:01:27
  away from us due to cosmic expansion, but the small
• 1:01:31
  additional motions caused by gravity, the way galaxies drift toward
• 1:01:36
  concentrations of mass. By subtracting the expansion and isolating those
• 1:01:42
  gravitational drifts, scientists could identify which galaxies were flowing toward
• 1:01:47
  the same gravitational basin. Laniakia is defined by that basin.
• 1:01:54
  Every galaxy inside it is in some sense falling in
• 1:01:58
  the same direction. Our galaxy included. This is where it
• 1:02:02
  starts getting stranger. Laniakia is not the only supercluster. It
• 1:02:08
  is one of many. The perseus pisces supercluster stretches in
• 1:02:13
  a different direction, a long filamentary chain of galaxies and
• 1:02:19
  clusters several hundred million light years long. The Coma supercluster
• 1:02:24
  is another. Then there is the Shapleigh Supercluster, the largest
• 1:02:30
  known concentration of galaxies within a billion light years of Earth.
• 1:02:35
  Shapley contains the mass of tens of thousands of galaxies
• 1:02:39
  packed into a region roughly six hundred fifty million light
• 1:02:44
  years away. Its gravitational influence reaches across distances that should,
• 1:02:50
  by any reasonable measure, make it irrelevant to us. It
• 1:02:55
  is not irrelevant. Its gravity measurably affects the motion of
• 1:03:00
  our entire local group. A structure hundreds of millions of
• 1:03:05
  light years away is tugging on the Milky Way right now.
• 1:03:10
  The numbers in this section have a way of becoming
• 1:03:12
  unmanageable quickly. Ten million light years for the local group,
• 1:03:17
  fifty three million for the distance to the Virgo cluster,
• 1:03:21
  five hundred twenty million for the span of Laniakia, six
• 1:03:26
  hundred fifty million for Shapley. These are not variations on
• 1:03:30
  the same theme. Each step is a genuine multiplication of scale,
• 1:03:36
  and the mind does not naturally track the difference between
• 1:03:39
  fifty million light years and five hundred million the way
• 1:03:43
  it tracks the difference between fifty miles and five hundred
• 1:03:48
  They feel similar. They are not. Somewhere inside this layered
• 1:03:52
  structure of clusters and superclusters, there is something that astronomers
• 1:03:57
  have known about for decades and still cannot look at directly.
• 1:04:03
  It is called the Greater Tractor. It sits somewhere between
• 1:04:07
  one hundred fifty and two hundred fifty million light years
• 1:04:12
  away in the direction of the Centaurus constellation. The Milky
• 1:04:17
  Way is moving toward it, Andromeda is moving toward it.
• 1:04:22
  The entire local group is moving toward it. Millions of
• 1:04:27
  other galaxies across the region are moving toward it. The
• 1:04:32
  velocities involved are not subtle, roughly one point four million
• 1:04:37
  miles per hour relative to the motion that cosmic expansion
• 1:04:41
  alone would predict. The problem is that the Greater Tractor
• 1:04:47
  lies almost exactly behind the densest part of the Milky
• 1:04:51
  Way's own disc From our perspective, the plane of our
• 1:04:55
  galaxy is thick with dust and gas, and that dust
• 1:04:59
  blocks of visible light almost completely in that direction. For
• 1:05:04
  a long time, astronomers could barely see anything there at all.
• 1:05:09
  Observations in X ray and radio wavelengths have revealed some
• 1:05:14
  of what is hiding behind the obscuring material. Massive galaxy
• 1:05:19
  clusters in the region known as the Normal Cluster appear
• 1:05:22
  to be part of the concentration, but a full picture
• 1:05:26
  remains difficult. The structure pulling millions of galaxies through space
• 1:05:32
  at over a million miles per hour is from our
• 1:05:35
  vantage point, hidden behind our own galaxies walls. The Greater Tractor,
• 1:05:42
  a gravitational anomaly somewhere between one hundred fifty and two
• 1:05:47
  hundred fifty million light years away, is pulling our entire
• 1:05:51
  galaxy toward it at one point four million miles per hour,
• 1:05:56
  and we cannot see it directly because it hides behind
• 1:05:59
  the dust of the Milky Way's own plane. Zoom far
• 1:06:04
  enough out from individual galaxies and superclusters, and the universe
• 1:06:09
  reveals a structure that nobody predicted and that still catches
• 1:06:14
  people off guard the first time they see it. The
• 1:06:18
  pattern is called the cosmic web. It is the largest
• 1:06:22
  known structure in the universe, and it is exactly what
• 1:06:26
  the name suggests, a web. Galaxies do not scatter randomly
• 1:06:32
  through space the way dust floats in a sunbeam. They
• 1:06:36
  cluster along thin strands of matter enormous filaments stretching hundreds
• 1:06:42
  of millions of light years from one end to the other.
• 1:06:46
  Where those filaments cross, galaxy clusters and superclusters pile up
• 1:06:51
  at the intersections, dense knots of matter holding billions of
• 1:06:56
  galaxies close together under their combined gravity. Between the filaments
• 1:07:01
  lies something else, entirely nothing, or close enough to nothing
• 1:07:07
  that the difference barely matters. These empty regions are called voids.
• 1:07:13
  They are not small gaps. They are vast, roughly spherical
• 1:07:18
  bubbles of space, containing almost no galaxies, almost no visible matter,
• 1:07:24
  almost nothing at all. The largest confirmed voids span hundreds
• 1:07:29
  of millions of light years across. The Boat's void is
• 1:07:32
  one of the best known. It measures roughly three hundred
• 1:07:36
  thirty million light years from one side to the other.
• 1:07:40
  To put that in terms already covered here, the distance
• 1:07:43
  from the Milky Way to Andromeda is about two point
• 1:07:47
  five million light years. The Boater's void is more than
• 1:07:51
  one hundred thirty times wider than that. A civilization sitting
• 1:07:57
  at its center would have to look an extraord dar
• 1:08:00
  distance in any direction before finding a neighboring galaxy. The
• 1:08:06
  night sky from inside the Boater's Void would contain far
• 1:08:10
  fewer galaxies visible even through powerful telescopes. Astronomers living there
• 1:08:16
  would have had a much harder time determining that other
• 1:08:19
  galaxies existed at all. The void is not an absence
• 1:08:24
  of space. It is space filled with almost nothing, enormous
• 1:08:30
  beyond easy reckoning. When cosmologists first mapped structures this large
• 1:08:35
  in the early nineteen eighties, the discovery was genuinely shocking.
• 1:08:40
  The assumption had been that matter, while clumped into galaxies
• 1:08:44
  and clusters, would be distributed with some rough uniformity across
• 1:08:49
  space at large enough scales. What the surveys found instead
• 1:08:54
  was this filamentary architecture, dense threads, empty bubbles, a skeletal
• 1:09:01
  pattern that repeated across hundreds of millions of light years.
• 1:09:06
  The first major void, the Booter's Void, was identified in
• 1:09:10
  nineteen eighty one. Robert Kirshner and his colleagues surveyed a
• 1:09:15
  patch of sky and found almost nothing where they expected
• 1:09:19
  to find galaxies. The structure was so unexpected that it
• 1:09:24
  took years before the broader picture came into focus. What
• 1:09:28
  makes the cosmic web stranger is what it looks like
• 1:09:32
  in simulation. Researchers running computer models of cosmic structure, formation,
• 1:09:39
  feeding in the known laws of physics and the initial
• 1:09:42
  conditions of the early universe consistently produced the same result.
• 1:09:48
  The matter organizes itself into exactly this kind of web,
• 1:09:53
  and when you look at the output of those simulations,
• 1:09:56
  rendered visually zoomed out to scale where the filaments and
• 1:10:01
  voids become clear, the image is difficult to forget. It
• 1:10:07
  resembles a biological structure. The filaments branch and intersect in
• 1:10:12
  patterns that look almost exactly like a neuron, the branching
• 1:10:17
  network of a nerve cell rendered at cosmic scale, or
• 1:10:22
  like the threads of a mycelial network spreading through soil.
• 1:10:26
  The resemblance has been noted, written about, and occasionally overinterpreted.
• 1:10:32
  It is worth being clear this is coincidence, not design.
• 1:10:38
  The same mathematical forces that govern how gravity pulls matter
• 1:10:42
  together on cosmic scales happen to produce branching, filamentary patterns,
• 1:10:48
  which also happen to appear in biological systems operating under
• 1:10:53
  completely different physical rules. The universe did not organize itself
• 1:10:59
  to look like a bri It organized itself according to
• 1:11:03
  gravity and dark matter, and the initial fluctuations seeded in
• 1:11:08
  the first moments after the Big Bang. The resemblance is striking.
• 1:11:14
  It is also meaningless beyond the coincidence. The dark matter
• 1:11:19
  connection matters here. The filaments of the cosmic web are
• 1:11:24
  not made primarily of galaxies. Galaxies are the visible component,
• 1:11:30
  the lit portion of a much larger structure built from
• 1:11:33
  dark matter. The dark matter web formed first its gravitational
• 1:11:39
  influence gathered ordinary matter along those invisible threads over billions
• 1:11:45
  of years. Galaxies formed preferentially where the dark matter was densest.
• 1:11:52
  The visible cosmic web we map through galaxy surveys is
• 1:11:56
  essentially a tracing of an underlying dark matter skeleton that
• 1:12:00
  we cannot observe directly. The voids are not just empty
• 1:12:05
  of galaxies, they are empty of dark matter as well,
• 1:12:09
  which is why galaxies never formed there in the first place.
• 1:12:14
  From the scale of individual stars to galaxy clusters to
• 1:12:18
  superclusters to the cosmic web, the universe has turned out
• 1:12:23
  to be structured at every level, but the web is
• 1:12:28
  the top of the hierarchy above it. On scales larger
• 1:12:32
  than a billion light years or so, the universe begins
• 1:12:36
  to look roughly the same in every direction. The web
• 1:12:40
  pattern continues, but no larger organized structure has been confirmed
• 1:12:45
  beyond it. This is the scale at which matter stops
• 1:12:49
  organizing itself into distinct features and becomes statistically a kind
• 1:12:55
  of uniform foam. On the largest observable scales, the universe
• 1:13:01
  looks less like a uniform sea of stars and more
• 1:13:04
  like foam. Dense filaments of galaxies surrounding enormous empty bubbles
• 1:13:11
  hundreds of millions of light years across. The observable universe
• 1:13:17
  is not the same thing as the whole universe, and
• 1:13:20
  the difference between those two concepts is where things start
• 1:13:24
  to get genuinely strange. Everything described before this point, the
• 1:13:29
  cosmic web, the filaments, the voids, the superclusters, stretching hundreds
• 1:13:35
  of millions of light years across, all of it, sits
• 1:13:39
  inside a boundary, not a wall, not an edge where
• 1:13:43
  space runs out. A horizon, the kind of horizon that
• 1:13:48
  exists not because the universe stops there, but because light
• 1:13:52
  does not move infinitely fast. The universe is roughly thirteen
• 1:13:58
  point eight billion years old. Light travels at one hundred
• 1:14:02
  eighty six thousand miles per second. Put those two facts together,
• 1:14:08
  and a limit appears on its own. We can only
• 1:14:12
  observe objects whose light has had enough time to reach
• 1:14:15
  us since the Big Bang. Everything beyond that distance sits
• 1:14:20
  in permanent darkness, not because it does not exist, but
• 1:14:25
  because its light has not arrived yet. Here is where
• 1:14:30
  the strangeness begins. You might expect the edge of the
• 1:14:33
  observable universe to sit thirteen point eight billion light years away,
• 1:14:39
  one year of travel for every year the universe has existed.
