Latest / Boring Science / Why We Can Never Escape the Great Attractor

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Why We Can Never Escape the Great Attractor · Boring Science

• 0:00
  Speaker 1: Something is pulling our entire galaxy toward it at one
• 0:03
  point three million miles per hour, and no one alive
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  has ever felt it. Not a tremor, not a pull,
• 0:11
  nothing that registers in any instrument you have ever touched,
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  or any sense you were born with. Right now, every
• 0:20
  star in the Milky Way, every planet orbiting those stars,
• 0:25
  every ocean and mountain and living thing on this particular
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  rock is hurtling towards something enormous, at a speed that
• 0:33
  makes every human achievement in propulsion look like standing still.
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  A commercial jet moves at roughly six hundred miles per hour.
• 0:44
  The fastest spacecraft ever launched by human hands peaked at
• 0:49
  around four hundred thirty thousand miles per hour. The motion
• 0:54
  carrying our entire galaxy toward this object is three times
• 0:59
  faster than them that, and nobody feels it. Nobody has
• 1:04
  ever felt it. If you find comfort in these gentle
• 1:08
  explorations of space and science, I'd be grateful if you'd
• 1:12
  consider subscribing and giving this video alike. The object doing
• 1:22
  the pulling does not have a clean edge. It is
• 1:25
  not a star, not a black hole, not a single galaxy.
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  It is a region of space, a gravitational basin so
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  massive that its influence stretches across hundreds of millions of
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  light years in every direction, bending the paths of more
• 1:43
  galaxies than anyone has fully counted. Astronomers gave it a
• 1:48
  name that has stuck for nearly four decades. They called
• 1:53
  it the Greater Tractor. The name is accurate in a
• 1:57
  way that most scientific names are not. It attracts. There
• 2:02
  is not much else to say about it that does
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  not immediately become difficult to believe. Here is what makes
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  the situation strange. We cannot see it directly. The Greater
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  Tractor sits behind one of the most impenetrable obstructions in
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  observational astronomy, hidden by the thick dust and gas of
• 2:22
  our own galaxies. Disc We can measure its gravitational fingerprint
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  on everything around it. We can trace the motion it
• 2:31
  produces across enormous volumes of space. We can calculate roughly
• 2:37
  how massive it must be. But pointing a telescope at
• 2:41
  it and getting a clean image, the way you would
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  point a camera at a mountain that has never been
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  fully possible, we are being pulled towards something we cannot
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  look at directly, and we will never arrive. This is
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  the other piece of the situation that takes a moment
• 2:59
  to settle. The universe is expanding. The space between us
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  and the Greater Tractor is growing, even as our galaxy
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  races toward it. Dark energy, the force driving that expansion,
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  is winning. On the largest scales. Our approach speed and
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  the recession speed of the Greater Tractor are in a race,
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  and the expansion is accelerating. Billions of years from now,
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  our galaxy will still be falling toward the Greater Tractor.
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  The distance will never reach zero. We are permanently in
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  transit toward a destination that permanently recedes. This is not
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  a minor footnote in cosmology. It is a description of
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  where we actually live, what our actual address in the
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  universe implies about our motion through it. The Greater Tractor
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  is the gravitational center of a supercluster containing more than
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  one hundred thousand galaxies. The Milky Way is one of them.
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  Everything in that supercluster is draining slowly toward the same point,
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  like water pulling toward a drain that cannot be reached
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  because the floor is tilting away at the same rate.
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  The water flows. The story of how astronomers discovered this,
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  how they pieced together the evidence from peculiar velocities and
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  cosmic background radiation and eventually managed to peer through the
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  obscuring disk of our own galaxy, is one of the
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  more methodical detective stories in modern science. It began in
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  Earnest in nineteen eighty six with a group of seven
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  researchers who set out to map galaxy motions and ended
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  up finding something that changed the scale of the known universe.
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  The picture gets stranger the deeper you go, and the
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  first place to dart is understanding why our galaxy refuses
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  to hold still. The universe is expanding. Every galaxy you
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  can name is moving away from every other galaxy. Not
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  because galaxies are flying through space like shrapnel from an explosion,
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  but because the space between them is growing the distance
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  itself stretches. Think of dots drawn on a balloon being inflated.
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  None of the dots moves across the surface, but every
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  dot gets farther from every other dot as the rubber expands.
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  This is the standard picture, and for most of the
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  universe it is accurate enough. But the Milky Way is
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  not behaving. The way the balloon model predicts. On top
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  of the expansion, our galaxy has an extra motion, a
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  motion that has nothing to do with the universe growing,
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  A motion that points in a specific direction toward a
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  specific region of sky at a speed that becomes genuinely
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  difficult to process once you sit with the numbers. That
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  extra motion is called a peculiar velocity, and the Milky
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  Ways is roughly six hundred kilometers per second, not per
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  hour per second. Every second that passes our entire galaxy
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  has moved six hundred kilometers closer to whatever is pulling it.
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  Every star in the galaxy, every planet around, every star,
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  every ocean, every mountain range, every person who has ever
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  drawn a breath on this world, all of it moving
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  together silently at six hundred kilometers per second, and not
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  one of us has ever felt it. To feel a motion,
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  you need acceleration or friction. Something has to push against you.
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  The Milky Way's peculiar velocity is smooth and constant enough
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  that there is nothing to feel, no wind, no g force,
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  no sensation whatsoever. You are currently aboard something the size
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  of a galaxy, crossing distances that would take light thousands
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  of years to cover, and your body has no mechanism
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  to register any of it. This is one of those
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  facts that sound straightforward when written down, and becomes quietly
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  unsettling the longer you think about it. Consider what six
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  hundred kilometers per second actually means in human terms. The
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  fastest crude spacecraft ever launched, the Apollo ten command module,
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  on its return from the Moon, reached about eleven kilometers
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  per second. That speed felt like an achievement. It was
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  the fastest any human beings had ever traveled. The Milky
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  Way's peculiar velocity is more than fifty four times faster
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  a commercial aircraft covers roughly two hundred fifty meters per second.
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  The galaxy is moving two thousand, four hundred times faster
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  than that. In the five seconds it took you to
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  read this paragraph, our galaxy has crossed roughly three thousand kilometers.
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  Not our solar system, the entire galaxy, all one hundred
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  thousand light years of it, dragged along for the ride.
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  Now hold the direction in mind. This is not a
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  random drift. Peculiar velocities can, in principle, point anywhere. Random
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  thermal motion in a gas produces velocities that point in
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  every direction equally canceling out. The Milky Way's motion does
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  not cancel out with the motion of its neighbors. The
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  galaxies surrounding hours are moving in roughly the same direction.
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  The galaxies farther out are moving in the same direction.
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  The peculiar velocities of galaxies across a region spanning hundreds
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  of millions of light years all share a common component,
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  a shared drift toward a single region of sky that
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  is not random, that is, gravity operating on a scale.
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  The textbooks of the mid twentieth century had not accounted
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  for the idea that this motion even exists required careful
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  measurement to establish. If you simply look at a galaxy
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  through a telescope and measure how fast it is receding,
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  you get a combination of two things, the expansion of
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  space and the peculiar velocity. Separating the two requires an
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  independent distance measurement, something that does not depend on how
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  fast the galaxy is moving. Without that, you cannot tell
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  whether a galaxy is farther away because the universe is bigger,
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  or because it happens to be moving away from you.
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  When astronomers developed reliable ways to measure distances, independently. They
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  found residuals in the velocity data. Galaxies were not exactly
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  where the expansion model said they should be. The leftover
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  motion after subtracting the expansion was not random noise. It
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  was a systematic signal something coherent was happening across an
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  enormous volume of space. Here is the picture as it stood.
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  By the early nineteen eighties, astronomers had accumulated enough data
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  to see that the Milky Way, the local group, the
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  surrounding clusters, and galaxies stretching out for roughly three hundred
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  million light years in a particular direction, were all sharing
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  a common extra velocity, all the same speed. Objects at
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  different distances and angles showed different magnitudes, but all pointed
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  more or less toward the same region of sky. The
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  pattern was consistent with a gravitational source. Something massive enough
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  to exert a measurable pull across a volume of space
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  that large must have been extraordinary. The Milky Way alone
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  contains roughly a trillion solar masses. Whatever was doing the
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  pulling had to dwarf that figure by an enormous margin
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  to exert this kind of influence acra of such distances.
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  No known structure at that time accounted for it. The
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  Virgo Cluster, the nearest large galaxy cluster to the Milky Way,
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  was a known gravitational influence, but the Virgo Cluster was
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  not nearly large enough to explain a bulk flow of
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  this magnitude. The numbers pointed somewhere else, somewhere farther, somewhere,
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  in a direction that would turn out to be one
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  of the most frustrating regions of the sky for an
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  astronomer to study. Something was out there, something massive and unseen,
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  and it was pulling on everything within hundreds of millions
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  of light years with a grip that nobody had predicted.
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  In nineteen eighty six, a team of seven astronomers did
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  something that sounds straightforward on paper and turned out to
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  change the way we understand the architecture of the universe.
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  Their names were David Berstein, Roger Davies, Alan Dressler, Sandra Faber,
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  Donald Lynden Bell, Roberto Turlovich, and Gary Wegner. They would
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  come to be called the Seven Samurai, not by themselves
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  but by the astronomical community that followed their work with
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  increasing disbelief. What they set out to do at least
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  on the surface was measure distances not to a handful
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  of galaxies, not to a few dozen, but to four
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  hundred of them, elliptical galaxies specifically scattered across an enormous
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  swath of the sky. The ambition of this alone was
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  enough to raise eyebrows in nineteen eighty six. Measuring distances
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  to four hundred galaxies was not a weekend project. It
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  was years of observing time, years of data reduction, years
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  of arguing over methodology in rooms full of people who
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  disagreed about almost everything except there the importance of getting
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  this right. The method they used is called the d
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  N Sigma relation. The name is less important than what
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  it does. It exploits a physical property of elliptical galaxies
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  that turns out to be surprisingly reliable. The size of
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  an elliptical galaxy, measured in a specific way, correlates tightly
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  with the speed at which stars inside it are moving.
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  Fast moving stars in a large, bright galaxy slower moving
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  stars in a smaller, dimmer one. Because this relationship holds
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  across elliptical galaxies, generally, if you measure how fast the
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  stars in a galaxy are moving, you can calculate how
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  physically large the galaxy must be. And if you know
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  how large it actually is, and you compare that to
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  how large it appears in the sky, you can work
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  out how far away it is distance without needing to
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  know anything about how f as the galaxy is moving
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  through space. This is the key. The Dn Sigma relation
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  gives you an independent distance estimate. It does not care
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  about redshift. It does not assume anything about the galaxy's motion.
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  It just uses the physics of the stars inside it
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  to tell you how far away you are. This matters
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  enormously because of what you do next. Once you have
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  an independent distance, you measure the galaxy's redshift separately. Redshift
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  tells you how fast the galaxy appears to be moving
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  away from you. Combining the expansion of the universe with
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  any additional motion the galaxy has on top of that,
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  subtract the expansion, and what is left over is the
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  peculiar velocity, the part of the motion that cannot be
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  explained by the universe simply growing. That remainder is what
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  the seven Samurai were after, not the expansion, the deviation
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  from it. Four hundred galaxies, each one a data point,
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  each one carrying inside it a small signal about whether
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  it was moving faster or slower toward or away from
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  the observer than pure expansion would predict. The work took years.