• 1:14:44
  That is the intuitive answer, and it is wrong. The
• 1:14:49
  actual boundary sits at roughly forty six point five billion
• 1:14:53
  light years in every direction. The observable universe is therefore
• 1:14:58
  a sphere about no ninety three billion light years across.
• 1:15:03
  The reason for the discrepancy is something that took cosmologists
• 1:15:08
  decades to fully absorb. Space itself has been expanding the
• 1:15:13
  entire time that ancient light was traveling toward us. A
• 1:15:18
  photon fired from some distant region thirteen billion years ago
• 1:15:22
  began its journey when that region was much closer. As
• 1:15:27
  the photon traveled, space stretched underneath it. The source of
• 1:15:31
  that light was carried farther away by the expansion even
• 1:15:35
  as the light moved in our direction. By the time
• 1:15:39
  the photon finally arrived, the object that launched it had
• 1:15:43
  been carried billions of additional light years out. The light
• 1:15:48
  crossed thirteen billion years of time. The source is now
• 1:15:53
  forty six billion light years away. Think about what that
• 1:15:58
  means in practical terms. When astronomers look at the most
• 1:16:02
  distant objects they can detect, they are not seeing things
• 1:16:07
  forty six billion light years away as they are now.
• 1:16:11
  They are seeing those regions as they were billions of
• 1:16:14
  years ago, when the universe was young and those locations
• 1:16:19
  were far closer to our own. The image that arrives
• 1:16:23
  is ancient. The reality it describes has aged by billions
• 1:16:28
  of years since the light left. Distance at these scales
• 1:16:33
  is also time. Looking outward is looking backward, and the
• 1:16:38
  two are inseparable. Every point in the universe has its
• 1:16:42
  own observable sphere, its own horizon. An observer ten billion
• 1:16:49
  light years from Earth sits at the center of a
• 1:16:52
  completely different bubble of visibility. That bubble overlaps with ours
• 1:16:58
  only partially in the overlapping region. Both observers could in
• 1:17:04
  principle detect the same ancient light, but each observer can
• 1:17:09
  also see regions entirely hidden from the other. Objects close
• 1:17:14
  to the far edge of our observable universe might sit
• 1:17:18
  comfortably inside the observable universe of a civilization across the cosmos,
• 1:17:24
  and those observers might have no knowledge of our local
• 1:17:28
  group at all. Neither perspective is more correct. Neither is
• 1:17:33
  the true view. Both are partial, Both are centered on
• 1:17:37
  a specific point in space and limited by the same physics.
• 1:17:42
  This matters because it dismantles a particular assumption that is
• 1:17:46
  easy to carry without noticing. The assumption is that there
• 1:17:51
  is somewhere a complete picture, a view from outside that
• 1:17:57
  shows the whole. There is not. The observable universe is
• 1:18:02
  not a portion of the universe that has been cordoned
• 1:18:05
  off for observation while the rest waits. It is simply
• 1:18:09
  the region that has had time to communicate with us.
• 1:18:13
  The rest exists on its own terms, indifferent to our horizon,
• 1:18:18
  and unreachable by any instrument we could ever build. Scientists
• 1:18:23
  estimate that the total universe, the full extent of space,
• 1:18:27
  including everything beyond our horizon, is almost certainly far larger
• 1:18:32
  than the ninety three billion light year sphere we can observe.
• 1:18:37
  Some models place the total size at something like ten
• 1:18:41
  to the twenty third power times larger than the observable portion.
• 1:18:46
  Others suggest the universe may be genuinely infinite. Both possibilities
• 1:18:53
  are consistent with the physics we have measured, Neither can
• 1:18:57
  be directly tested. The absurd, irvable universe gives us no
• 1:19:01
  clear signal about what lies beyond its edge, the same
• 1:19:05
  way the horizon visible from a ship gives no information
• 1:19:09
  about what lies over it. What the observable universe does
• 1:19:13
  offer is a defined, measurable arena, a region with a
• 1:19:19
  known size, a known age, a countable number of structures
• 1:19:24
  within it. Sit two trillion galaxies, one septilian stars, a
• 1:19:30
  cosmic web of filaments threading through voids, all of it
• 1:19:34
  visible in principle from Earth, all of its sending light
• 1:19:39
  that we can collect and study. Beyond the horizon, the
• 1:19:43
  universe simply continues. Whether it continues for a finite distance
• 1:19:49
  or forever, The answer has the same practical consequence. We
• 1:19:54
  cannot know what is there, and nothing there can know
• 1:19:58
  what is here. There is no vantage point from which
• 1:20:02
  the entire universe can be seen at once. Every observer
• 1:20:07
  anywhere in the cosmos sees their own bubble, and the
• 1:20:11
  bubbles only partially overlap. The farthest thing in the observable
• 1:20:16
  universe is not a galaxy. It is not a quasar,
• 1:20:20
  not a dying star, not any object with a name
• 1:20:24
  or a position in a catalog. It is a wall
• 1:20:27
  of light, a glow left over from a moment so
• 1:20:31
  early in cosmic history that the universe had not yet
• 1:20:35
  produced a single star. That glow is called the cosmic
• 1:20:39
  microwave background, and it sits at the outer edge of
• 1:20:43
  everything we can ever hope to see. To understand what
• 1:20:48
  the CMB actually is, you have to go back to
• 1:20:52
  the beginning, not to the Big Bang itself, but to
• 1:20:56
  about three hundred eighty thousand years after it. The early
• 1:21:00
  universe was not the cold, dark, mostly empty place we
• 1:21:04
  described in the last section. It was a dense, blindingly
• 1:21:09
  hot plasma, a roiling soup of particles and energy so
• 1:21:14
  tightly packed that light could not travel more than a
• 1:21:17
  tiny distance before being absorbed and scattered again. Photons existed,
• 1:21:24
  but they were essentially trapped bouncing between particles so rapidly
• 1:21:29
  that the universe was opaque. There was no sky, There
• 1:21:34
  was no view the universe was a fog so thick
• 1:21:38
  that nothing could see through it. Then the universe cooled
• 1:21:43
  enough for something to change. Electrons and protons combined to
• 1:21:48
  form the first hydrogen atoms. When that happened, the photons
• 1:21:53
  that had been constantly scattered suddenly had nothing left to
• 1:21:57
  scatter against. They streamed outward freely in every direction for
• 1:22:03
  the first time. That moment is called recombination, and it
• 1:22:09
  is the earliest event in cosmic history that light can
• 1:22:13
  carry information about Every photon released at that moment has
• 1:22:18
  been traveling ever since. What we observe today as the
• 1:22:23
  CMB is that ancient light. It has been traveling for
• 1:22:28
  thirteen point eight billion years, and during that journey, the
• 1:22:33
  expansion of the universe has stretched it, stretched it dramatically.
• 1:22:38
  What will once energetic visible light photons have been pulled
• 1:22:42
  into microwave wavelengths by the growth of space itself. The
• 1:22:47
  CMB today registers at about two point seven degrees kelvin,
• 1:22:53
  That is two point seven degrees above absolute zero, the faintest,
• 1:22:58
  coldest whisper of something that was once unimaginably hot. It
• 1:23:03
  is everywhere, not just in one direction, every point in
• 1:23:08
  the sky. If you had a microwave detector instead of eyes,
• 1:23:12
  would glow with this same ancient light. The CMB fills
• 1:23:17
  the entire observable universe the way a room fills with
• 1:23:21
  sound after a bell has been struck. It comes from
• 1:23:24
  all directions equally, with only the faintest variations temperature differences
• 1:23:30
  of about one part in one hundred thousand subtle ripples
• 1:23:34
  in the density of the early universe that eventually grew
• 1:23:38
  into the galaxies and superclusters we described before. Those ripples
• 1:23:43
  matter enormously. They are the fingerprints of the early cosmos.
• 1:23:49
  Physicists have mapped them in extraordinary detail, and the patterns
• 1:23:54
  they produce are one of the strongest conformations we have
• 1:23:58
  of the entire standard model of cosmology. The CMB is
• 1:24:04
  not just a distant glow. It is the best data
• 1:24:07
  set we have about the early universe, an ancient photograph
• 1:24:12
  taken three hundred eighty thousand years after the beginning of time.
• 1:24:17
  But here is the thing that makes the CMB different
• 1:24:21
  from every other object described in this video. Every galaxy,
• 1:24:26
  every quasar, every distant structure we have discussed is something
• 1:24:30
  we could theoretically observe better with a more powerful telescope
• 1:24:35
  with better instruments, with more time, the CMB is different.
• 1:24:41
  It is not a technological limit, it is a physical one.
• 1:24:46
  Before three hundred eighty thousand years after the Big Bang,
• 1:24:50
  the universe was opaque. No photon from that era survived
• 1:24:55
  to reach us because no photon could travel freely in
• 1:24:59
  that vironment. The information from those first three hundred eighty
• 1:25:05
  thousand years does not exist in the form of light
• 1:25:09
  anywhere in the observable universe. It was erased not by
• 1:25:15
  distance but by the state of the universe itself. This
• 1:25:19
  means there are events that almost certainly happened, the formation
• 1:25:23
  of the first particles, the physics of the Big Bang itself,
• 1:25:27
  the nature of whatever came before it, that light cannot
• 1:25:31
  bring us. We can infer what happened using physics and mathematics.