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  Observing time on major telescopes had to be scheduled and
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  fought for. Data had to be collected at multiple facilities
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  across multiple observing seasons. Measurements had to be cross checked
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  against each other. The DN sigma relation, while reliable, introduced
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  its own uncertainties, and those uncertainties had to be understood, quantified,
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  and accounted for before any conclusions could be trusted. There
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  was also the question of Malmquist bias, a statistical artifact
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  that creeps into galaxy surveys when brighter objects are preferentially
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  included in a sample, skewing the results in ways that
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  can look like real physical signals. The Seven Samurai knew
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  about this, correcting for it was not straightforward. When they
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  finally compiled the results and plotted the peculiar velocities of
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  their four hundred galaxies across the sky, something stopped them.
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  The data was not random. It was not the scattered
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  noise you would expect if the galaxies were simply jostling
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  around in isolated groups, each doing its own thing. The
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  peculiar velocities had a pattern, a coherent one, across an
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  enormous volume of space, hundreds of millions of light years across.
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  Galaxies were moving in the same direction, not every single one,
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  and not in a perfectly uniform way, but the signal
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  was there, and it was unmistakable, a bulk flow an
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  entire region of the universe drifting in concert toward a
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  specific point on the sky. The velocities they measured ranged
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  from roughly positive seven hundred kilometers per second to negative
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  seven hundred kilometers per second, depending on where in the
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  sky you looked relative to that convergence point. Galaxies on
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  the near side of the structure between us and whatever
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  was pulling everything were moving away from us faster than
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  expansion alone could account for. Galaxies on the far side
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  were moving toward us, slowed by the pull of the
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  same source. The pattern was the signature of a gravitational well,
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  something massive sitting at a specific location, pulling everything in
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  its neighborhood toward it, The mathematical signature of what you
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  would expect if there were a concentration of mass enormous
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  enough to do storught. The velocity field of galaxies across
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  hundreds of millions of light years. They gave it a name,
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  the Great Attractor. The name was partly a description and
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  partly an admission of ignorance. They could not see it.
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  They could not point to a single galaxy or cluster
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  and say, there, that is the thing doing this. What
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  they had was a velocity field, a pattern in how
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  galaxies were moving, pointing like a compass needle toward a
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  region of sky. The name reflected what the data required,
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  something attracting on a great scale, with enough mass to
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  pull the known universe around it like a slow tide.
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  Something was out there in the direction of the constellation's
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  normal and triangular nostrala, behind the obscuring disk of our
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  own galaxy, and it was massive enough to set hundreds
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  of thousands of galaxies in motion toward it simultaneously. The
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  paper the seven Samurai published landed in the astronomical community
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  like a stone dropped into still water. Some colleagues were
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  skeptical the systematic uncertainties in the d N sigma method
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  were real, and a result this dramatic invited scrutiny, But
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  the data held up under repeated examination, and the debate
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  shifted from whether the bulk flow was real to what
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  on Earth could be causing it. They had found the
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  gravitational engine behind the bulk flow, given it a name,
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  and immediately run into the problem that would define greater
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  tractor research for the next decade. They could not see it.
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  Every galaxy in the universe is doing two things at once.
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  The first thing is straightforward. The universe is expanding, and
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  every galaxy is being carried outward with it. This is
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  not motion through space in any ordinary sense. It is
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  space itself, stretching, the way dots painted on a balloon
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  move apart from each other. When you blow it up.
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  The fartherer galaxy is from us, the faster it appears
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  to recede. This relationship was mapped by Edwin Hubble in
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  the late nineteen twenties, and it holds across the entire
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  observable universe. It is called the Hubble flow, and for
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  a long time astronomers treated it as the dominant fact
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  about how galaxies move. The second thing is the one
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  that caused decades of confusion. On top of this expansion,
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  galaxies have their own individual motions, small deviations from the
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  smooth Hubble flow caused by the gravitational pull of nearby
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  mass concentrations. These deviations are called peculiar velocities. The word
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  peculiar here is used in its older scientific sense, meaning
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  particular or specific, not strange, but given what these velocities revealed,
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  strange would not be wrong either. Measuring a peculiar velocity
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  requires two separate measurements. First, you need to know how
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  fast a galaxy appears to be moving away from you,
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  which comes from its red shift, the stretching of light
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  caused by recession. That number is easy to get. Second,
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  you need an independent measurement of how far away the
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  galaxy actually is, so you can calculate how fast it
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  should be receding based on Hubble expansion alone. The difference
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  between what you measure and what expansion predicts is the
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  peculiar velocity. Get the distance wrong, and the peculiar velocity
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  you calculate is fiction. This is why the Seven Samurai's
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  careful work with elliptical galaxy distances mattered. They were trying
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  to get the second number right. When they did, the
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  pattern was unmistakable. Galaxies across a vast region of space
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  were not moving in random directions. They were streaming. Depending
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  on where in the sky you looked, the peculiar velocity
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  of a galaxy might be plus seven hundred kilometers per second,
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  meaning it was moving toward a common point faster than
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  expansion predicted, or minus seven hundred kilometers per second, meaning
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  it was lagging behind dragged back against the flow. The
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  range ran from roughly negative to positive seven hundred kilometers
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  per second, and the direction of that range was not random.
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  It was organized. It pointed somewhere. Now, how do you
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  know this pattern is real and not just noise in
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  your measurements. This is where the cosmic microwave background becomes indispensable.
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  The CMB is the oldest light in the universe. About
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  three hundred eighty thousand years after the Big Bang, the
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  universe cooled enough for electrons and protons to combine into
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  neutral hydrogen, and suddenly the universe became transparent. The light
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  released at that moment has been traveling ever since. Stretched
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  by expansion into microwave wavelengths, it fills all of space equally.
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  From any truly stationary point in the universe, the CMB
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  should look almost perfectly uniform in every direction. The same
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  temperature of about two point seven kelvin no matter where
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  you aim your detector, but if you are moving, the
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  CMB does not look uniform. It looks very slightly warmer
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  in the direction you are traveling toward, and very slightly
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  cooler in the direction you are traveling away from. This
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  is the Doppler effect applied to the after glow of
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  the Big Bang itself. The effect is tiny. We are
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  talking about temperature differences measured in thousandths of a degree.
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  But it is real, and it is measurable, and it
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  tells you something precise. It tells you your velocity relative
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  to the rest frame of the early universe. When astronomers
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  measured the CMB with enough precision to detect this asymmetry,
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  the result was striking. The Milky Way has a net
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  motion of roughly six hundred kilometers per second in a
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  specific direction relative to to the CMB rest frame. One
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  side of our sky tests fractionally warmer, the other side
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  tests fractionally cooler. The CMB is not telling us about expansion.
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  Expansion is uniform and cancels out. It is telling us
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  about something else. It is telling us that our entire
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  galaxy has an additional velocity a peculiar velocity, pushing it
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  toward a particular region of space. This gave astronomers something
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  more powerful than a map of galaxy velocities. It gave
• 26:38
  them a reference. The CMB is the universe's own coordinate system,
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  a fixed backdrop against which any motion can be measured cleanly.
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  And against that backdrop, the Milky Way is moving fast,
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  and it is not moving alone. The galaxy clusters the
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  seven Samurai had surveyed were also moving, all in roughly
• 27:03
  the same direction, their peculiar velocities adding up to a
• 27:07
  coherent bulk flow across hundreds of millions of light years.
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  The signal was not in one galaxy or two galaxies.
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  It was in a region of space so large that
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  the only explanation was something massive sitting at the far
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  end of it, pulling on everything within range. Something was
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  generating that pull. The numbers pointed to a mass concentration
• 27:34
  enormous enough to drag galaxies across hundreds of millions of
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  light years, accelerating everything in its basin toward the same
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  gravitational sink. The CMB confirmed that the motion was real.
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  The galaxy surveys confirmed that it extended across a staggering
• 27:53
  volume of space. The fingerprint was there, clear and organized,
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  repeating at every scale. Astronomers could measure the direction this
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  motion pointed. The source of the gravity driving hundreds of
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  millions of light years of galaxies toward the same spot
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  was sitting directly behind. The worst possible obstruction in the
• 28:18
  observable sky. The most inconveniently placed obstacle in observational astronomy
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  is not a distant nebula or a far off galaxy cluster.
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  It is our own galaxy. When the Seven Samurai published
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  their results in nineteen eighty six and pointed at a
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  region of sky and said, something enormous is there, the
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  first instinct of the astronomical community was to look, point
• 28:47
  a telescope in that direction, find out what is doing
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  the pulling. This is normally how science works. You identify
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  a gravitational effect, you trace it back to its source,
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  and you look at the source directly. What happened instead
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  was that astronomers pointed their telescopes toward the greater tractor
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  and ran directly into a wall of their own galaxy.
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  The Milky Way is not a transparent structure. From above,
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  it would look like a flat disc of stars and
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  gas spiraling outward from a dense central bulge. From inside,
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  which is where we are, it looks like a bright
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  band of light cutting across the night sky. That band
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  is beautiful. It is also for observational astronomy something close
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  to catastrophic when the object you need to study happens
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  to sit behind it. The disc of the Milky Way
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  is packed with interstellar dust. These are not the dust
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  bunnies under a sofa. These are vast clouds of microscopic
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  solid particles, silicates, and carbon co distributed throughout the galactic
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  plane in quantities thick enough to block visible light almost completely.
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  When light from a distant galaxy tries to pass through
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  this material, it does not make it through. The photons scatter,
• 30:18
  they absorb, they disappear into the cloud. By the time
• 30:23
  a ground based optical telescope looks toward the galactic plane,
• 30:28
  it is staring at a ceiling with no way to
• 30:30
  see past it. Astronomers call this region the zone of avoidance.
• 30:37
  The name is accurate. For most of the history of
• 30:40
  extra galactic astronomy, surveys simply skipped this part of the sky.
• 30:46
  There was no point including it. Anything behind the disc
• 30:50
  was invisible. Galaxy catalogs developed holes in the shape of
• 30:55
  the Milky Way's plane, blank regions where the data simply
• 30:59
  st This was accepted as a limitation and mostly ignored
• 31:05
  because the galaxies visible in the unobscured sky provided enough
• 31:09
  science to occupy generations of researchers. Then the greater tractor
• 31:15
  was discovered, and the zone of avoidance stopped being a
• 31:19
  minor inconvenience and became the central problem in an entire field.
• 31:25
  Think of it this way. Imagine you are trying to
• 31:28
  find the source of a sound that is clearly coming
• 31:31
  from somewhere in a fog bank. You can hear it
• 31:34
  on your instruments. You can measure how the sound bends
• 31:38
  the motion of everything around it. You can map with
• 31:43
  precision the direction the sound is coming from. But the
• 31:47
  fog stretches across exactly the region you need to look into,
• 31:52
  and you cannot see anything in it at all. You
• 31:55
  can tell from the pattern of deflections around the fog
• 31:58
  that the source is somewhere in the middle of it.
• 32:02
  You just cannot get your eyes on it. That is
• 32:05
  what astronomers were dealing with after nineteen eighty six. The
• 32:10
  gravitational fingerprint of the Greater Tractor was unmistakable. Its direction
• 32:16
  was known, its approximate distance was estimated, But the region
• 32:22
  of sky it occupied was sitting directly inside the zone
• 32:26
  of avoidance, hidden behind roughly ten to twenty degrees of
• 32:31
  latitude on either side of the galactic plane, a band
• 32:36
  covering something close to twenty percent of the entire sky
• 32:41
  in the specific direction of the Greater Tractor. The obscuration
• 32:46
  was severe enough that entire galaxy clusters could be sitting
• 32:51
  in plain sight and never appear in any optical catalog.
• 32:57
  This was not a minor gap. Researchers throughout the late
• 33:01
  nineteen eighties and into the nineteen nineties worked to map
• 33:05
  the surrounding territory. They measured galaxy velocities in the regions
• 33:11
  flanking the zone of avoidance. They traced the flow lines.
• 33:15
  They built increasingly detailed models of what the gravitational source
• 33:20
  must look like, based entirely on the distortions it was
• 33:24
  producing in the galaxies around it. The models pointed consistently
• 33:29
  to the same location. The mass concentration had to be there,
• 33:35
  in the zone in the gap, in the one patch
• 33:39
  of sky. Their instruments could not reach. What made this
• 33:44
  particularly maddening is the asymmetry of the situation. Astronomers were
• 33:50
  not trying to study a small, faint, marginal object. They
• 33:55
  were trying to study what appeared to be the dominant
• 33:58
  gravitational structure within high hundreds of millions of light years,
• 34:02
  the engine driving the bulk motion of our entire cosmic neighborhood.
• 34:08
  By any measure, this was the most important thing in
• 34:11
  the local universe to understand, and it was hiding behind
• 34:16
  the one obstacle nobody had thought to prepare for. Because
• 34:20
  nobody had anticipated that the target would end up precisely there,
• 34:27
  some researchers attempted optical workarounds. They searched for windows in
• 34:32
  the dust gaps where the obscuration was thinner, places where
• 34:37
  careful observation might allow a partial view through the zone.
• 34:42
  A few objects were found this way. They helped build
• 34:46
  a partial picture, but the core of the greater tractor
• 34:49
  region remained stubbornly out of reach for conventional telescopes. The
• 34:55
  problem was not going to be solved by looking harder
• 34:58
  in visible light. The only path through was to stop
• 35:02
  using instruments designed for visible light and start looking at
• 35:07
  the universe in a completely different way. X rays cannot
• 35:12
  be stopped by the same material that makes the zone
• 35:15
  of avoidance impenetrable to visible light. The dust and gas
• 35:20
  clogging the Milky Way's disc are enormously effective at absorbing
• 35:24
  and scattering optical photons, the kind of light human eyes
• 35:28
  are built to detect, the kind that ground based telescopes
• 35:32
  had relied on for centuries. But X rays are a
• 35:36
  different class of radiation entirely. They carry far more energy.
• 35:42
  They punch through interstellar dust the way hard radiation punches
• 35:47
  through thin cloth. When astronomers began putting X ray detectors
• 35:52
  in orbit above Earth's atmosphere, where X rays can actually
• 35:57
  be detected, they were and did something that optical astronomers
• 36:02
  in the nineteen eighties could only dream about, a way
• 36:07
  to look through the obstruction that had been hiding the
• 36:10
  Greater tractor for decades. This was not an overnight development.
• 36:15
  X ray satellites had to improve considerably before they could
• 36:19
  map feint distant galaxy clusters in a region as crowded
• 36:24
  and obscured as the Greater Tractor direction. Early instruments lacked
• 36:30
  the resolution and sensitivity needed to pick individual clusters out
• 36:34
  of the noise. But by the late nineteen nineties the
• 36:39
  technology had advanced enough to make systematic surveys worthwhile. A
• 36:44
  dedicated program called the Clusters in the Zone of Avoidance
• 36:48
  Project CIZA set out to map exactly what was hiding
• 36:53
  behind the galaxy's disc The goal was specific find galaxy clusters,
• 37:00
  catalog their positions and rough masses, build a picture of
• 37:04
  the large scale structure in the one region of sky
• 37:08
  that optical surveys had been forced to leave blank. What
• 37:12
  CIZA and similar X ray surveys revealed was a region
• 37:17
  of space far more populated than the blank patches on
• 37:21
  optical maps had suggested. Clusters that had no optical counterpart
• 37:27
  in any catalog. Structures that had been invisible for the
• 37:31
  simple reason that nobody had been able to look at them,
• 37:35
  began appearing in the X ray data. The zone of
• 37:39
  avoidance was not empty. It had simply been waiting for
• 37:44
  the right instrument. Then came the revision that stopped astronomers cold.
• 37:49
  In two thousand five, researchers working with data from the
• 37:53
  CIZA survey published a result that forced a fundamental rethink
• 38:00
  the mass of the greater tractor. They reported was roughly
• 38:04
  one tenth of what the original nineteen eighty six estimates
• 38:08
  had placed it at. Not a slight correction, not a
• 38:12
  refinement at the margins, a reduction by a factor of ten.
• 38:18
  If the original picture had been of a gravitational engine
• 38:22
  with the mass of roughly ten quadrillion suns, the new
• 38:27
  picture placed it closer to one quadrillion, still staggering by
• 38:32
  any human measure, but dramatically smaller than what the peculiar
• 38:37
  velocity maps had seemed to imply. This raised an obvious
• 38:41
  and uncomfortable question. If the actual mass was only a
• 38:45
  tenth of the estimate, why had the peculiar velocities pointed
• 38:50
  so insistently toward this region. Part of the answer was
• 38:54
  a statistical artifact called Malmquist bias. When astronomers survey the
• 39:00
  sky for galaxies, they inevitably find more of the bright ones.
• 39:06
  That is not a flaw in technique. It is a
• 39:09
  simple consequence of how detection works. Faint objects at great
• 39:14
  distances fall below the threshold of any instrument, the survey
• 39:20
  only catches the luminous fraction. The problem is that this
• 39:24
  skews the sample in a systematic way. Galaxies that appear
• 39:30
  in the catalog are not a random cross section of
• 39:33
  the real population. They are drawn from the brighter end
• 39:37
  of the distribution. When you calculate distances and peculiar are
• 39:43
  velocities using this biased sample, you overestimate the mass concentrations
• 39:50
  responsible for the observed motions. The original greater Tractor mass
• 39:55
  figures had been built partly on a foundation of this effect.
• 39:59
  When the X ray surveys counted mass more directly through
• 40:04
  the temperature and distribution of hot gas in galaxy clusters,
• 40:09
  rather than through galaxy brightness, the number came down sharply.
• 40:14
  This did not mean the greater Tractor was an illusion.
• 40:17
  The bulk flow was real, the peculiar velocities were real.
• 40:23
  Something out there was pulling, but the revised picture was
• 40:27
  more complex than a single monolithic mass concentration. The pulling
• 40:32
  was being done by a combination of structures spread across
• 40:36
  a larger region of space, with no single dominant object
• 40:42
  of the scale the early models had assumed. The X
• 40:46
  ray surveys were not deflating the mystery so much as
• 40:50
  redistributing it. What they did find in the heart of
• 40:54
  the obscured region was a specific object, massive enough to
• 40:58
  serve as a gravitational anchor for the whole area. The
• 41:03
  Normal cluster, cataloged as ACO three six two seven, showed
• 41:09
  up clearly in X ray observations as the dominant mass
• 41:12
  concentration in the Greater Tractor direction, sitting approximately two hundred
• 41:18
  twenty million light years away behind the dust that had
• 41:23
  hidden it for decades. It was the most massive galaxy
• 41:27
  cluster in the local universe within that region of sky.
• 41:32
  Optical follow up using near infrared wavelengths that could partially
• 41:37
  penetrate the dust confirmed what the X ray data had indicated.
• 41:43
  There was a real enormous structure there. After years of
• 41:48
  working with inferred gravitational effects and blank patches on maps,
• 41:54
  astronomers finally had something concrete tea awaim their instruments at.
• 41:59
  This was a pivotal moment in Greater Tractor research. The
• 42:04
  problem had gone from an invisible gravitational disturbance with no
• 42:08
  known source to a known object with measurable properties. The
• 42:13
  Normal Cluster was real, locatable, and enormous. It was the
• 42:18
  most tangible thing in a region that had spent the
• 42:21
  better part of two decades resisting every attempt at direct observation.
• 42:28
  And what the X ray surveys found at the center
• 42:31
  of the Greater Tractor region was not the smooth, featureless
• 42:35
  mass concentration the original models had predicted. It was something
• 42:40
  with structure texture its own strange drama at the heart
• 42:47
  of the Greater Tractor region. Since the Norma cluster, and
• 42:51
  the first time you look at the numbers describing it,
• 42:55
  your instinct is to check whether they are correct. The
• 42:58
  cluster carries the formal day designation AC three six two seven.
• 43:05
  It sits roughly two hundred twenty million light years away
• 43:10
  behind the zone of avoidance in the direction of the
• 43:13
  constellation Norma. For decades, the dust and gas of our
• 43:18
  own galaxies disc hid it almost completely. When X ray
• 43:22
  observations finally cut through, what they revealed was the most
• 43:27
  massive galaxy cluster in the local universe. The mass of
• 43:32
  the Normal cluster is approximately one thousand trillion solar masses.
• 43:37
  That number needs unpacking because as written it means almost nothing.
• 43:43
  Our Sun is not a small star. Nearly all of
• 43:46
  the Solar System's mass, approximately ninety nine percent, belongs to
• 43:51
  our Sun. One thousand trillion of those suns packed into
• 43:56
  a single gravitational structure higher Milky Way, every star in it,
• 44:03
  every gas cloud, every dark corridor between the spiral arms
• 44:08
  contains roughly one trillion solar masses. The Normal Cluster outweighs
• 44:13
  the Milky Way by a factor of one thousand. If
• 44:18
  you somehow collected a thousand complete copies of our galaxy,
• 44:22
  every star, and every planet in all of them, and
• 44:26
  compressed them into a single bound system, you would approach
• 44:30
  what is sitting at the center of the Greater Tractor region.
• 44:35
  That is what the X ray satellites found hiding behind
• 44:39
  the zone of obscuration. Pause to consider what follows carefully.
• 44:44
  The light crossing the Normal Cluster right now, the light
• 44:48
  from one galaxy hitting the outer edge of another as
• 44:51
  they slowly merge, left those galaxies two hundred twenty million
• 44:56
  years ago to reach us. At that time, the dominant
• 45:01
  animals on Earth were early dinosaurs. That is how far
• 45:06
  away something is while still being by the standards of
• 45:11
  the Greater Tractor. Essentially the local neighborhood. But the Normal
• 45:16
  Cluster is not the whole story. It is the dense core,
• 45:20
  the most massive single structure, the hub around it extends
• 45:25
  something larger. Astronomers call it the norma wall and sometimes
• 45:31
  the Greater Tractor Wall. It is a filamentary sheet of
• 45:34
  galaxies and clusters spreading out from the Normal Cluster in
• 45:38
  multiple directions, crossing the sky behind the zone of avoidance,
• 45:44
  extending across the constellations of Centaurus and Vela. Other clusters
• 45:50
  are embedded along this wall. PAVO two lies along it.