• 1:25:37
  We can build models and test predictions, but we cannot
• 1:25:41
  observe it directly. The wall is absolute. Astronomers have detected
• 1:25:46
  gravitational waves, which travel differently from light and pass through
• 1:25:51
  the early universe's plasma without being scattered. Future detectors may
• 1:25:57
  eventually reveal something of what happened and before recombination, but
• 1:26:03
  in terms of electromagnetic radiation. In terms of anything traveling
• 1:26:07
  at the speed of light, the CMB is the last word,
• 1:26:12
  the surface of last scattering, as physicists call it. Is
• 1:26:16
  the farthest back in time that light will ever take us.
• 1:26:20
  The CMB is not the edge of the universe. It
• 1:26:24
  is the edge of what light can ever tell us,
• 1:26:27
  a wall built not from distance, but from time. In
• 1:26:32
  nineteen twenty nine, Edwin Hubble made an observation that changed everything,
• 1:26:38
  and it was this, the universe is expanding. He was
• 1:26:42
  studying the light from distant galaxies and notice something systematic
• 1:26:47
  in the data. The light from almost every galaxy was
• 1:26:51
  shifted toward the red end of the spectrum. Redshift, in
• 1:26:56
  this context meant the light waves were being stretched, and
• 1:27:01
  the reason they were being stretched was that the galaxies
• 1:27:04
  themselves were moving away, not just a few of them,
• 1:27:09
  all of them in every direction. Hubble looked the galaxies
• 1:27:14
  were receding. The farther away a galaxy was the faster
• 1:27:19
  it was moving. This relationship distance proportional to recession speed
• 1:27:26
  became known as Hubble's law, and it rewrote the way
• 1:27:30
  physics understood the cosmos. The natural image this produces in
• 1:27:35
  most people's minds is an explosion, a single point, a detonation,
• 1:27:41
  and then matter flying outward in all directions through empty space.
• 1:27:47
  That picture is wrong, and the reason it is wrong
• 1:27:50
  matters more than it might seem. Galaxies are not moving
• 1:27:55
  through space the way shrapnel moves through air. Space itself
• 1:27:59
  is expanding. The fabric of the universe is stretching, and
• 1:28:04
  galaxies are embedded in that fabric, carried along passively as
• 1:28:09
  the distances between them grow. No galaxy is rushing away
• 1:28:14
  from us under its own power. The space between us
• 1:28:17
  and them is simply getting larger. This distinction has a
• 1:28:22
  specific and important consequence. When space expands, it does not
• 1:28:27
  pull everything apart. Equally, gravity is strong enough at small
• 1:28:33
  scales to hold structures together in spite of the expansion.
• 1:28:38
  The Earth and the Moon are not drifting apart. The
• 1:28:42
  Solar System is not slowly dissolving, the Milky Way is
• 1:28:47
  not losing its satellite galaxies to the growing universe. Gravity
• 1:28:53
  binds these things together, and the expansion of space has
• 1:28:57
  no meaningful effect on objects are already gravitationally locked. What
• 1:29:04
  the expansion does affect are the vast unbound stretches of
• 1:29:08
  space between galaxy clusters, the enormous empty corridors that make
• 1:29:14
  up most of the universe's volume. In those regions, there
• 1:29:18
  is nothing to resist the expansion, and so the distances
• 1:29:22
  grow without limit. The scale at which this becomes important
• 1:29:28
  is hard to feel intuitively. Within the local group the
• 1:29:33
  Milky Way, Andromeda and the several dozen smaller galaxies bound
• 1:29:38
  to them gravitationally, the expansion is irrelevant. Those galaxies are
• 1:29:44
  held together. But beyond the local group, across the hundreds
• 1:29:49
  of millions of light years separating us from other clusters
• 1:29:53
  and superclusters, the expansion is ongoing and relentless. The Virgo
• 1:29:59
  cluster is receding, the Coma cluster is receding. The most
• 1:30:04
  distant galaxy clusters are receding faster still, and this leads
• 1:30:10
  to one of the stranger conclusions in all of cosmology.
• 1:30:14
  Some galaxies are currently moving away from us faster than
• 1:30:18
  the speed of light. This sounds like it should be impossible.
• 1:30:22
  In ordinary physics, nothing moves faster than light, but that
• 1:30:27
  rule applies to objects moving through space, not to space
• 1:30:31
  itself expanding. The expansion of space is not governed by
• 1:30:36
  the speed limit that applies to matter and radiation. Space
• 1:30:41
  can expand at any rate, and at distances large enough.
• 1:30:46
  The cumulative effect of that expansion means that the total
• 1:30:51
  recession speed of a distant galaxy, the rate at which
• 1:30:55
  the space between us and it is growing, exceeds three
• 1:30:59
  hundred thousand kilometers per second. The galaxy is not moving
• 1:31:04
  through space at that speed. The space between us is
• 1:31:08
  simply growing that fast. The practical consequence is stark light
• 1:31:14
  from those galaxies is traveling toward us, but the space
• 1:31:17
  between us and them is growing faster than the light
• 1:31:20
  can cross it. The light makes no progress. It will
• 1:31:24
  never arrive. Right now, there are galaxies in the universe
• 1:31:29
  whose light has been traveling toward Earth since before our
• 1:31:33
  solar system formed, and that light will never reach us
• 1:31:37
  because the space it has to cross keeps growing faster
• 1:31:41
  than it can travel. This defines something called the cosmic
• 1:31:46
  event horizon. It sits at roughly sixteen billion light years
• 1:31:51
  from Earth. Galaxies beyond that boundary are receding faster than light,
• 1:31:57
  and no signal from them, no light, no radio wave,
• 1:32:01
  no information of any kind will ever reach Earth. They
• 1:32:06
  are not merely distant. They are permanently and irrevocably cut
• 1:32:11
  off the separation is not a matter of waiting long
• 1:32:14
  enough or building a better telescope. The geometry of an
• 1:32:19
  expanding universe makes the disconnection absolute. Even if a civilization
• 1:32:25
  beyond the event horizon sent a signal today at the
• 1:32:28
  speed of light aimed directly at Earth, that signal would
• 1:32:33
  spend eternity traveling toward us and never close the gap.
• 1:32:39
  What makes this more unsettling is that the event horizon
• 1:32:43
  is not fixed. If the expansion of the universe continues
• 1:32:48
  to accelerate, which is what current measurements suggest it is doing,
• 1:32:53
  the horizon will gradually tighten. Galaxies that are currently observable
• 1:33:00
  will eventually cross the threshold and go dark, not because
• 1:33:04
  they have changed or died, but because the expanding space
• 1:33:08
  between us and them will finally be carrying them away
• 1:33:12
  faster than their light can reach us. The universe is
• 1:33:16
  not just large, It is actively growing in a way
• 1:33:20
  that makes parts of it permanently inaccessible, and that boundary
• 1:33:25
  is moving closer rather than farther as time goes on.
• 1:33:30
  Beyond roughly sixteen billion light years, the expansion of space
• 1:33:35
  is carrying galaxies away from us faster than light, and
• 1:33:39
  no signal from those regions will ever reach Earth, not
• 1:33:43
  in a billion years, not in a trillion. Measuring exactly
• 1:33:49
  how fast the universe is expanding turns out to be
• 1:33:53
  one of the most contentious problems in modern physics. The
• 1:33:57
  number that describes this rate has a name, the Hubble constant,
• 1:34:02
  after Edwin Hubble, whose observations in nineteen twenty nine first
• 1:34:07
  revealed the expansion. It measures how quickly galaxies recede from
• 1:34:12
  us as a function of distance. Specifically, it tells you
• 1:34:18
  how many kilometers per second a galaxy accelerates away from
• 1:34:22
  us for every megaparsec of distance between that galaxy and Earth.
• 1:34:27
  One megaparsec is about three point three million light years.
• 1:34:32
  The number sounds technical, what it represents is fundamental. It
• 1:34:38
  is the heartbeat of the universe, the single value that
• 1:34:41
  tells you how fast everything is flying apart. The problem
• 1:34:46
  is that nobody can agree on what that number actually is.
• 1:34:51
  There are two main ways to measure it. The first
• 1:34:55
  uses the cosmic microwave background, the faint ancient radiation left
• 1:35:00
  over from roughly three hundred eighty thousand years after the
• 1:35:04
  Big Bang. Scientists can analyze the subtle temperature patterns in
• 1:35:09
  that radiation run them through the standard model of cosmology,
• 1:35:14
  and calculate what the Hubble constant must have been in
• 1:35:17
  the early universe and what it should be today. That
• 1:35:22
  method gives a value of approximately sixty eight kilometers per
• 1:35:27
  second per megaparsec. The second method works in the opposite direction.
• 1:35:33
  Astronomers look at nearby objects whose distances can be measured directly,
• 1:35:39
  variable stars called cephides and type one A supernovae, which
• 1:35:45
  reliably explode at the same intrinsic brightness and can therefore
• 1:35:49
  serve as standard candles. Measuring how dim they appear tells
• 1:35:55
  you how far away they are. Comparing their distance to
• 1:35:58
  their recession speed gives you the Hubble constant directly from
• 1:36:02
  the local universe. That method gives a value of approximately
• 1:36:07
  seventy three kilometers per second per megaparsec. Five kilometers per
• 1:36:12
  second per megaparsec. That sounds like a rounding error, it
• 1:36:18
  is not. The gap between sixty eight and seventy three
• 1:36:23
  represents a disagreement of roughly seven to eight percent. In
• 1:36:28
  a field where precision is everything, where the measurements have
• 1:36:32
  been refined over decades using some of the most sophisticated
• 1:36:36
  instruments ever built. A persistent seven per cent discrepancy between
• 1:36:41
  two independent methods does not go away quietly. It grows louder,
• 1:36:48
  and that is exactly what has happened. As both techniques
• 1:36:51
  have improved, as the data sets have grown larger and
• 1:36:55
  the error bars have shrunk, the two numbers have not converged.
• 1:37:00
  They have stayed stubbornly, embarrassingly apart. This is called the
• 1:37:06
  Hubble tension, and it has been intensifying for roughly a decade.
• 1:37:11
  The reason this matters so much is that both measurements
• 1:37:15
  are now highly precise. Each team has checked its work,
• 1:37:21
  The systematic errors have been examined, corrected for, and examined again.
• 1:37:27
  The CEPHEID based measurements were extensively cross checked using the
• 1:37:32
  James web Space telescope, which confirmed the results produced by
• 1:37:37
  the older Hubble Space telescope observations. The CMB measurements from
• 1:37:44
  the Plank satellite have been tested against multiple independent analyzes.