• 45:55
  The Centaurus Crux cluster is part of it. The cluster
• 45:58
  designated Sea Eza J one three two four point seven
• 46:04
  minus five seven three six extends the structure further still.
• 46:09
  The Normal Wall is not a collection of independent objects
• 46:13
  that happen to be near each other. It is a coherent,
• 46:17
  large scale structure, a gravitational family whose members share a
• 46:22
  common origin in the density fluctuations of the early universe.
• 46:27
  Taken together, the estimated diameter of this wall spans roughly
• 46:32
  three hundred million light years. The Milky Way, for comparison,
• 46:38
  is about one hundred thousand light years across. The Normal
• 46:43
  Wall is three thousand times wider than our entire galaxy.
• 46:48
  None of this is the most important thing about the
• 46:50
  Normal Cluster. The most important thing is what you cannot see.
• 46:56
  The one thousand trillion solar masses of visible material the
• 47:00
  colliding ellipticals, the radio bright collision zones, the gas halos
• 47:06
  detectable in X rays. All of that represents the luminous fraction.
• 47:12
  The part that telescopes can actually register. Underneath it, threading
• 47:17
  through it, extending far beyond its visible edges in every direction,
• 47:23
  is a dark matter halo whose mass dwarfs what any
• 47:27
  instrument has directly observed. The visible cluster is something like
• 47:32
  the foam on top of a wave. The wave itself.
• 47:36
  The true gravitational engine is the dark matter concentration. We
• 47:41
  cannot see it. We infer its presence from the way
• 47:45
  visible matter moves from the depth of the gravitational well
• 47:50
  from the way light bends around the region. But without it,
• 47:55
  the numbers would not work. A cluster of one thousand
• 47:59
  trillion solar masses invisible material would require a dark matter
• 48:04
  envelope several times larger to produce the gravitational effects actually measured.
• 48:11
  The normal cluster was the anchor, the most concrete object
• 48:15
  in a region that had spent decades resisting observation, but
• 48:20
  it was only the visible face of something whose true
• 48:23
  mass was mostly invisible. The numbers used to describe the
• 48:27
  greater tractor stop making intuitive sense almost immediately. That is
• 48:33
  the Universe's way of telling you that your intuitions were
• 48:36
  built for the wrong scale. The normal cluster alone holds
• 48:40
  roughly one thousand trillion solar masses. That figure already strains comprehension.
• 48:48
  But the Normal Cluster is only the visible core of
• 48:51
  a much larger region. The region as a whole carries
• 48:55
  a mass estimated at around ten thousand trillion trillion soular masses.
• 49:01
  Written out that is a ten followed by sixteen zeros.
• 49:07
  There is no everyday comparison that survives contact with the
• 49:11
  number like that. You have to build your way up
• 49:15
  to it slowly, the way you might let your eyes
• 49:18
  adjust to darkness. Start with something familiar. Our Sun is
• 49:25
  a fairly ordinary star. It is not especially massive, not
• 49:30
  especially large. The Milky Way contains roughly one trillion stars
• 49:36
  in that general range. When you add up all the stars,
• 49:40
  all the gas, all the dust, all the dark matter
• 49:44
  holding the galaxy together, you get a total mass of
• 49:48
  approximately one trillion solar masses. One trillion. That is the
• 49:54
  full weight of everything in the galaxy. You live inside,
• 49:58
  every star you have ever se in the night sky,
• 50:02
  every nebula, ever photographed, every black hole astronomers have ever measured.
• 50:09
  The Milky Way is not small, and the Greater Tractor
• 50:13
  Region outweighs it by a factor of roughly ten thousand.
• 50:17
  Ten thousand milky ways stacked together would match the gravitational
• 50:23
  weight of the structure pulling us across the cosmos Right now.
• 50:28
  The diameter is just as difficult to hold in your mind.
• 50:32
  The Milky Way stretches approximately one hundred thousand light years
• 50:37
  from one edge to the other. That distance is already
• 50:40
  so large that light traveling at three hundred thousand kilometers
• 50:45
  per second needs a full hundred thousand years to cross it.
• 50:50
  The Greater Tractor region spans an estimated three hundred million
• 50:54
  light years. To cross the Greater Tractor from one side
• 50:58
  to the other at the speed of light would take
• 51:01
  three thousand times longer than the entire current age of
• 51:05
  the universe. Put differently, the Greater Tractor is roughly three
• 51:11
  thousand milky ways wide. If you place the milky Way
• 51:16
  inside the Greater Tractor Region and try to find it,
• 51:20
  it would be invisible, not because it is faint, but
• 51:25
  because it would occupy a volume so small relative to
• 51:29
  the whole that it would be statistically irrelevant. Here is
• 51:33
  something worth sitting with. Light leaving one edge of the
• 51:36
  Greater Tractor Region today headed toward the other edge will
• 51:42
  not arrive until long after the Earth has been swallowed
• 51:45
  by an aging sun. The structure is so wide that
• 51:49
  no signal, no message, no observation made from one side
• 51:55
  could reach the other within a time frame that means
• 51:58
  anything to human civilize ation. The Greater Tractor does not
• 52:03
  fit inside human time. It barely fits inside cosmic time.
• 52:09
  Now comes the stranger part of all that mass, ten
• 52:14
  thousand trillion trillion solar masses, roughly eighty five percent of
• 52:19
  it produces no light whatsoever. It does not emit, it
• 52:25
  does not absorb, it does not reflect. Every telescope ever built,
• 52:31
  from Galileo's first tube to the mirrors of the James
• 52:35
  Web Space Telescope is completely blind to it. This is
• 52:40
  dark matter in the Greater Tractor region. It is not
• 52:44
  a minor component or a marginal correction to the visible mass.
• 52:49
  It is the overwhelming majority of what is there. The galaxies,
• 52:55
  the clusters, the colliding ellipticals of the normal cluster, radiating
• 53:00
  their radio waves across millions of light years, all of
• 53:05
  that visible material accounts for only about fifteen percent of
• 53:10
  what is doing the pulling. What we can see is
• 53:13
  an afterthought. The real structure is invisible. Think of it
• 53:18
  this way. Imagine an enormous sphere of completely transparent material,
• 53:24
  something with no color, no shine, no surface you could
• 53:28
  ever detect with your eyes. Now scatter a thin layer
• 53:33
  of glowing dust across the outside of that sphere. What
• 53:37
  you would see, looking from a distance is the dust.
• 53:42
  That is what astronomers see when they look at the
• 53:44
  Great Attractor Region. The galaxies are the dust. The sphere
• 53:50
  they are draped across is the dark matter halo underneath.
• 53:54
  The halo is immeasurably larger and heavier than everything visible combined.
• 54:01
  This means the map's astronomers made of the Greater Tractor
• 54:04
  Region invisible light were telling a deeply incomplete story. The
• 54:10
  X ray surveys that identified the normal cluster, the optical
• 54:14
  catalogs that traced the normal wall across multiple constellations, they
• 54:20
  were mapping the dust on the surface, the structure doing
• 54:24
  the actual gravitational work, the thing exerting that ten thousand
• 54:29
  milky way pull across hundreds of millions of light years,
• 54:34
  was sitting underneath the visible galaxies, silent and undetectable by
• 54:40
  any direct means. Astronomers inferred its presence from its effects,
• 54:47
  the way you might infer the existence of a strong
• 54:50
  wind by watching everything in the room move in the
• 54:54
  same direction without being able to feel the air itself.
• 54:58
  What made this realizeation so significant was not just its strangeness.
• 55:04
  It was what it implied about the architecture of our
• 55:07
  local universe. A structure this massive, this wide, hiding most
• 55:13
  of its substance in invisible matter, drawing in galaxies from
• 55:18
  hundreds of millions of light years in every direction. That
• 55:23
  is not an accident of cosmic geography. Gravity does not
• 55:27
  accidentally build something that large out of random noise. What
• 55:32
  the Greater Tractor represented once the scale came into focus
• 55:37
  was a node, a place where the large scale structure
• 55:42
  of the universe converges, where filaments of matter and dark
• 55:46
  matter meet and pile up, where the gravitational logic of
• 55:51
  the cosmos makes a kind of focal point. This was
• 55:55
  not a random anomaly in an empty corner of the universe.
• 56:00
  It was the center of something organized on a scale
• 56:03
  that required a completely new map. In September of twenty fourteen,
• 56:08
  a team of astronomers published a paper that quietly redrew
• 56:12
  the map of the universe as far as it concerns us.
• 56:16
  The lead author was Brent Tulley, an astronomer at the
• 56:20
  University of Hawaii who had spent decades studying the large
• 56:24
  scale distribution of galaxies. His collaborators included researchers from France, Israel,
• 56:32
  and the United States. What they published was not a
• 56:36
  new observation of a distant object. It was something more
• 56:41
  fundamental than that. They had found a new way to
• 56:45
  ask a very old question, where do we actually live?
• 56:49
  And how large is it? The question sounds simple, it
• 56:53
  is not. For most of the twentieth century, the concept
• 56:58
  of a supercluster was looseed. Astronomers use the word to
• 57:03
  describe regions of space where galaxies seemed to cluster more
• 57:07
  densely than average, but the boundaries were subjective. There was
• 57:13
  no clean method to say where one supercluster ended and
• 57:17
  another began. You could draw the lines almost anywhere and
• 57:22
  find someone who would argue with you. Tully's team changed
• 57:26
  this by switching the question entirely. Instead of asking where
• 57:30
  the galaxies are, they asked where the galaxies are going.
• 57:35
  The technique they used was built around galaxy flow patterns.
• 57:40
  Every galaxy has a peculiar velocity, a motion above and
• 57:45
  beyond the simple expansion of the universe. That motion is
• 57:49
  caused by gravity. Galaxies are pulled toward mass concentrations, which
• 57:56
  means that if you map which direction thousands of galaxies
• 57:59
  are moving, you can reverse engineer the gravitational landscape beneath them.
• 58:05
  Tully's team compiled data on the peculiar velocities of roughly
• 58:10
  eight thousand galaxies across a vast region of space and
• 58:15
  use that data to draw the gravitational watershed lines. A
• 58:20
  watershed in geography is a boundary that separates water flowing
• 58:25
  into one basin from water flowing into another. The team
• 58:30
  applied the same concept to galaxies. Any galaxy whose flow
• 58:35
  pointed inward toward the same gravitational minimum was part of
• 58:40
  the same supercluster. Any galaxy whose flow pointed away draining
• 58:46
  toward a different basin was not. The result was a
• 58:51
  supercluster defined not by proximity or density, but by gravitational allegiance.
• 58:58
  What they found was enormous. The structure they mapped spans
• 59:03
  roughly five hundred million light years. That number is worth
• 59:08
  holding for a moment. The Milky Way is about one
• 59:12
  hundred thousand light years across. The distance to the nearest
• 59:16
  large galaxy, Andromeda, is about two point five million light years.
• 59:23
  The distance to the Virgo Cluster, the nearest major cluster,
• 59:27
  is about sixty five million light years. Lania KaiA contains
• 59:33
  all of this and extends roughly eight times farther still
• 59:37
  in every direction from its center. It holds more than
• 59:41
  one hundred thousand individual galaxies. The total mass is estimated
• 59:47
  at around one hundred million billion times the mass of
• 59:51
  our Sun. The Milky Way, with its few hundred billion stars,
• 59:57
  is one small tributary in this structure. They named it
• 1:00:01
  Lania KaiA. The word comes from Hawaiian Lani means heaven,
• 1:00:07
  a kaya means immense or spacious, immense heaven. The name
• 1:00:13
  was not chosen arbitrarily. Tully had strong connections to Hawaii
• 1:00:18
  through his work at the university there, and the language
• 1:00:22
  has a tradition of naming astronomical features. The name fits.
• 1:00:29
  There is something appropriate about a structure this large being
• 1:00:33
  described with a word that gestures toward the sky rather
• 1:00:37
  than toward any earthly measure. The paper came with a
• 1:00:41
  visualization that got considerable attention when it was published. Researchers
• 1:00:47
  rendered the flow lines of thousands of galaxies as streams,
• 1:00:51
  each one a thin tendril curving through three dimensional space.
• 1:00:57
  What the image showed was something that looked looked unmistakably
• 1:01:01
  like a drainage system. The streams flowed inward from every direction,
• 1:01:07
  curling and converging toward a single point. Some streams were
• 1:01:12
  short and nearly straight. Others traced long, arching paths across
• 1:01:18
  hundreds of millions of light years before bending toward the center.
• 1:01:24
  All of them pointed toward the same place. That place
• 1:01:28
  was the greater attractor. The Milky Way appears in that
• 1:01:32
  visualization as one small stream among thousands located in the
• 1:01:38
  outer regions of the basin, already moving inward, already fall ing.
• 1:01:45
  Lanyakir is not, however, gravitationally bound. This is a critical distinction.
• 1:01:52
  Being part of the same gravitational basin is not the
• 1:01:56
  same thing as being locked together permanently. The outer regions
• 1:02:00
  of Laniakia are far enough apart that the expansion of
• 1:02:04
  the universe will eventually overcome the gravitational pull, and the
• 1:02:09
  structure will slowly be pulled apart over billions of years.
• 1:02:15
  The inward flow we see today reflects the current balance
• 1:02:19
  between gravity and expansion. It is not a permanent state.
• 1:02:25
  What Tully's paper captured was a snapshot, a moment in
• 1:02:30
  the long history of this structure when inward flow still
• 1:02:34
  dominates across its full extent. The redefinition also clarified something
• 1:02:39
  that had been somewhat muddled in the older literature. When
• 1:02:43
  earlier researchers referred to the Virgo Supercluster, they were describing
• 1:02:48
  a region that Laniakia now contains as one of its
• 1:02:52
  subsidiary components. The Virgo Supercluster, which had been considered our
• 1:02:58
  home supercluster for decades, turned out to be one lobe
• 1:03:02
  of something considerably larger. Our address in the universe had
• 1:03:07
  gained another line. We live in the Milky Way in
• 1:03:12
  the local group in the Virgo Supercluster region in Laniakia,
• 1:03:18
  all of it flowing toward the center, which raised a
• 1:03:22
  question that the new map made unavoidable. If everything in
• 1:03:26
  Laniakia is draining toward the greater tractor, what is pulling
• 1:03:30
  the greater tractor? The answer turned out to be something
• 1:03:35
  even larger. When researchers mapped the flow lines of Laniakia
• 1:03:39
  and trace them inward toward the Greater Tractor, they had
• 1:03:43
  assumed they were approaching the end of the chain. The
• 1:03:47
  greater tractor was the gravitational center, the low point in
• 1:03:52
  the basin, the destination toward which everything nearby was draining.
• 1:03:57
  That picture was clean, it was organized. It was also incomplete,
• 1:04:04
  because when astronomers looked at which direction the Greater Tractor
• 1:04:07
  itself was moving, the answer came back the same as before.
• 1:04:13
  It was falling, not randomly, not drifting, but falling in
• 1:04:18
  a specific direction towards something sitting roughly six hundred fifty
• 1:04:23
  million light years away in nearly the same region of sky.
• 1:04:28
  That something is called the Shapleigh Supercluster, and the numbers
• 1:04:33
  describing it make the Greater Tractor look modest. The Shaply
• 1:04:37
  Supercluster contains more than eight thousand individual galaxies, not eight
• 1:04:42
  thousand stars, galaxies, each one a gravitational system, holding anywhere
• 1:04:49
  from tens of millions to hundreds of billions of stars
• 1:04:53
  inside it. The combined mass of the Shaply Supercluster exceeds
• 1:04:58
  ten million billion in times the mass of our Sun.
• 1:05:02
  Written out that number is a ten, followed by twenty
• 1:05:06
  one zeros. The Great Attractor, for all the weight attached
• 1:05:11
  to its name, is substantially less massive. Within a radius
• 1:05:16
  of roughly one billion light years from the Milky Way,
• 1:05:20
  nothing rivals shaply. It is the most massive concentration of
• 1:05:25
  galaxies known to exist in our corner of the universe,
• 1:05:29
  and it has been pulling on everything around it for
• 1:05:32
  billions of years without anyone on Earth knowing it was there.
• 1:05:36
  The discovery changed how astronomers interpreted the Milky Way's peculiar velocity.
• 1:05:43
  The original picture, the one the seven Samurai had built
• 1:05:46
  in nineteen eighty six, attributed our galaxy's motion almost entirely
• 1:05:52
  to the Greater Attractor. That was the dominant gravitational source.
• 1:05:58
  That was the explanation. But as surveys improved and researchers
• 1:06:03
  were able to look farther out and account for more
• 1:06:06
  of the mass distribution in the local universe, something shifted
• 1:06:11
  in the numbers. Our peculiar velocity. It turned out was
• 1:06:16
  not coming from a single source. About forty four percent
• 1:06:21
  of it could be attributed to the Greater Tractor, and
• 1:06:25
  the laniar Kia structure around it. The remaining fifty six
• 1:06:29
  percent was coming from farther away. It was coming from Shapleigh.
• 1:06:34
  More than half of the gravitational pull moving our galaxy
• 1:06:38
  at six hundred kilometers per second through space has nothing
• 1:06:42
  to do with the object that was identified as the cause.
• 1:06:46
  It is coming from something even larger, sitting even farther away,
• 1:06:51
  in a direction we have been looking past for decades.
• 1:06:56
  This realization forced a significant revision to the store. The
• 1:07:01
  Seven Samurai had not, in fact mapped the influence of
• 1:07:05
  a single gravitational engine. They had mapped the combined effect
• 1:07:10
  of a hierarchy, two enormous structures pulling on the same
• 1:07:15
  region of space from different distances, their influences blending into
• 1:07:20
  a single velocity signal that pointed somewhere between them. The
• 1:07:25
  greater Tractor is real, Its gravitational effect is real, but
• 1:07:31
  it was never the full explanation. What makes this stranger
• 1:07:35
  is the chain It implies. Our galaxy is falling toward
• 1:07:40
  the greater tractor. The greater tractor is falling toward Shapley.
• 1:07:46
  Each object in this sequence is itself a component of
• 1:07:50
  something larger caught in the gravity of something it cannot escape.
• 1:07:56
  There is something almost uncomfortable about the pattern where you
• 1:08:00
  trace it clearly, you might expect, at some level of
• 1:08:04
  the hierarchy to find a resting point, some structure massive
• 1:08:09
  enough that nothing pulls it further along. Shaply for a
• 1:08:15
  time seemed like a candidate. It is the largest known
• 1:08:19
  concentration within a billion light years. Nothing near it appears
• 1:08:25
  to have the mass needed to drag it out of position,
• 1:08:29
  But the universe is large enough that even this ceiling
• 1:08:33
  has a ceiling. Surveys of galaxy motions on scales larger
• 1:08:37
  than Shaply reveal bulk flows that extend beyond it, suggesting
• 1:08:42
  that even more distant mass concentrations are influencing the motion
• 1:08:46
  of the entire local region of the universe. The names
• 1:08:50
  for these structures, the Sloane Great Wall, the Hercules, Corona
• 1:08:55
  Borealis Great Wall, begin to accumulate. Each one represents an
• 1:09:01
  over density, a concentration of galaxies and dark matter pulling
• 1:09:07
  on everything around it at a distance where individual galaxies
• 1:09:11
  are too faint to see without the largest telescopes ever built.
• 1:09:17
  Whether there is a true end to this chain, some
• 1:09:20
  outermost gravitational source beyond which nothing pulls is a question
• 1:09:26
  that observational cosmology cannot yet answer cleanly. What astronomers can
• 1:09:32
  say is that the local picture, the one centered on
• 1:09:35
  the Greater Tractor on Laniakia, on the bulk flow the
• 1:09:40
  seven Samurai measured, is embedded in a larger gravitational landscape
• 1:09:45
  that does not stop at the edges of what any
• 1:09:48
  single survey has mapped. Shaply is the dominant known influence
• 1:09:53
  beyond the Greater Tractor, and its contribution to our peculiar
• 1:09:57
  velocity is larger than the Greater Tractor zone. That fact
• 1:10:02
  alone was enough to reshape how cosmologists think about large
• 1:10:06
  scale structure in the nearby universe. The motion of our
• 1:10:12
  galaxy is not the signature of one object. It is
• 1:10:16
  the fingerprint of a nested system of gravitational pulls, each
• 1:10:21
  embedded in the next, reaching outward to distances where the
• 1:10:26
  edge of the observable universe starts to become relevant. The
• 1:10:30
  Shaply supercluster sits about six hundred fifty million light years away.
• 1:10:36
  Light left it when complex animal life on Earth was
• 1:10:39
  only beginning to take hold. That light is arriving now
• 1:10:44
  carrying information about a structure whose gravity has been shaping
• 1:10:48
  the motion of our galaxy for longer than multicellular life
• 1:10:53
  has existed on this planet. The Greater Tractor is falling
• 1:10:57
  toward it at a speed comp to our own peculiar velocity.
• 1:11:03
  Our galaxy is falling toward the Greater Attractor, and something
• 1:11:09
  beyond Shapley, something not yet fully mapped, may be pulling
• 1:11:14
  on all of it together. Every time astronomers thought they
• 1:11:18
  had found the ceiling of the hierarchy, another floor turned
• 1:11:22
  up above it. About eighty five percent of the mass
• 1:11:26
  responsible for pulling the Milky Way toward the Greater Attractor
• 1:11:30
  cannot be seen. Not because it is too far away,
• 1:11:35
  not because it is hidden behind the zone of avoidance.
• 1:11:38
  It cannot be seen because it does not interact with
• 1:11:42
  light at all. It does not emit light, absorb light,
• 1:11:47
  or reflect light. Every telescope ever built in every wavelength,
• 1:11:53
  from radio waves to gamma rays, returns nothing from this material,
• 1:11:58
  and yet it outweighs everything we have ever observed by
• 1:12:02
  a factor of roughly five to one. This is dark matter,
• 1:12:07
  and without it, the Greater Tractor would not exist, the
• 1:12:12
  normal cluster would not exist, and the Milky Way itself
• 1:12:16
  would long ago have torn itself apart. The first serious
• 1:12:21
  evidence appeared in the nineteen seventies, when astronomers began measuring
• 1:12:25
  how fast stars move around the centers of their galaxies.