• 1:37:50
  Neither side has found a mistake large enough to explain
• 1:37:53
  the gap. If one method were simply noisier or more
• 1:37:57
  prone to error, then improving that method should bring the
• 1:38:01
  two values closer together. The opposite is happening more precision
• 1:38:08
  sharper disagreement. The leading explanation, and the one that makes
• 1:38:13
  cosmologists most uncomfortable, is that the standard model of cosmology
• 1:38:18
  is incomplete. The Standard Model, sometimes called Lambda CDM, is
• 1:38:25
  the framework that describes the universe's composition and evolution. It
• 1:38:31
  accounts for dark energy, dark matter, ordinary matter, and radiation.
• 1:38:37
  It fits almost everything we observe with extraordinary accuracy, almost everything.
• 1:38:44
  The Hubble tension is one place where it fails, and
• 1:38:48
  a failure at the level of the most fundamental constant
• 1:38:52
  is not a failure anyone can easily explain away. Some
• 1:38:56
  researchers have proposed that dark energy is not a fit constant,
• 1:39:01
  but varies over time. If the energy density of the
• 1:39:05
  universe's expansion changed in the early universe even slightly, it
• 1:39:10
  could shift the CMB based predictions enough to close the gap.
• 1:39:16
  Others have suggested undiscovered particles, new forms of dark matter
• 1:39:22
  that behave differently in the early universe, or subtle effects
• 1:39:26
  in the way supernovae explode that have not been fully
• 1:39:30
  accounted for. None of these proposals has gained clear empirical support.
• 1:39:37
  None has closed the gap. Each requires adding something new
• 1:39:42
  to a model that has otherwise worked remarkably well, and
• 1:39:46
  none of them comes without its own complications. What this
• 1:39:51
  amounts to is a crack running through the foundation of cosmology,
• 1:39:57
  not a catastrophic collapse, but of visible, persistent fracture. In
• 1:40:02
  our most successful description of the universe, Edwin Hubble discovered
• 1:40:07
  that the universe was expanding, and a century later the
• 1:40:11
  field he created cannot agree on how fast. The two
• 1:40:15
  most precise ways we have of measuring the universe's expansion
• 1:40:19
  rate consistently disagree with each other, and the disagreement is
• 1:40:24
  getting worse, not better, as the measurements improve. Everything in
• 1:40:30
  this video up to this point, every star, every galaxy,
• 1:40:35
  every supercluster, every structure in the cosmic web, adds up
• 1:40:40
  to about five percent of what the universe actually contains.
• 1:40:45
  That number deserves a moment. Every supernova ever photographed, every
• 1:40:51
  spiral galaxy in the Hubble Deep Field, every filament threading
• 1:40:56
  across the cosmic web, every atom in every planet orbiting,
• 1:41:01
  every star in two trillion galaxies, all of it combined
• 1:41:06
  is five percent. The other ninety five percent is something else, entirely,
• 1:41:12
  something we cannot see, cannot touch, and in most cases
• 1:41:17
  cannot explain. The universe we have been describing is the
• 1:41:21
  small part. The universe we are about to describe is
• 1:41:25
  almost everything else. The first major invisible ingredient is dark matter,
• 1:41:33
  which makes up roughly twenty seven percent of the universe's
• 1:41:36
  total mass energy content. It does not emit light, It
• 1:41:41
  does not absorb light. It does not reflect it, or
• 1:41:45
  scatter it, or interact with it in any way that
• 1:41:49
  our instruments have ever been able to confirm. A beam
• 1:41:53
  of light passes through a region full of dark matter
• 1:41:57
  and arrives on the other side completely un changed, no shadow,
• 1:42:02
  no delay, no signature. We have never captured a single
• 1:42:07
  dark matter particle in any laboratory on Earth, despite decades
• 1:42:11
  of experiments built specifically to find one. Detectors buried miles underground,
• 1:42:18
  cooled to temperatures near absolute zero, designed to register the
• 1:42:23
  faintest possible interaction. Nothing dark matter passes through them like
• 1:42:30
  they are not there. What we know about dark matter
• 1:42:34
  comes entirely from what it does to other things. Gravity.
• 1:42:38
  That is the only tool dark matter uses, and it
• 1:42:42
  is the only tool we have to study it. The
• 1:42:46
  first serious clue came from galaxies themselves, when astronomers measure
• 1:42:51
  how fast stars orbit the center of a galaxy, they
• 1:42:55
  find something that does not match the physics. Stars at
• 1:42:59
  the outer edges of galaxies orbit just as fast as
• 1:43:03
  stars closer to the center, sometimes faster. This should not happen.
• 1:43:10
  If a galaxy contained only the matter we can see,
• 1:43:13
  gravity would weaken with distance from the center, and stars
• 1:43:18
  in the outer regions would orbit more slowly, the way
• 1:43:22
  the outer planets in our Solar System orbit the Sun
• 1:43:26
  more slowly than the inner ones. They do not orbit
• 1:43:30
  more slowly. They orbit at nearly the same speed, regardless
• 1:43:36
  of how far they are from the center. The only
• 1:43:39
  explanation that holds up is that there is additional mass
• 1:43:43
  spread throughout and beyond the visible part of every galaxy,
• 1:43:47
  mass we cannot see, pulling on the visible stars and
• 1:43:52
  keeping their orbits faster than they should be. This invisible
• 1:43:56
  mass is dark matter. The evidence do not stop with
• 1:44:00
  individual galaxies. Galaxy clusters bend light. This is gravitational lensing,
• 1:44:08
  a prediction of general relativity confirmed many times over. When
• 1:44:13
  astronomers measure how much a cluster bends the light passing
• 1:44:17
  near it, they can calculate how much mass is doing
• 1:44:21
  the bending. The answer is always much larger than the
• 1:44:25
  visible mass inside the cluster. Much larger clusters contain hundreds
• 1:44:30
  to thousands of times more dark matter than ordinary matter,
• 1:44:34
  and the ordinary matter itself is mostly not stars but
• 1:44:38
  hot gas. The stars visible inside clusters are a small
• 1:44:43
  fraction even of the ordinary five percent. Then there is
• 1:44:48
  the cosmic web itself, the filaments and voids we described earlier.
• 1:44:54
  Those enormous structures spanning hundreds of millions of light years
• 1:44:58
  could not have formed so quickly after the Big Bang.
• 1:45:03
  The universe was nearly smooth. Ordinary matter alone would not
• 1:45:08
  have had enough gravitational pull to collapse into galaxies and
• 1:45:13
  galaxy clusters fast enough to produce the structures we observe
• 1:45:17
  today within thirteen point eight billion years. The math simply
• 1:45:23
  does not work without additional mass pulling things together. Dark
• 1:45:29
  matter provides that pull. It was already collapsing into filaments
• 1:45:33
  while the universe was still too hot for ordinary matter
• 1:45:36
  to form neutral atoms, giving gravity a head start. By
• 1:45:41
  the time ordinary matter could fall together under gravity, dark
• 1:45:45
  matter had already built the scaffolding galaxies formed inside dark
• 1:45:51
  matter halos, enormous invisible clouds of dark matter that seeded
• 1:45:57
  the locations where visible structure events grew. The most striking
• 1:46:03
  direct evidence came from something called the Bullet Cluster, two
• 1:46:08
  galaxy clusters that collided roughly one hundred fifty million years ago.
• 1:46:14
  When clusters collide, the gas inside them slows down and
• 1:46:19
  piles up from the collision. The dark matter, interacting with
• 1:46:24
  almost nothing, passed through the collision without slowing. Astronomers mapped
• 1:46:29
  the mass distribution of the merged system through gravitational lensing.
• 1:46:34
  The mass was separated from the gas. The gas was
• 1:46:38
  in the middle, the mass was on the outside, where
• 1:46:42
  the dark matter had already coasted through. The two components
• 1:46:47
  had separated in space. That is as close to a
• 1:46:50
  direct observation of dark matter as science has ever produced.
• 1:46:55
  Dozens of candidates have been proposed, weakly interacting massive particles, axioms,
• 1:47:03
  sterile neutrinos, primordial black holes formed in the early universe.
• 1:47:09
  None have been confirmed, none have been ruled out entirely.
• 1:47:14
  The experiments continue. Dark matter is the scaffolding on which
• 1:47:20
  every visible structure in the universe is arranged, and nobody
• 1:47:24
  knows what it is. In nineteen ninety eight, two separate
• 1:47:28
  teams of astronomers looking at distant exploding stars expected to
• 1:47:33
  measure how quickly the universe's expansion was slowing down, and
• 1:47:38
  instead found that it was speeding up. The entire premise
• 1:47:42
  of the observations was built on an assumption so reasonable
• 1:47:46
  it barely qualified as an assumption. Gravity pulls things together,
• 1:47:51
  so the expansion from the Big Bang must be losing momentum.
• 1:47:56
  Every physical instinct pointed that way. Both teams, working independently
• 1:48:03
  and in competition with each other, were using type one
• 1:48:06
  A supernova as measuring tools. These explosions are useful because
• 1:48:13
  they all detonate with roughly the same intrinsic brightness, which
• 1:48:17
  means you can calculate how far away one is by
• 1:48:21
  measuring how dim it appears. The more distant the supernova,
• 1:48:27
  the fainter it looks. The two teams pointed their telescopes
• 1:48:32
  at supernova billions of light years away, expecting the distant
• 1:48:37
  ones to be slightly brighter than a decelerating universe would predict. Instead,
• 1:48:44
  they we redimmer, not by a little. The universe's expansion
• 1:48:50
  had been speeding up, not slowing down. Both teams sat
• 1:48:54
  with that result for a long time before publishing it,
• 1:48:58
  because the implication was deep uncomfortable. Something was pushing space apart,
• 1:49:04
  and nobody had any idea what it was. That something
• 1:49:08
  was given a name, dark energy. The name is essentially
• 1:49:14
  a label for ignorance. It does not describe what the
• 1:49:18
  thing is. It describes what it does. Dark energy drives
• 1:49:23
  the accelerating expansion of the universe, opposing gravity on the
• 1:49:27
  largest scales, and beyond that, the description runs out of
• 1:49:32
  solid ground quickly. Unlike dark matter, which at least behaves
• 1:49:38
  like a substance with mass that clusters around galaxies and
• 1:49:42
  shape structure, dark energy does not appear to clump. It
• 1:49:48
  does not concentrate anywhere. The best evidence currently available suggests
• 1:49:53
  it is not a thing distributed through space at all.
• 1:49:57
  It appears to be a property of space self, a
• 1:50:01
  kind of intrinsic pressure built into the vacuum that does
• 1:50:05
  not weaken. As space expands. More space means more dark energy,
• 1:50:10
  which drives more expansion, which produces more space. The cycle
• 1:50:16
  runs without limit. This makes dark energy deeply unlike anything
• 1:50:20
  else in physics. Normal matter and energy dilute as the
• 1:50:25
  universe grows. Pour the same amount of water into a
• 1:50:29
  larger and larger container, and the concentration drops. Dark energy
• 1:50:35
  does not behave this way. As the volume of the
• 1:50:39
  universe increases, the total amount of dark energy appears to
• 1:50:44
  increase with it. This is what makes the acceleration self sustaining.
• 1:50:50
  It is not running down, it is building. The number
• 1:50:54
  attached to dark energy is sixty eight percent. Sixty eight
• 1:50:58
  percent of the total mass mass energy content of the
• 1:51:01
  observable universe is this invisible, structureless accelerating force that was
• 1:51:08
  not discovered until nineteen ninety eight and is still not
• 1:51:12
  understood in any fundamental sense. The atoms making up every star,
• 1:51:17
  every planet, every galaxy, every structure in the cosmic web
• 1:51:22
  represent about five percent of the universe's content. Dark matter
• 1:51:27
  holds another twenty seven percent. Dark energy holds the remaining
• 1:51:32
  sixty eight percent, and has the distinction of being the
• 1:51:36
  dominant feature of reality while being the thing about which
• 1:51:40
  the least is known. The most common ingredient in the
• 1:51:44
  universe is the one with no confirmed explanation. There are
• 1:51:49
  competing theoretical frameworks for what dark energy might be. Albert Einstein,
• 1:51:55
  in constructing his equations of general relativity in nineteen seventeen,
• 1:52:00
  included a term he called the cosmological constant, a kind
• 1:52:04
  of built in repulsive pressure meant to balance gravity and
• 1:52:09
  keep the universe static. He later called it his greatest blunder.
• 1:52:15
  After Hubble's observations showed the universe was expanding, the cosmological constant,
• 1:52:21
  it turns out, may not have been a blunder at all.
• 1:52:25
  The leading current model treats dark energy as exactly this constant,
• 1:52:31
  a fixed property of space with a value that does
• 1:52:34
  not change over time. Other models proposed that dark energy
• 1:52:39
  is a dynamic field that evolves slowly, stronger in some
• 1:52:44
  epochs than others. These are called quintessence models. So far,
• 1:52:51
  no observation has been able to distinguish clearly between the two,
• 1:52:55
  and the question of which is correct has enormous consequences
• 1:52:59
  for how the universe ends. If dark energy is constant,
• 1:53:05
  the future of the universe is already written. The expansion
• 1:53:09
  continues to accelerate. Galaxies within gravitationally bound groups like the
• 1:53:14
  local group will eventually pull inward and merge, as the
• 1:53:19
  Milky Way and Andromeda are already doing. But galaxies outside
• 1:53:24
  those gravitationally bound islands will recede faster and faster. Eventually
• 1:53:32
  they will cross the cosmic event horizon, the boundary beyond
• 1:53:36
  which recession speed exceeds the speed of light, and their
• 1:53:40
  light will never reach us again. The night sky will
• 1:53:44
  gradually empty in tens of billions of years. Any civilization
• 1:53:49
  still alive in the Milky Way would look outward and
• 1:53:53
  see almost nothing. The rest of the universe would have
• 1:53:57
  vanished behind an expanding wall of unreachable distance. Further still,
• 1:54:04
  stars exhaust their fuel on time scales of billions to
• 1:54:08
  trillions of years, depending on their mass. The smallest red
• 1:54:12
  dwarfs burn so slowly they will last for trillions of years,
• 1:54:16
  but they too will eventually go dark. When they do,
• 1:54:21
  no new stars will ignite, because the gas reservoirs needed
• 1:54:25
  to form them will have thinned past the point of collapse.
• 1:54:29
  The universe will enter a phase sometimes called the degenerate era,
• 1:54:34
  populated largely by cold stellar remnants, white dwarfs cooling toward
• 1:54:39
  absolute zero, neutron stars, and black holes slowly radiating away
• 1:54:45
  their mass over incomprehensible time scales. The cosmic web, those
• 1:54:51
  vast filaments of galaxies that stretch across hundreds of millions
• 1:54:55
  of light years, will long since have dissolved dark na energy,
• 1:55:00
  whatever it turns out to be is steering the universe
• 1:55:04
  toward a future of increasing darkness and isolation, one in
• 1:55:09
  which even the cosmic web we described earlier will eventually
• 1:55:14
  cease to exist. For most of the twentieth century, the
• 1:55:19
  standard estimate held that the observable universe contained somewhere around
• 1:55:24
  two hundred billion galaxies. That number stood for decades. It
• 1:55:30
  was printed in textbooks, cited in papers, repeated in documentaries.
• 1:55:36
  Two hundred billion galaxies. Most people who heard it probably
• 1:55:41
  felt the same thing, that it was already too large
• 1:55:45
  to process, already beyond the reach of ordinary comprehension. It
• 1:55:51
  turned out to be wrong, not slightly wrong, wrong by
• 1:55:55
  a factor of ten. In twenty sixteen, a team led
• 1:55:59
  by Christopher Consulis at the University of Nottingham published a
• 1:56:03
  study that quietly re wrote the census of the observable universe.
• 1:56:09
  The method was careful. They took deep field images from
• 1:56:13
  the Hubble Space Telescope, some of the most detailed photographs
• 1:56:17
  of distant space ever taken, and converted what they saw
• 1:56:22
  into three dimensional maps. What they were looking for were
• 1:56:27
  the galaxies that previous surveys had missed, not because the
• 1:56:33
  telescopes were poorly aimed because the galaxies were too small,
• 1:56:38
  too faint, too far away to show up in any
• 1:56:42
  count we had previously attempted. The early universe was dense
• 1:56:47
  with them, small dim collections of stars that existed in
• 1:56:51
  enormous numbers when the cosmos was young and have since
• 1:56:55
  either merged into larger structures or faded beyond detection. Once
• 1:57:00
  Consalisa's team accounted for those missing populations, the total climbed
• 1:57:05
  from two hundred billion to roughly two trillion. Two trillion.
• 1:57:10
  The number is worth sitting with for a moment, even
• 1:57:13
  knowing that sitting with it will not make it feel
• 1:57:16
  any more real. If you tried to count to two
• 1:57:19
  trillion at the rate of one galaxy per second, without sleeping,
• 1:57:24
  without stopping, the count would take about sixty three thousand years.
• 1:57:30
  That is approximately twelve times the entire recorded span of
• 1:57:34
  human civilization, ticked out one galaxy at a time, and
• 1:57:39
  you still would not be done. Every dynasty that ever
• 1:57:43
  rose and fell, every language that was born and went extinct,
• 1:57:49
  every war and every harvest, and every generation that came
• 1:57:53
  and went. Twelve of those full spans of history back
• 1:57:58
  to back, count what is already out there, Each of
• 1:58:02
  those two trillion galaxies contains stars, not a handful billions
• 1:58:08
  of them in the smaller ones, hundreds of billions in
• 1:58:12
  galaxies like the Milky Way, trillions in the largest ellipticals.
• 1:58:17
  Run that multiplication in any direction, and the output stops
• 1:58:22
  behaving like a number. It becomes a symbol for something
• 1:58:26
  the brain was simply not built to hold. The total
• 1:58:30
  stellar count implied by two trillion galaxies is the figure
• 1:58:35
  that comes up in the next part of this but
• 1:58:38
  even approaching it requires first accepting what the galaxy count
• 1:58:42
  alone means. Here is something worth holding alongside the number.
• 1:58:48
  The galaxies that push the count from two hundred billion
• 1:58:51
  to two trillion are not ancient relics that no longer exist.
• 1:58:57
  They represent what the universe looked like in its earth
• 1:59:00
  early chapters, when matter was still clumping, when the first
• 1:59:04
  structures were still assembling out of gas and dark matter.
• 1:59:09
  The universe was, in some measurable sense, more crowded then.
• 1:59:15
  Many of those small early galaxies have since merged, swallowed
• 1:59:20
  by larger neighbours, folded into the structures we observe today.
• 1:59:25
  The visible universe now contains fewer larger galaxies than it
• 1:59:31
  once did, which means that two trillion is actually a
• 1:59:35
  conservative description of how many distinct galaxies the universe has
• 1:59:40
  ever contained. What the twenty sixteen revision also made plain
• 1:59:46
  was something that cuts directly against the assumption that modern
• 1:59:50
  astronomy had more or less finished mapping the contents of
• 1:59:53
  the observable universe. It had not. The Hubble Space Telescope,
• 1:59:59
  which which had been operating for more than two decades
• 2:00:02
  by then and had produced some of the most celebrated
• 2:00:06
  images in the history of science, was still missing the
• 2:00:10
  majority of galaxies in the observable universe. Most of what
• 2:00:15
  is out there was invisible to it. Not because the
• 2:00:19
  galaxies were beyond the edge of the observable universe, but
• 2:00:23
  simply because they were too dim. They existed within the
• 2:00:28
  region we can, in principle observe and fell below the
• 2:00:32
  threshold of what our instruments could detect. That is, the
• 2:00:36
  state of the count as of the most recent revision,
• 2:00:40
  two trillion is the current best estimate, and the researchers
• 2:00:45
  who produced it were clear that it is almost certainly
• 2:00:48
  still and underestimate. The galaxies too faint for even deep
• 2:00:53
  field imaging to resolve, remain unaccounted for. The James Webb
• 2:00:59
  Space telescope, far more sensitive than Hubble, has already begun
• 2:01:04
  to reveal structures in the early universe that were not
• 2:01:08
  expected and were not visible before. Each improvement in instrumentation
• 2:01:15
  tends to push the count upward, which suggests the final number,
• 2:01:20
  if there is one, is higher still two trillion galaxies,
• 2:01:25
  each containing hundreds of billions of stars, and the revised
• 2:01:29
  count itself is probably still an underestimate because even our
• 2:01:34
  best telescopes cannot see the faintest ones. The number of
• 2:01:39
  stars in the observable universe is estimated at roughly one septilion,
• 2:01:45
  which is a one followed by twenty four zeros. Write
• 2:01:49
  that number out and it looks like this, one quintillion
• 2:01:54
  twenty four zeros. The human brain did not evolve to
• 2:01:59
  process a number that large. It processes it the same
• 2:02:03
  way it processes any number past a few thousand, which
• 2:02:06
  is to say, it stops processing and simply registers the
• 2:02:11
  word enormous and moves on. But the comparison that makes
• 2:02:16
  this number slightly more tangible comes from somewhere much closer
• 2:02:20
  to home. There are roughly seven point five quintillian grains.