• 1:12:30
  The expectation was straightforward. Stars near the center of a galaxy,
• 1:12:36
  where most of the visible mass is concentrated, should orbit quickly.
• 1:12:41
  Stars far from the center, where visible mass thins out,
• 1:12:45
  should orbit more slowly. This is how the planets in
• 1:12:49
  our Solar system behave. Mercury moves fast, Neptune moves slow.
• 1:12:55
  The math is clean. But when VERA. Reuben and her
• 1:12:59
  colleagues meet measured actual stellar rotation curves in spiral galaxies,
• 1:13:04
  the curves came out flat. Stars at the far edges
• 1:13:08
  of galaxies were moving just as fast as stars near
• 1:13:12
  the center. They should have been flung outward into empty
• 1:13:17
  space by that speed, like water spraying off a spinning wheel.
• 1:13:22
  The i were not. Something was holding them in, something
• 1:13:28
  massive distributed throughout the entire galaxy, including the outer regions,
• 1:13:34
  and completely invisible. The visible matter alone could not account
• 1:13:39
  for the gravity needed to keep those stars on their orbits.
• 1:13:43
  Rotation curves were only the beginning. Gravitational lensing provided a
• 1:13:49
  second line of evidence. When light from a distant object
• 1:13:53
  passes close to a massive structure, gravity bends the light,
• 1:13:58
  distorting the image in predictable ways. Astronomers can work backward
• 1:14:04
  from the distortion to calculate how much mass is doing
• 1:14:08
  the bending. When they did this with galaxy clusters, the
• 1:14:13
  mass required to produce the observed lensing was several times
• 1:14:17
  larger than the mass visible in stars and gas. Something
• 1:14:22
  invisible was bending the light. There was simply too much
• 1:14:27
  gravitational effect for the visible matter to explain the structure
• 1:14:31
  of galaxy clusters made the case even harder to dismiss.
• 1:14:36
  Clusters like the normal cluster, contain hot gas that moves
• 1:14:41
  between galaxies. The temperature and motion of that gas reflects
• 1:14:46
  the gravitational environment it is sitting in. When astronomers measured
• 1:14:51
  the temperature of the gas through X ray observations, they
• 1:14:56
  found it was far too hot to be held in
• 1:14:58
  by the gravity of the visible galaxies alone. The cluster
• 1:15:03
  should have been blowing itself apart. The gas could only
• 1:15:07
  be that hot and still be retained if there was
• 1:15:11
  a much larger mass holding everything in place. Every method
• 1:15:17
  pointed to the same conclusion. Most of the mass in
• 1:15:21
  every galaxy, every cluster, and every supercluster is invisible for
• 1:15:28
  the Greater Tractor. This changes the picture significantly. When the
• 1:15:33
  Seven Samurai detected the bulk flow in nineteen eighty six,
• 1:15:38
  they were measuring the gravitational pull of a structure whose
• 1:15:41
  total mass is somewhere around ten thousand trillion solar masses.
• 1:15:47
  The visible component of that the galaxies you can count,
• 1:15:52
  the hot gas you can detect with X ray satellites,
• 1:15:55
  the stars you can resolve, represents perhaps fifteen percent of
• 1:16:00
  the total. The remaining eighty five percent is a dark
• 1:16:05
  matter halo of staggering proportions distributed through the region in
• 1:16:10
  a way no optical telescope can map. The norma cluster,
• 1:16:15
  the normal wall, the individual galaxies streaming toward the center.
• 1:16:20
  These are surface features. They are the thin visible coating
• 1:16:25
  on an object whose true bulk is entirely hidden. Think
• 1:16:29
  of it this way. If you could somehow remove every
• 1:16:33
  galaxy in the Greater Tractor region, every star, every cloud
• 1:16:38
  of gas and dust, the gravitational structure would still be there.
• 1:16:43
  The dark matter halo would remain, and it would still
• 1:16:46
  be pulling on the Milky Way at essentially the same rate.
• 1:16:51
  The galaxies are passengers, not the engine. What the Seven
• 1:16:56
  Samurai actually found in nineteen eighty six as mostly the
• 1:17:00
  gravitational signature of something that has never been seen and
• 1:17:05
  has not yet been detected in any laboratory on Earth,
• 1:17:09
  despite decades of experiments designed specifically to find it. This
• 1:17:14
  matters for understanding not just the greater tractor, but the
• 1:17:18
  entire large scale structure of the universe, the filaments, the voids,
• 1:17:25
  the walls of galaxies, the massive nodes where superclusters accumulate.
• 1:17:30
  None of this pattern emerged from ordinary matter alone. In
• 1:17:35
  the early universe, before the first stars formed, dark matter
• 1:17:40
  began clumping under its own gravity. Because it did not
• 1:17:45
  interact with light, it was not pushed around by radiation
• 1:17:49
  pressure the way ordinary matter was. It could collapse freely,
• 1:17:55
  pulling itself together into halos while the visible matter was
• 1:17:59
  still too hot and too agitated to follow. By the
• 1:18:04
  time ordinary matter cooled enough to settle, dark matter had
• 1:18:09
  already constructed a scaffolding across the entire observable universe. Galaxies
• 1:18:16
  formed where the dark mat er was densest filaments traced
• 1:18:22
  the dark matter filaments underneath them. Voids formed where dark
• 1:18:27
  matter was thin. The cosmic web we described earlier is
• 1:18:31
  at its foundation a dark matter web with visible matter
• 1:18:36
  painted onto it. The Greater Tractor is one of the
• 1:18:39
  densest nodes of that web, within several hundred million light
• 1:18:43
  years of Earth. Its gravity is real, its pull is measurable.
• 1:18:49
  But what you are being pulled by is mostly something
• 1:18:52
  for which we have no complete physical explanation, something we
• 1:18:57
  have never held in a detector, and something that makes
• 1:19:01
  up the overwhelming majority of all the matter in the universe.
• 1:19:06
  Without the dark matter scaffolding, none of the visible structure
• 1:19:10
  we are describing would exist in the form it does.
• 1:19:14
  The Greater Tractor included. There is a force working in
• 1:19:19
  the opposite direction from gravity on the largest scales, and
• 1:19:23
  given enough time, it wins. To understand what this means
• 1:19:27
  for the Greater Tractor, you have to go back to
• 1:19:31
  nineteen ninety eight, when two independent teams of astronomers were
• 1:19:36
  doing something that sounded routine. They were measuring the brightness
• 1:19:41
  of distant supernovae the catastrophic explosions of dying stars to
• 1:19:47
  map how the expansion of the universe was changing over time.
• 1:19:52
  The standard assumption was straightforward gravity should be slow in
• 1:19:57
  the expansion down the universe verse began with the Big Bang.
• 1:20:02
  Matter was flung outward and gravity should be pulling that
• 1:20:06
  matter back, decelerating the whole thing like a ball thrown
• 1:20:10
  upward against the pull of Earth. The question was not
• 1:20:15
  whether the expansion was slowing, The question was how fast.
• 1:20:20
  The answer came back wrong. Both teams, working independently with
• 1:20:26
  different telescopes and different data sets, found the same result.
• 1:20:31
  The distant supernova were dimmer than they should have been,
• 1:20:35
  not slightly dimmer, measurably consistently dimmer, and in astronomy, dimmer
• 1:20:42
  means farther away. The supernovae were farther away than a
• 1:20:47
  decelerating universe had any right to put them. The expansion
• 1:20:52
  was not slowing down, It was speeding up. Something was
• 1:20:57
  pushing the universe apart at an accelerating rate, and nobody
• 1:21:01
  had predicted it. The teams spent months checking their data,
• 1:21:06
  looking for errors, looking for systematic mistakes in calibration. The
• 1:21:12
  result held. In nineteen ninety eight, the discovery was announced,
• 1:21:17
  and cosmology has not been the same since. The force
• 1:21:21
  responsible has a name, dark energy, and that name is
• 1:21:26
  essentially an admission that nobody knows what it is. What
• 1:21:31
  astronomers do know is this dark energy makes up roughly
• 1:21:36
  sixty eight percent of the total energy content of the universe.
• 1:21:41
  Ordinary matter, the stars, the planets, the gas clouds, everything
• 1:21:46
  built from atoms accounts for only about five percent. Dark matter.
• 1:21:52
  The invisible gravitational scaffolding described in the previous section accounts
• 1:21:57
  for around twenty seven percent. The universe is, in terms
• 1:22:03
  of its energy budget, overwhelmingly made of something that has
• 1:22:07
  never been detected in a laboratory and has no agreed
• 1:22:11
  upon physical explanation. Dark energy is the largest single component
• 1:22:17
  of the cosmos, and it is a mystery. What dark
• 1:22:21
  energy does is push. On small scales, gravity is stronger.
• 1:22:28
  A galaxy cluster holds together because the mass packed inside
• 1:22:32
  it generates enough gravitational attraction to overwhelm the push of
• 1:22:36
  dark energy. At those distances, the Milky Way and Andromeda
• 1:22:42
  are gravitationally bound. The local group, the few dozen galaxies
• 1:22:47
  sitting nearest to us are bound together. The Virgo cluster
• 1:22:52
  holds together. Gravity wins at those scales because the distances
• 1:22:57
  are short and the densities are high, But as you
• 1:23:01
  zoom out, the balance shifts. At scales of a few
• 1:23:06
  hundred million light years or more, the expansion, driven by
• 1:23:10
  dark energy begins to win over the gravitational pull tying
• 1:23:15
  structures together. Objects that are not tightly bound by gravity
• 1:23:20
  at those distances do not stay together. They drift apart.
• 1:23:25
  The expansion carries them away from each other faster than
• 1:23:29
  gravity can pull them back. This is where Laniakia becomes
• 1:23:32
  a problem. Laniakia, the supercluster containing the Milky Way, the
• 1:23:38
  Greater Tractor, and more than one hundred thousand other galaxies
• 1:23:43
  spread across five hundred million light years, is not gravitationally bound.
• 1:23:49
  It looks like a coherent structure when you map the
• 1:23:52
  flow of galaxies toward the Greater Tractor. It has a center,
• 1:23:57
  a basin, streams of galaxy draining inward like rivers toward
• 1:24:02
  a sea. But it is not held together the way
• 1:24:06
  a galaxy cluster is held together. The outer regions of
• 1:24:11
  Laniakeia are already being pulled apart by the expansion of
• 1:24:15
  the universe. Given enough time, the structure will not survive
• 1:24:20
  as a unit. The rivers will reverse, the streams will
• 1:24:25
  scatter for the Milky Way. This means something specific. Right now,
• 1:24:31
  we are moving toward the greater tractor at roughly six
• 1:24:34
  hundred kilometers per second. Every second of every day, our
• 1:24:39
  galaxy closes that distance by six hundred kilometers. The pull
• 1:24:44
  is real, it is measurable, and it has been operating
• 1:24:48
  for billions of years. But the greater Tractor is not
• 1:24:52
  sitting still waiting for us to arrive. The space between
• 1:24:57
  us and that region is expanding at the distance of
• 1:25:01
  the greater tractor somewhere between one hundred fifty and two
• 1:25:06
  hundred fifty million light years. The recession due to cosmic
• 1:25:11
  expansion is already significant. Our approach speed and the expansion
• 1:25:18
  rate are working against each other, and dark energy is
• 1:25:22
  accelerating the expansion continuously. The gap is not shrinking as
• 1:25:28
  fast as the raw velocity would suggest. In some projections,
• 1:25:34
  it may not shrink at all, depending on how dark
• 1:25:37
  energy behaves on these time scales. Think of it as
• 1:25:40
  running toward a door that is sliding away from you
• 1:25:43
  down a corridor. You are running. You are getting closer
• 1:25:48
  in the sense that you are covering ground, but the
• 1:25:51
  corridor is stretching as you run, and the door is accelerating.