• 2:02:25
  Of sand on Earth, not just beach sand, but every desert,
• 2:02:30
  every river bed, every sandbar, every grain at the bottom
• 2:02:35
  of every ocean, all of it together seven and a
• 2:02:39
  half followed by eighteen zeros. That is the total, and
• 2:02:44
  the number of stars in the observable universe exceeds that
• 2:02:48
  figure by a factor of approximately one milliar n. Sit
• 2:02:53
  with that for a moment. Every grain of sand on
• 2:02:56
  every beach you have ever walked on, every hour glass
• 2:03:00
  that has ever been flipped, every castle ever built by
• 2:03:04
  a child and dissolved by a tide, all of that
• 2:03:08
  sand from every coast on every continent is still one
• 2:03:13
  million times fewer than the number of stars sitting in
• 2:03:17
  the observable universe right now. The comparison is supposed to
• 2:03:22
  make the number feel smaller. It doesn't really. What it
• 2:03:27
  does is make the sand feel smaller too, and then
• 2:03:30
  you are left with both quantities feeling equally beyond reach.
• 2:03:35
  The Milky Way alone holds somewhere between one hundred billion
• 2:03:39
  and four hundred billion stars. Estimates vary because enormous sections
• 2:03:45
  of the galaxy are screened behind dust clouds that current
• 2:03:49
  telescopes cannot fully penetrate. But even at the conservative end,
• 2:03:55
  one hundred billion stars in a single galaxy is already
• 2:03:59
  a figure that defies intuition. Now multiply that by two
• 2:04:04
  trillion galaxies, the math produces the Septilian. Each one of
• 2:04:09
  those stars is a sun, not a point of light.
• 2:04:13
  A sun a nuclear furnace burning at millions of degrees,
• 2:04:18
  likely surrounded by its own system of orbiting worlds, shaped
• 2:04:22
  by its own gravity, living out its own life cycle
• 2:04:26
  over millions or billions of years. Most of those stars,
• 2:04:31
  current evidence strongly suggests, have planets. The Kepler Space Telescope
• 2:04:37
  surveyed a small patch of the Milky Way and found
• 2:04:40
  planets so common that astronomers now consider them the default
• 2:04:45
  rather than the exception. Nearly every star it observed appeared
• 2:04:50
  to host at least one scale that rate across one
• 2:04:54
  septilian stars, and the number of planets in the observable
• 2:04:58
  universe climbs into ranges that make the star count look
• 2:05:02
  modest by comparison. Rocky planets, gas giants, ice worlds, ocean
• 2:05:09
  worlds distributed across two trillion galaxies in numbers that again
• 2:05:15
  have stopped being numbers in any useful human sense. Here
• 2:05:21
  is where the scale becomes something stranger than just large.
• 2:05:25
  If life can arise anywhere, If the chemistry that produced
• 2:05:29
  the first self replicating molecules on Earth four billion years
• 2:05:33
  ago is not a miracle but a process, something that
• 2:05:37
  follows from physics and probability given the right conditions, then
• 2:05:42
  a universe containing this many stars, this many planets, this
• 2:05:47
  many billions of years of available time should be teeming
• 2:05:51
  with it, not just life in a few places, life
• 2:05:56
  in staggering abundance. Civilizations arise, developing technology, spreading signals out
• 2:06:03
  into space. The galaxy has been here for thirteen billion years.
• 2:06:09
  If other civilizations followed a trajectory even roughly similar to ours,
• 2:06:14
  some of them had billions of years head start, Their signals,
• 2:06:19
  their structures, their presence should be unmistakable. The sky is quiet,
• 2:06:27
  completely quiet. Decades of listening from early radio telescope surveys
• 2:06:33
  through the modern SETI programs have produced no confirmed signal
• 2:06:37
  of artificial origin, not a whisper, not an anomaly that
• 2:06:42
  held up under scrutiny. The silence is not for lack
• 2:06:46
  of looking. The silence is the data. There are explanations
• 2:06:52
  life might be far rarer than the statistics suggest. Intelligent
• 2:06:57
  life rarer. Still, the window during which a civilization produces
• 2:07:02
  detectable signals might be narrow. Space might simply be too large,
• 2:07:08
  the signals too faint, our instruments too limited, and our
• 2:07:12
  search to brief each of these is plausible. None of
• 2:07:17
  them is settled. What is settled is the count one
• 2:07:22
  septilian stars, planets around most of them, a universe nearly
• 2:07:28
  fourteen billion years old, filled to a degree that no
• 2:07:32
  earlier generation of humans could have calculated or imagined, and
• 2:07:36
  from all of it, across every frequency, across every direction
• 2:07:41
  in the sky, nothing A universe containing one septilian stars,
• 2:07:48
  most of them probably hosting planets, almost certainly harboring conditions
• 2:07:53
  suitable for chemistry, and we have detected not a single signal,
• 2:07:59
  not one can confirmed indication that any of it has
• 2:08:03
  produced anything that thinks the observable universe contains roughly ten
• 2:08:09
  to the power of eighty atoms, a number with eighty
• 2:08:13
  zeros after it, and despite that count being essentially incomprehensible,
• 2:08:19
  the universe is almost completely empty. That figure has a name.
• 2:08:24
  The physicist Arthur Eddington first attempted to estimate it in
• 2:08:29
  the early twentieth century, and the result has carried his
• 2:08:33
  name ever since the Eddington number. It is a count
• 2:08:37
  so large that if you try to write it out,
• 2:08:40
  longhand the zeros alone would fill several pages. And yet
• 2:08:46
  the universe built from all those atoms is, by almost
• 2:08:50
  any measure nothing. Here is what nothing actually looks like.
• 2:08:55
  In numbers, the average density of ordinary matter across the
• 2:08:59
  observable universe works out to roughly six hydrogen atoms per
• 2:09:04
  cubic meter, not six per cubic centimeter, not six per liter.
• 2:09:11
  Six per cubic meter, roughly the volume of a large refrigerator.
• 2:09:16
  A single cubic meter of air in an ordinary room
• 2:09:19
  contains something close to ten septilian atoms. The universe, on average,
• 2:09:25
  is about one trillion trillion times less dense than the
• 2:09:29
  air currently moving through your lungs. The emptiness is not
• 2:09:34
  a feature of deep space alone. It is the dominant
• 2:09:37
  condition of everything that exists. This is hard to hold
• 2:09:42
  in mind because the objects we associate with the universe, stars, planets,
• 2:09:47
  nebulai seems substantial. They feel like things, but zoom in
• 2:09:53
  far enough on any of them and the solidity dissolves.
• 2:09:57
  A hydrogen atom is mostly empty. Spe the nucleus sits
• 2:10:02
  at the center like a marble inside a cathedral, and
• 2:10:06
  the electron orbits somewhere out near the walls. If you
• 2:10:10
  scaled a hydrogen atom up until its nucleus was the
• 2:10:14
  size of a grain of sand, the electron would be
• 2:10:17
  circling roughly half a mile away. Everything between the nucleus
• 2:10:22
  and that electron is vacuum. The matter that seems solid
• 2:10:26
  is nearly all gap. Extend this inward logic outward, and
• 2:10:31
  the picture becomes stranger. Even the densest regions of the universe,
• 2:10:36
  the cares of stars, the hearts of molecular clouds, the
• 2:10:40
  crowded centers of galaxies are thinly populated compared to anything
• 2:10:46
  a laboratory on Earth could call a material. The best
• 2:10:50
  vacuum ever created in a laboratory still contains millions of
• 2:10:54
  atoms per cubic centimeter. The space between stars in the
• 2:10:59
  mid Silky Way contains, on average about one atom per
• 2:11:03
  cubic centimeter. The deep voids between galaxy filaments contain far
• 2:11:09
  less than that. The universe is not a room with
• 2:11:13
  some furniture. It is an enormous empty cathedral with a
• 2:11:18
  few specks of dust drifting through it. What makes this
• 2:11:21
  stranger still is that the atoms themselves, all ten to
• 2:11:26
  the eightieth of them, account for only a fraction of
• 2:11:29
  what the universe contains. Ordinary matter, the kind built from
• 2:11:34
  protons and neutrons and electrons, the kind that makes planets
• 2:11:39
  and people and every object anyone has ever touched, represents
• 2:11:43
  approximately five percent of the universe's total energy content. The
• 2:11:49
  other ninety five percent is dark matter and dark energy,
• 2:11:54
  neither of which is made of atoms at all, neither
• 2:11:58
  interacts with light, neither shows up in any direct measurement
• 2:12:03
  of the kind that produce the Eddington number. The atom
• 2:12:07
  count is a count of five percent. So the universe
• 2:12:11
  contains ten to the power of eighty atoms, and those
• 2:12:15
  atoms are spread across ninety three billion light years of
• 2:12:19
  space at an average density that makes the best laboratory
• 2:12:23
  vacuum look crowded, and the atoms themselves are mostly empty,
• 2:12:28
  And the atoms together represent only five percent of what
• 2:12:33
  the universe actually is. Each layer of that statement makes
• 2:12:38
  the previous one more extreme. The emptiness compounds. There is
• 2:12:44
  something disorienting about arriving at this from the direction of stars.
• 2:12:49
  The previous count one septilian stars each one burning, each
• 2:12:55
  one enormous by human standards, each one surrounded by orbiting worlds,
• 2:13:01
  suggested a universe packed with matter and fire and complexity.
• 2:13:06
  And in one sense it is but one septilian stars
• 2:13:11
  distributed across ninety three billion light years of three dimensional space,
• 2:13:16
  still leaves the overwhelming majority of every cubic meter containing
• 2:13:21
  nothing at all. The fire is real, The fire is
• 2:13:25
  just very, very far apart. The matter that makes up
• 2:13:29
  every star, every planet, every person, and every grain of
• 2:13:34
  dust is five percent of the universe's content, and is
• 2:13:38
  itself almost entirely empty space. The observable universe is not
• 2:13:43
  the universe. Everything described so far, the two trillion galaxies,
• 2:13:50
  the cosmic web, the ninety three billion light year sphere
• 2:13:54
  of space. We can, in principle observe that is only
• 2:13:58
  the portion of reality from which light has had time
• 2:14:02
  to reach us since the Big Bang. It is not
• 2:14:05
  the whole thing. It is the part we can see.
• 2:14:09
  The distinction sounds minor, It is not minor. The universe
• 2:14:15
  beyond our cosmic horizon may dwarf everything inside it, the
• 2:14:19
  way the ocean dwarfs a single drop pulled from its surface.
• 2:14:25
  The boundary exists because of a simple fact, light travels
• 2:14:30
  at a fixed speed. The universe is thirteen point eight
• 2:14:34
  billion years old. This means we can only receive light
• 2:14:39
  from objects whose light has had time to reach us
• 2:14:42
  across that entire span. Anything farther away has had no
• 2:14:46
  time to send us a signal we could yet receive.
• 2:14:50
  This creates a horizon, a hard edge on observable reality,
• 2:14:55
  not because space ends there, but because time runs out.
• 2:15:00
  Beyond that boundary, space continues. Galaxies almost certainly continue. The
• 2:15:07
  geometry of the universe does not stop at the edge
• 2:15:10
  of what we can see. Our vision stops. The universe
• 2:15:14
  does not. Here is where inflation enters. In the nineteen eighties,
• 2:15:21
  physicist Alan Guth proposed that in the first fraction of
• 2:15:25
  a second after the Big Bang, a window so brief
• 2:15:28
  that writing the number out requires more than thirty zeros.
• 2:15:32
  After the decimal point, the universe underwent a period of
• 2:15:36
  exponential expansion, not the steady expansion that has continued ever since,
• 2:15:43
  something far more violent. Space expanded faster than light, doubling
• 2:15:49
  in size again and again in an almost incomprehensibly short time.
• 2:15:55
  A region smaller than a proton was stretched to something
• 2:15:58
  far larger than anything we can observe today, all before
• 2:16:03
  the universe was even a full second old. If inflation happened,
• 2:16:08
  and the evidence from the structure of the cosmic microwave
• 2:16:12
  background strongly suggests it did, then the total size of
• 2:16:16
  the universe is staggering even by the standards we have
• 2:16:20
  been using. Current estimates suggest the entire universe maybe at
• 2:16:25
  least ten to the twenty third times larger than the
• 2:16:29
  observable portion. Write that out. Our observable universe ninety three
• 2:16:36
  billion light years across, containing two trillion galaxies, is smaller
• 2:16:42
  than the full universe by a factor that makes the
• 2:16:46
  word larger feel inadequate. The full universe, if inflation is correct,
• 2:16:52
  is so much bigger than what we can see that
• 2:16:55
  the observable universe inside it is roughly equivalent to a
• 2:16:59
  single atom inside the observable universe itself, the numbers have
• 2:17:05
  stopped working. That is not a rhetorical point. At a
• 2:17:10
  certain scale, the figures become formally meaningless to human cognition.
• 2:17:16
  Ten to the twenty third is a quantity the brain
• 2:17:20
  cannot distinguish from ten to the twenty fourth in any
• 2:17:23
  intuitive sense. Both are simply the word enormous dressed in
• 2:17:29
  different notation. What matters is the direction the math is
• 2:17:34
  pointing bigger, bigger by amounts that have no anchor in
• 2:17:39
  anything a human being has ever experienced or can experience.
• 2:17:44
  The expansion of space that carries distant galaxies beyond our
• 2:17:48
  event horizon applies to the full universe as well. There
• 2:17:52
  are regions of space that were causally connected to our
• 2:17:56
  region at the very beginning, that shared the same initial
• 2:17:59
  can diditions, the same burst of inflation, and that have
• 2:18:03
  since been carried so far away that no signal from
• 2:18:07
  them can ever reach us. Not because of how far
• 2:18:11
  they are in the way we usually mean distance, because
• 2:18:15
  space between us and them is expanding faster than light
• 2:18:19
  can cross it. The light they emit moves toward us
• 2:18:23
  at full speed, the space between us grows faster still
• 2:18:28
  that light will never arrive. Those regions are not distant
• 2:18:33
  in the way that Andromeda is distant. They are permanently,
• 2:18:37
  irrecoverably beyond reach, causially severed, gone, not by destruction but
• 2:18:44
  by geometry. What that means in practice is this, there
• 2:18:49
  are almost certainly stars out there, galaxies out there, entire
• 2:18:54
  structures as complex as the cosmic web we mapped inside
• 2:18:57
  the observable universe, all of it existing, evolving, possibly producing planets,
• 2:19:04
  possibly producing chemistry, and none of it able to send
• 2:19:08
  us any information now or ever. No light, no signal,
• 2:19:14
  no gravitational wave, nothing. The separation is total. Some of
• 2:19:20
  those regions would look if we could somehow see them,
• 2:19:24
  roughly like our own region of space. Inflation stretched the
• 2:19:28
  same initial conditions across all of it. But the physical constants,
• 2:19:34
  the specific arrangements of matter, the history of structure formation
• 2:19:38
  in those regions, all of that evolved independently. We have
• 2:19:44
  no access to the record. We cannot check. The universe
• 2:19:48
  beyond the horizon is not hidden in the way a
• 2:19:51
  distant mountain is hidden by fog. Where you know something
• 2:19:55
  is there and could reach it with enough time and effort,
• 2:19:59
  It is sidden in a way that is final. The
• 2:20:02
  expansion of space has sealed it off as completely as
• 2:20:06
  if it did not exist, except that it does. There
• 2:20:11
  are regions of space that have existed since the beginning
• 2:20:14
  of time that may contain stars and galaxies and possibly life,
• 2:20:20
  that will never be in causal contact with us, not
• 2:20:24
  in a trillion years, not in any amount of time
• 2:20:28
  that the universe has available to offer. Some cosmologists, following
• 2:20:33
  the mathematics of inflation and probability to their logical conclusion,
• 2:20:38
  have arrived at the possibility that the universe we have
• 2:20:41
  just described is not the universe, but merely one universe
• 2:20:47
  among an essentially uncountable number. The idea begins with inflation.
• 2:20:53
  In the first fraction of a second after the Big Bang,
• 2:20:57
  the universe underwent a period of exponential expansion so violent
• 2:21:02
  and so fast that the numbers describing it barely qualify
• 2:21:06
  as numbers. Alan Guth proposed this in nineteen eighty. The
• 2:21:12
  theory was designed to solve specific problems in cosmology, why
• 2:21:17
  the universe looks so uniform in every direction, why space
• 2:21:22
  appears so geometrically flat. Inflations solve those problems elegantly, but
• 2:21:28
  it came with a consequence that Gooth and others did
• 2:21:31
  not initially advertise. If inflation started, the mathematics suggests it
• 2:21:38
  never fully stopped. Not everywhere. Pockets of space would drop
• 2:21:43
  out of the inflationary process at different times, each pocket
• 2:21:47
  expanding into what would look from the inside like a
• 2:21:51
  complete universe. Our universe is one of those pockets. The
• 2:21:57
  process that created it is still running somewhere beyond our horizon,
• 2:22:02
  creating others. This is the inflationary multiverse. The bubble universes
• 2:22:09
  it produces are not copies of each other. Each one
• 2:22:14
  drops out of inflation with its own initial conditions, potentially
• 2:22:19
  its own physical constants. The speed of light, the strength
• 2:22:23
  of gravity, the mass of an electron in another bubble.
• 2:22:28
  None of these have to match what we measure here.
• 2:22:31
  Some bubbles may collapse immediately, some may expand forever, Some
• 2:22:37
  may generate stars, planets, chemistry. Most probably do not. The
• 2:22:44
  range of possible universes is as large as the range
• 2:22:47
  of possible physics, which is to say, we have no
• 2:22:50
  idea how large it actually is. There is a second
• 2:22:55
  version of the multiverse that comes from a different direction. Entirely,
• 2:23:00
  it does not require inflation. It requires only that space
• 2:23:05
  is genuinely infinite, and that matter within it is arranged
• 2:23:10
  in some finite number of configurations. If space goes on
• 2:23:14
  without limit, and if quantum mechanics constrains the possible arrangements
• 2:23:19
  of matter within any finite volume, then every arrangement that
• 2:23:24
  can happen will happen, not eventually, infinitely, many times right
• 2:23:30
  now scattered across an infinite space. This is sometimes called
• 2:23:36
  the level one multiverse. It is in some ways the
• 2:23:41
  most conservative version of the idea, because it requires no
• 2:23:45
  new physics at all. It requires only that the universe
• 2:23:50
  is very, very large. If that is true, then somewhere
• 2:23:56
  separated from us by a distance so large it makes
• 2:23:59
  the observable universe's ninety three billion light years look like
• 2:24:03
  a single step. There is a region of space where
• 2:24:07
  every atom is arranged exactly as it is here, not
• 2:24:11
  approximately exactly. This means a copy of Earth, a copy
• 2:24:16
  of you, a version of this moment playing out in
• 2:24:20
  perfect detail, not as a metaphor or a thought experiment,
• 2:24:24
  as a straightforward consequence of infinite space and finite physics.
• 2:24:29
  The copy has your memories, It is having the same thoughts.
• 2:24:34
  It has no idea. It is a copy, and neither
• 2:24:37
  would you, because there is no original. The mathematics of
• 2:24:42
  infinite universes does not distinguish between them. These ideas sit
• 2:24:46
  in a strange place inside physics. They are not fringe
• 2:24:51
  claims made by people outside the mainstream. They follow from
• 2:24:55
  theories that have substantial evidence behind them, but they are
• 2:24:59
  all so by construction untestable. A bubble universe that lies
• 2:25:05
  beyond our cosmic horizon cannot be observed. A copy of
• 2:25:10
  Earth separated from us by distances that exceed the observable
• 2:25:15
  universe cannot send us a signal. The multiverse makes no
• 2:25:20
  prediction that can be checked against data in any experiment
• 2:25:24
  currently conceivable. Some physicists argue this disqualifies it as science entirely.
• 2:25:33
  Others argue that a theory's testability is separate from its truth,
• 2:25:39
  and that the mathematics pointing toward the multiverse is too
• 2:25:43
  clean to dismiss. The debate has not moved much in decades.
• 2:25:48
  What has moved is the precision of the measurements that
• 2:25:51
  support inflation, and with each improvement, the inflationary multiverse becomes
• 2:25:57
  harder to avoid as a consequence not proven, not confirmed,
• 2:26:03
  but harder to simply wave away. There is also a version,
• 2:26:08
  rooted in quantum mechanics, in which every quantum event that
• 2:26:12
  can go one of several ways goes all of them,
• 2:26:15
  each outcome branching into a separate universe. This is the
• 2:26:20
  many world's interpretation, proposed by Hugh Everett in nineteen fifty seven.