• 1:25:56
  You will not reach it. The universe has built destination
• 1:26:01
  into the motion of our galaxy, and then quietly ensured
• 1:26:05
  that the destination remains unreachable. What this means in concrete
• 1:26:09
  terms is that the Milky Way will spend billions of
• 1:26:13
  years moving toward the Greater Tractor and will never close
• 1:26:17
  the distance to zero. The gravitational pull will not vanish suddenly.
• 1:26:24
  It will weaken gradually as the distance grows, as the
• 1:26:28
  expansion carries the Greater Tractor region farther and farther from us,
• 1:26:33
  as the geometry of the universe dilutes the grip of
• 1:26:36
  that enormous concentration of mass and dark matter. There is
• 1:26:41
  no arrival. There is only approach, and then eventually a
• 1:26:46
  slow fading of the pull itself. For a structure that
• 1:26:50
  has been shaping the motion of our galaxy since before
• 1:26:53
  the Earth existed. That is a strange kind of ending.
• 1:26:58
  Not a collision, not to merger, not a dramatic conclusion,
• 1:27:04
  just an increasing distance, a weakening signal, a gravitational whisper
• 1:27:09
  fading below the threshold of any influence. The countdown that
• 1:27:15
  governs our relationship to the Greater Tractor is not the
• 1:27:19
  countdown to arrival. It is the countdown to the point
• 1:27:24
  where the pull becomes too weak to matter, and that
• 1:27:27
  clock runs on a different time scale than anything in
• 1:27:31
  human experience. Every structure in the universe is simultaneously a
• 1:27:36
  container for smaller things and a component of something larger.
• 1:27:41
  This is not a poetic observation. It is a description
• 1:27:45
  of how matter is actually organized, layer by layer, from
• 1:27:49
  the scale of a solar system out to the scale
• 1:27:53
  of superclusters spanning half a billion light years. And once
• 1:27:58
  you start tracing the layers from the inside out, something
• 1:28:02
  unsettling becomes clear. Every level of the hierarchy is falling
• 1:28:08
  toward the next one up. There is no stationary floor
• 1:28:12
  start close. The Milky Way is not alone. It belongs
• 1:28:18
  to a collection of galaxies called the Local Group, roughly
• 1:28:22
  fifty members in total, most of them small dwarf galaxies
• 1:28:26
  clustered around two dominant structures, the Milky Way and the
• 1:28:31
  Andromeda galaxy. The Local Group spans about ten million light
• 1:28:36
  years by cosmic standards. That is a tight cluster. The
• 1:28:42
  Milky Way and Andromeda are already gravitationally bound to each other,
• 1:28:48
  already moving toward each other, already on a collision course
• 1:28:52
  that will play out in roughly four point five billion years.
• 1:28:58
  Everything in the local group is held together. Nothing escapes.
• 1:29:04
  This is the first container, but the local group is
• 1:29:07
  not sitting still relative to everything beyond it. The entire
• 1:29:12
  collection Milky Way Andromeda all fifty members, is being drawn
• 1:29:18
  toward the Virgo Cluster, a dense knot of more than
• 1:29:22
  one thousand, three hundred galaxies sitting about sixty five million
• 1:29:27
  light years away. The Virgo Cluster is one of the
• 1:29:31
  most massive structures in the nearby universe, containing enough mass
• 1:29:36
  to pull our entire local group toward it with measurable force.
• 1:29:42
  The motion is slow relative to the distances involved. It
• 1:29:47
  will not result in a collision on any timescale worth discussing.
• 1:29:52
  But the pull is real and it shows up in
• 1:29:55
  the data. The local group is not drifting randomly. It
• 1:30:00
  is drifting toward Virgo. Zoom out one level further, and
• 1:30:04
  the Virgo cluster reveals itself as the center of something bigger.
• 1:30:10
  The Virgo Cluster, the local group, and the surrounding region
• 1:30:14
  of galaxies spread across roughly one hundred ten million light
• 1:30:18
  years formed the Virgo supercluster, also called the Local supercluster.
• 1:30:25
  For decades, astronomers treated the Virgo supercluster as the largest
• 1:30:30
  gravitational unit our galaxy belonged to. It was a reasonable conclusion.
• 1:30:36
  The structure was real, the mass was real, and Virgo
• 1:30:41
  sat at its center pulling everything inward. Then twenty fourteen
• 1:30:46
  happened and the map got redrawn. The twenty fourteen paper
• 1:30:51
  that defined Laniakia didn't add new objects to the universe.
• 1:30:56
  It showed that the Virgo supercluster was not a freeze
• 1:31:00
  standing structure, but a tendril of something vastly larger. Laniakia
• 1:31:05
  spans roughly five hundred million light years, contains more than
• 1:31:10
  one hundred thousand galaxies, and has a total mass on
• 1:31:14
  the order of one hundred million billion suns. The Virgo
• 1:31:19
  supercluster is one piece of it. The Local Group is
• 1:31:24
  one piece of the Virgo supercluster. The Milky Way is
• 1:31:28
  one piece of the local group. Each container fits inside
• 1:31:33
  the next one, and none of them are static. The
• 1:31:36
  flow lines that defined Laniyakia's boundary showed something striking. Galaxies
• 1:31:43
  throughout this enormous volume are not moving randomly. They are
• 1:31:47
  draining inward toward the center. The way water drains toward
• 1:31:52
  the lowest point in a basin. That lowest point is
• 1:31:56
  the greater tractor. What does it mean to say that
• 1:31:59
  one one hundred thousand galaxies are all drifting toward the
• 1:32:03
  same spot. Consider the distances involved. A galaxy on the
• 1:32:09
  far edge of Laniakia, two hundred fifty million light years
• 1:32:14
  from the center, is still caught in that inward pull.
• 1:32:19
  Light from that galaxy takes two hundred fifty million years
• 1:32:23
  to reach us, which means the dinosaurs were not yet
• 1:32:27
  extinct when that light left its source, and the galaxy
• 1:32:31
  that produced it is right now moving toward the same
• 1:32:36
  gravitational basin. We are moving toward. Everything within half a
• 1:32:42
  billion light years is sharing a direction. Your address in
• 1:32:46
  the universe, properly written out, has more lines than most
• 1:32:50
  people expect. A planet in a solar system, in a galaxy,
• 1:32:56
  in a galaxy, group in a supercluster, in inside a
• 1:33:00
  larger supercluster, all of it converging toward a gravitational center.
• 1:33:06
  And each line on that address is itself falling toward
• 1:33:09
  the line below it. The local group falls toward Virgo.
• 1:33:15
  Virgo falls with us toward the greater tractor. The greater tractor,
• 1:33:20
  along with all of Laniakia, falls toward the Shapleigh supercluster,
• 1:33:25
  six hundred and fifty million light years away. Even the
• 1:33:29
  container falls towards something. The Shapleigh supercluster is the most
• 1:33:34
  massive structure within roughly a billion light years of Earth.
• 1:33:39
  It contains more than eight thousand galaxies and outweighs the
• 1:33:43
  Greater Tractor by a considerable margin. It is pulling on
• 1:33:48
  Laniakia as a whole, bending the trajectory of every galaxy
• 1:33:52
  inside it, including hours. Calculations suggest that approximately fifty six
• 1:33:59
  percent of the Milky Way's peculiar velocity, the motion above
• 1:34:04
  and beyond simple cosmic expansion, comes from Shapleigh's gravity. The
• 1:34:10
  Greater Tractor accounts for most of the rest. We are
• 1:34:14
  not just falling toward one thing. We are falling toward
• 1:34:18
  a thing that is itself falling toward a larger thing,
• 1:34:22
  and that larger thing sits inside the cosmic web, surrounded
• 1:34:27
  by structures we have only recently begun to map. The
• 1:34:31
  cognitive experience of working through this hierarchy is difficult to
• 1:34:36
  describe cleanly. There is a moment when the nesting stops
• 1:34:40
  feeling like a neat organizational chart and starts feeling like
• 1:34:45
  something vitigenous. You realize that there is no privileged level,
• 1:34:50
  no natural stopping point where structure ends and emptiness begins.
• 1:34:56
  Every scale reveals another scale. Every the container is contained,
• 1:35:02
  and the motion runs all the way up the chain.
• 1:35:05
  Nothing is exempt from the pull of the next level out.
• 1:35:10
  The greater tractor sits at one of those convergence points,
• 1:35:14
  a node where the threads of the cosmic web pull tight,
• 1:35:19
  a gravitational crossroads, drawing in everything for hundreds of millions
• 1:35:24
  of light years in every direction. If you could somehow
• 1:35:27
  step back to a distance from which the entire observable
• 1:35:31
  universe fits in your field of view, what you would
• 1:35:35
  see is not a smooth fog of galaxies uniformly distributed
• 1:35:40
  through space. What you would see is a web long
• 1:35:45
  luminous filaments stretching across hundreds of millions of light years,
• 1:35:50
  branching and intersecting at dense glowing nodes. Between those filaments,
• 1:35:56
  almost nothing, enormous dark void, some spanning hundreds of millions
• 1:36:02
  of light years, where matter was pulled away long ago
• 1:36:07
  and never returned. The structure looks organic, it looks almost biological.
• 1:36:15
  Scientists call it the cosmic web, and it is the
• 1:36:18
  largest known structure in the universe. This was not always expected.
• 1:36:24
  For much of the twentieth century, the working assumption was
• 1:36:28
  that on large enough scales, matter would smooth out look
• 1:36:33
  far enough in any direction, and galaxies should be roughly
• 1:36:37
  evenly distributed. The universe should look more or less the
• 1:36:42
  same from any vantage point. This idea has a name,
• 1:36:47
  the cosmological principle, and it is still broadly true on
• 1:36:51
  the very largest scales. But on scales of hundreds of
• 1:36:55
  millions of light years, the universe is anything but small booth.
• 1:37:01
  It is lumpy, stringy, and punctuated by vast emptiness in
• 1:37:07
  a way that surprised many of the researchers who first
• 1:37:11
  mapped it. The first strong predictions of this web structure
• 1:37:14
  came not from telescopes but from computers. In the nineteen eighties,
• 1:37:20
  cosmologists running early numerical simulations of structure formation found that
• 1:37:26
  if you started with the right initial conditions and let
• 1:37:30
  gravity run for billions of years, matter did not spread evenly.
• 1:37:35
  It collapsed first into sheets, then into filaments, then into
• 1:37:41
  dense clumps. At the points where filaments crossed, the simulations
• 1:37:46
  showed a web forming almost inevitably, driven by nothing more
• 1:37:50
  exotic than gravity working on small initial differences in density.
• 1:37:57
  The pattern was striking enough that researchers took it seriously
• 1:38:01
  even before observations could confirm it. What the simulations could
• 1:38:06
  not produce on their own was the right amount of structure.