• 2:26:27
  In this picture, the number of universes is not just uncountable,
• 2:26:33
  but growing with every particle interaction everywhere in every existing
• 2:26:38
  universe simultaneously. The scale implied by this version exceeds the
• 2:26:43
  inflationary multiverse in the same way the observable universe exceeds
• 2:26:49
  a single atom. What holds across all versions of this
• 2:26:53
  idea is the same. The observable universe, which already contains
• 2:26:59
  two trillion galaxies and one septilian stars and structures spanning
• 2:27:05
  hundreds of millions of light years, shrinks into something barely
• 2:27:10
  worth measuring when placed against what the multiverse implies. Whether
• 2:27:16
  the multiverse is physics or philosophy, the honest answer is
• 2:27:21
  that the universe we have spent this entire video describing
• 2:27:26
  may be a single atom in something larger. Still, given
• 2:27:30
  the scale of the universe two trillion galaxies, one septilian stars,
• 2:27:36
  thirteen point eight billion years of time, the statistical case
• 2:27:41
  for life elsewhere is not just strong, it borders on
• 2:27:45
  impossible to argue against. The Milky Way alone has been
• 2:27:50
  around for roughly thirteen billion years. Our Solar system is
• 2:27:55
  only four point six billion years old. That gap more
• 2:28:00
  than eight billion years is time in which other civilizations
• 2:28:05
  could have risen built technology, spread across their galaxy and
• 2:28:10
  gone extinct before our sun had even ignited. The raw
• 2:28:14
  ingredients were always there, carbon, nitrogen, oxygen, water. These are
• 2:28:21
  not rare commodities. They are among the most common molecules
• 2:28:25
  in the cosmos. Amino acids have been found inside meteorites.
• 2:28:31
  Complex organic chemistry happens in the frozen gas clouds between stars.
• 2:28:36
  The building blocks of life appear to be written into
• 2:28:39
  the fabric of chemistry itself. The numbers make the silence
• 2:28:43
  harder to explain, not easier. If even one in a
• 2:28:48
  billion stars hosts a planet with life, that is still
• 2:28:52
  a billion life bearing worlds in the Milky Way alone.
• 2:28:56
  If even one in a trillion of those develops something
• 2:29:00
  thing we would call a civilization, the galaxy should still
• 2:29:04
  be full of them. Scale the same math to two
• 2:29:08
  trillion galaxies, and the arithmetic becomes almost surreal. The probability
• 2:29:15
  of Earth being the only world where chemistry ever crossed
• 2:29:18
  the threshold into biology is not just small by any
• 2:29:22
  reasonable estimate, it approaches zero. Yet here we are listening
• 2:29:29
  and the sky says nothing. This is the Faermi paradox.
• 2:29:34
  In the summer of nineteen fifty, the physicist Enrico Fermi
• 2:29:39
  was having lunch with colleagues at Los Alamos when the
• 2:29:42
  conversation turned to the possibility of extraterrestrial life. Fairmi had
• 2:29:48
  been thinking through the numbers. The galaxy is old. Interstellar travel,
• 2:29:55
  while difficult, is not obviously impossible given enough time and
• 2:29:59
  technolog Even at a small fraction of the speed of light,
• 2:30:04
  a civilization with a few million years of technological development
• 2:30:09
  could spread across the Milky Way entirely. Faermi looked up
• 2:30:14
  from his lunch and reportedly asked a question with no
• 2:30:17
  particular drama attached to it, Where is everybody? The question
• 2:30:23
  has been formalized, debated, and examined from nearly every angle
• 2:30:28
  in the decades. Since none of the proposed answers are
• 2:30:32
  fully satisfying, Some are merely uncomfortable, others are genuinely disturbing.
• 2:30:39
  The most optimistic explanation is sometimes called the rare Earth hypothesis.
• 2:30:45
  Life may need conditions so specific the right star, the
• 2:30:50
  right orbit, a large stabilizing moon, a giant outer planet
• 2:30:56
  to deflect incoming asteroids, plate tectonics, a magnetic field of
• 2:31:01
  the right strength that the combination is vanishingly rare Earth
• 2:31:07
  may have won a lottery with extraordinary odds. Complex life
• 2:31:12
  on this view, is not an inevitable product of chemistry,
• 2:31:17
  but an accident that almost never happens. The universe would
• 2:31:21
  then be full of sterile rocks and the occasional single
• 2:31:25
  celled organism drifting in a subsurface ocean somewhere, but nothing
• 2:31:31
  that builds radio telescopes. A second class of explanations accepts
• 2:31:36
  that life is common, but argues that technological civilizations destroy
• 2:31:41
  themselves before becoming detectable. A species that evolves to the
• 2:31:46
  point of building machines is the same species that evolves tribalism, competition,
• 2:31:53
  and the capacity to develop weapons capable of ending its
• 2:31:56
  own existence. The time between discovering radio and discovering nuclear
• 2:32:02
  weapons for US was roughly fifty years. If that window
• 2:32:08
  closes violently for most civilizations, the galaxy could be littered
• 2:32:13
  with ruins, worlds that reached for the stars and burn
• 2:32:18
  themselves out before sending a signal anyone could hear. Then
• 2:32:24
  there is the explanation that tends to sit in the
• 2:32:27
  back of the room and make everyone slightly uneasy, The
• 2:32:32
  Great Filter. The idea proposed by economist Robin Hanson is
• 2:32:37
  that somewhere along the chain from simple chemistry to interstellar civilization,
• 2:32:43
  there is a step that almost nothing gets past, a
• 2:32:47
  barrier so difficult that the overwhelming majority of life, wherever
• 2:32:52
  it arises, fails to cross it. The unsettling question is
• 2:32:58
  where that filter sits. If it is behind us, If
• 2:33:02
  the hard step was the emergence of complex cellular life,
• 2:33:06
  or multicellular organisms, or something else that Earth happened to navigate,
• 2:33:13
  then the silence is expected and the future is open.
• 2:33:18
  If the filter is ahead of us, if there is
• 2:33:21
  something that consistently destroys civilizations at a stage of development
• 2:33:26
  we have not yet reached, then the silence is a warning,
• 2:33:30
  and the fact that we see no evidence of any
• 2:33:33
  civilization anywhere in the observable universe becomes the most frightening
• 2:33:38
  data point in science. We have been pointing radio telescopes
• 2:33:43
  at the sky since nineteen sixty Project Osma SETI, the
• 2:33:48
  Allan telescope array, decades of listening across millions of frequencies.
• 2:33:54
  One candidate signal, the WOW signal in nineteen seventy seven,
• 2:34:00
  lasted seventy two seconds and was never repeated. Nothing else
• 2:34:04
  has come close. A universe containing thirteen point eight billion
• 2:34:09
  years of history, two trillion galaxies, and a septilian stars,
• 2:34:15
  has produced, as far as we can tell, one species
• 2:34:19
  capable of asking The question. Whether that is because we
• 2:34:23
  are first, because we are alone, or because something keeps
• 2:34:28
  the answer from being found is a question the data
• 2:34:32
  has not resolved. Enrico Fermi asked where everybody was in
• 2:34:37
  nineteen fifty, and the universe has offered nothing since then
• 2:34:42
  that constitutes an answer. After all of that, the question
• 2:34:47
  that tends to sit in the background through most of
• 2:34:49
  this is the one that is hardest to answer. What
• 2:34:54
  does any of it mean? The numbers do not lead
• 2:34:57
  to meaninglessness. That conclusion requires a step the math never
• 2:35:02
  actually takes. What the scale of the universe demands is
• 2:35:06
  not despair, but a different kind of accounting, a more
• 2:35:11
  honest one. The atoms in your body were not always yours.
• 2:35:17
  They were forged under pressures and temperatures that no laboratory
• 2:35:21
  on Earth has ever reproduced, inside stars that burned for
• 2:35:26
  millions of years and then tore themselves apart in explosions
• 2:35:31
  bright enough to outshine entire galaxies. The calcium in your
• 2:35:36
  bones was made in a dying star. The iron in
• 2:35:40
  your blood was scattered across space in a supernova before
• 2:35:44
  gravity pulled it back together into a new solar system,
• 2:35:48
  then a planet, then eventually into you. You did not
• 2:35:53
  arrive in the universe from somewhere else. You assembled from it.
• 2:35:58
  The universe did not produce you as a side effect.
• 2:36:02
  It produced you as a continuation. This does not make
• 2:36:06
  every life equally vast. It does not flatten the difference
• 2:36:11
  between a century and a billion years. What it does
• 2:36:16
  is place you inside the story, rather than outside it.
• 2:36:21
  Most people hearing numbers like ninety three billion light years
• 2:36:26
  or one septilian stars feel a kind of shrinking. The
• 2:36:31
  self gets small against those figures. But the atoms that
• 2:36:35
  make up the self have already crossed those distances, already
• 2:36:40
  lived inside stellar cause, already survived the deaths of worlds
• 2:36:45
  older than our sun. The shrinking is an illusion built
• 2:36:49
  by a brain that evolved to track distances measured in
• 2:36:53
  footsteps and years measured in harvests. The universe operates on
• 2:36:59
  scale we were not built to feel. Intuitively, that is
• 2:37:03
  not indifference. That is just physics. A star does not
• 2:37:08
  know it is burning. A supernova does not know it
• 2:37:12
  is seeding the next generation of solar systems. Somewhere in
• 2:37:17
  the gap between a universe that does not know and
• 2:37:20
  a mind that does, something remarkable happened. The universe began
• 2:37:26
  to observe itself. That is not poetry. That is a
• 2:37:31
  description of what actually occurred when chemistry became biology and
• 2:37:36
  biology became thought. On a clear night, far enough from
• 2:37:41
  city lights, the sky opens in a way that city
• 2:37:44
  dwellers rarely see. Every point of light up there is
• 2:37:48
  a sun, not a lamp, not a symbol, not a
• 2:37:52
  pixel in a screen. A sun, most of them larger
• 2:37:57
  than ours, most of them old enough to have hosted
• 2:38:00
  planets for billions of years before our solar system formed.
• 2:38:05
  When you look at a star one thousand light years away,
• 2:38:09
  the light entering your eye left that star roughly one
• 2:38:12
  thousand years ago. It has been crossing space since before
• 2:38:16
  any living person's earliest ancestor was born. It crossed the
• 2:38:21
  entire distance of the observable Milky Way neighborhood, threading through
• 2:38:25
  vacuum and gas and dust to land in your eye
• 2:38:29
  on a specific night, in a specific place, at the
• 2:38:33
  specific moment you happen to look up. That is what
• 2:38:38
  you are doing. When you look at the night sky.
• 2:38:41
  You are receiving messages that have been in transit for centuries.
• 2:38:46
  You are watching a universe so large that we ran
• 2:38:50
  out of useful numbers for it, while still inside the
• 2:38:54
  observable portion, while still unable to see ninety seven percent
• 2:38:59
  of what almost certainly exists beyond our horizon every clear night.
• 2:39:06
  If you find a dark sky and look up, you
• 2:39:08
  are looking at ancient light from a cosmos so large
• 2:39:12
  that the number describing its size stopped meaning anything useful
• 2:39:17
  long before you finished reading it, and it has been
• 2:39:20
  there waiting for you to look the whole time