• 1:38:10
  Ordinary matter alone, the protons, electrons, and neutrons that make
• 1:38:16
  up stars and gas and dust did not collapse fast
• 1:38:20
  enough to build the web you actually see in the sky.
• 1:38:24
  Something else had to provide the scaffolding. That something was
• 1:38:28
  dark matter. Because dark matter does not interact with light,
• 1:38:34
  it began clustering under gravity earlier than ordinary matter did,
• 1:38:39
  before the universe had cooled enough for ordinary matter to follow.
• 1:38:44
  By the time ordinary matter could collapse, dark matter had
• 1:38:48
  already sketched out the structure, the filaments, the nodes, the voids.
• 1:38:55
  All of it was roughed in by dark matter first.
• 1:38:59
  Ordinary matter settled into those grooves, afterward, the way water
• 1:39:03
  fills the channel's carve the inter rock. The observational conformation
• 1:39:08
  came through large scale galaxy surveys. In the nineteen nineties
• 1:39:14
  and two thousands, Projects like the two DF Galaxy Redshift
• 1:39:18
  Survey and the Sloane Digital Sky Survey began mapping the
• 1:39:23
  three dimensional positions of hundreds of thousands of galaxies across
• 1:39:28
  enormous volumes of space. The web was unmistakable. Filaments ran
• 1:39:34
  for hundreds of millions of light years. Voids sat between them,
• 1:39:39
  some nearly empty of any visible matter. At the intersections
• 1:39:44
  of multiple filaments, dense galaxy clusters sat like knots in
• 1:39:49
  a net. The maps looked almost exactly like what the
• 1:39:54
  computer simulations had predicted decades earlier. The match between theory
• 1:39:59
  and si observation was one of the more striking conformations
• 1:40:03
  in modern cosmology. Where did the web ultimately come from?
• 1:40:08
  Go back far enough, and the answer lies in quantum mechanics.
• 1:40:13
  In the first fraction of a second after the Big Bang,
• 1:40:17
  the universe was not perfectly uniform. There were tiny, random
• 1:40:22
  fluctuations in density, regions slightly denser than average and regions
• 1:40:28
  slightly emptier, produced by quantum processes at almost incomprehensibly small scales.
• 1:40:36
  These fluctuations were then stretched across the universe during a
• 1:40:40
  period of rapid expansion called inflation. What began as quantum
• 1:40:46
  noise became the seeds of cosmic structure. Over the next
• 1:40:50
  several billion years, gravity amplified those seeds slightly denser regions
• 1:40:57
  attracted more matter, grew denser still, and eventually collapsed into
• 1:41:02
  the filaments and nodes of the cosmic web. The voids
• 1:41:07
  grew emptier as matter drained away from them toward the filaments.
• 1:41:13
  The web you see in galaxy survey maps today is
• 1:41:17
  the cumulative result of gravity working on quantum fluctuations for
• 1:41:22
  roughly thirteen billion years. The nodes of this web, the
• 1:41:27
  places where multiple filaments intersect, are where the largest gravitational
• 1:41:33
  concentrations in the universe sit. These are the superclusters, the
• 1:41:38
  places where the web pulls tight and matter has piled deepest.
• 1:41:44
  They are not random, They sit precisely where the geometry
• 1:41:49
  of the filaments forces them to be. When you have
• 1:41:53
  three or four massive filaments converging on the same region
• 1:41:56
  of space, the gravitational basin that intersection will accumulate mass
• 1:42:03
  over billions of years until it becomes something on the
• 1:42:07
  scale of the greater tractor. That is not a coincidence
• 1:42:11
  or an accident of initial conditions. In some narrow sense.
• 1:42:16
  It is the inevitable output of a process that was
• 1:42:20
  set in motion in the first moments of the universe.
• 1:42:25
  The Greater Tractor is one of the knots in that web,
• 1:42:29
  and its gravity is doing what gravity has always done
• 1:42:33
  and will always do, pulling the nearby universe toward the middle.
• 1:42:38
  Here is the strange part. Every second that passes the
• 1:42:43
  Milky Way moves roughly six hundred kilometers closer to the
• 1:42:46
  Greater Tractor. Not the Solar System, not just the Sun,
• 1:42:52
  the entire galaxy, every star in it, every planet orbiting
• 1:42:57
  those stars, every person who has lived covering six hundred
• 1:43:02
  kilometers of ground, every second pointed at the same destination,
• 1:43:07
  without interruption, without slowing down. That has been true for
• 1:43:13
  billions of years. It will be true tomorrow, and despite
• 1:43:18
  all of it, we will never get there. The reason
• 1:43:21
  has to do with something that is happening at the
• 1:43:23
  same time in the opposite direction. The universe is expanding.
• 1:43:29
  The space between the Milky Way and the Great a
• 1:43:32
  Tractor is not fixed. It is growing continuously because the
• 1:43:38
  fabric of space itself is stretching. This is not the
• 1:43:43
  Great Attractor moving away from us the way a car
• 1:43:46
  pulls ahead on a highway. The road itself is getting longer.
• 1:43:51
  Every megaparsec of distance between us and the attractor region
• 1:43:56
  is adding roughly seventy kilometers per second of resis session
• 1:44:01
  on top of whatever motion the galaxies have on their own.
• 1:44:05
  The greater tractor is somewhere between one hundred fifty and
• 1:44:10
  two hundred fifty million light years away. At that distance,
• 1:44:15
  the expansion of space is adding several thousand kilometers per
• 1:44:20
  second of separation between us. Our six hundred kilometers per
• 1:44:26
  second approach d O as not come close to canceling
• 1:44:30
  that out. We are running toward a destination that is
• 1:44:34
  receding faster than we can close. The gap. Now add
• 1:44:38
  dark energy. Since nineteen ninety eight, astronomers have known that
• 1:44:43
  the expansion of the universe is not slowing down. It
• 1:44:47
  is accelerating. Whatever dark energy is, it is pushing the
• 1:44:52
  universe apart at an ever increasing rate. That means the
• 1:44:57
  recession speed due to expansion is not a fixed number
• 1:45:01
  we are racing against. It is a growing number. Every
• 1:45:06
  billion years, the effective recession rate between us and structures
• 1:45:10
  at greater tractor distances gets a little larger. Our approach
• 1:45:15
  speed does not increase to match it. The gap between
• 1:45:20
  what we can do and what the universe is doing
• 1:45:23
  to the space in front of us quietly widens. Think
• 1:45:27
  of it this way. Imagine walking toward a door at
• 1:45:30
  the end of a corridor. You are making steady progress.
• 1:45:35
  You can see the door getting closer. Then you notice
• 1:45:38
  that the corridor itself is stretching. The door is still there.
• 1:45:43
  You are still walking, but the floor beneath you is
• 1:45:47
  being pulled backward even as your legs move forward. At first,
• 1:45:52
  you are fast enough that the distance still shrinks, but
• 1:45:56
  the corridor keeps stretching faster, and you're walking speed stays
• 1:46:01
  the same. Eventually, the door stops getting closer, and then
• 1:46:07
  slowly it starts to get farther away, even though you
• 1:46:12
  never stopped walking. That is roughly the situation we are in,
• 1:46:17
  on a scale that requires millions of light years instead
• 1:46:21
  of meters to describe the numbers bear this out in
• 1:46:25
  a rough way. The greater tractor sits approximately two hundred
• 1:46:30
  and fifty million light years out our peculiar velocity of
• 1:46:34
  six hundred kilometers per second. If it were the only
• 1:46:38
  thing happening, would mean we are cutting the distance at
• 1:46:41
  a rate that sounds fast until you do the conversion.
• 1:46:46
  Six hundred kilometers per second is about nineteen billion kilometers
• 1:46:51
  per year one light year is roughly nine point five
• 1:46:56
  trillion kilometers, so we are closing in about two thousand
• 1:47:01
  light years per year. At that rate, reaching something two
• 1:47:06
  hundred fifty million light years away would take more than
• 1:47:10
  one hundred billion years, well beyond the current age of
• 1:47:14
  the universe, and that calculation completely ignores the expansion working
• 1:47:21
  against us. In reality, the expansion is already adding separation
• 1:47:28
  faster than six hundred kilometers per second, which means the
• 1:47:32
  true distance to the grayer tea attractor region is not
• 1:47:36
  shrinking at all. It is growing, just more slowly than
• 1:47:40
  it would be if we had no peculiar velocity. There
• 1:47:43
  is something worth sitting with here. This is not a
• 1:47:47
  matter of insufficient technology or inadequate propulsion. No spacecraft, no
• 1:47:53
  matter how fast, launched in any direction, could reach the
• 1:47:57
  Greater Tractor before dark energy renders the journey impossible. The
• 1:48:03
  obstacle is not engineering, It is physics. The space between
• 1:48:08
  here and there is being manufactured faster than anything can
• 1:48:11
  cross it. The Greater Tractor is, in a precise sense,
• 1:48:16
  not merely distant. It is unreachable. Its gravity reaches us.
• 1:48:23
  Its effects on our motion have been real for billions
• 1:48:26
  of years. But the place itself sits behind an expanding
• 1:48:30
  barrier that deepens every second. What makes this genuinely strange
• 1:48:36
  is the word attractor. In physics and mathematics, an attractor
• 1:48:41
  is something that nearby states converge toward. Over time, Systems
• 1:48:46
  fall into attractors. They do not hover indefinitely at some
• 1:48:51
  fixed remove from them. A pendulum dragged to the side
• 1:48:55
  swings back to center and stays. A ball placed on
• 1:48:59
  a curved surface rolls to the bottom. The Greater Tractor
• 1:49:04
  is different. It exerts gravitational pull across hundreds of millions
• 1:49:10
  of light years, sets an entire supercluster's worth of galaxies
• 1:49:15
  moving toward its center, and then, because the universe is
• 1:49:20
  expanding underneath, everything never actually collects what it is pulling.
• 1:49:26
  The galaxies of Laniakia are draining toward this gravitational minimum
• 1:49:31
  the way water drains toward a plug hole, except the
• 1:49:34
  plug hole is slowly moving away from the water. The
• 1:49:38
  geometry does not allow convergence. It allows only an eternal,
• 1:49:43
  futile approach. Our galaxy has been making this approach since
• 1:49:49
  before the Sun formed. The Milky Way was already falling
• 1:49:54
  toward the Greater Tractor. When the first stars in it
• 1:49:57
  began to burn, and it has not stopped. The motion
• 1:50:02
  will continue through everything that happens to this galaxy, through
• 1:50:07
  the merger with Andromeda, through the long cooling of the universe,
• 1:50:12
  through changes that stretch across time scales with no useful
• 1:50:16
  human analogy. The approach is real. The gravity is real,
• 1:50:22
  the motion is real. The arrival is not. The Milky
• 1:50:27
  Way has been falling toward the Greater Tractor for billions
• 1:50:30
  of years and will keep falling for billions more, and
• 1:50:33
  it will never arrive. Eventually, dark energy wins completely. Right now,
• 1:50:41
  the numbers still favor the greater tractor. The pull is real.
• 1:50:46
  The six hundred kilometers per second is real. The flow
• 1:50:50
  lines threading through Laniakia, the rivers of galaxies draining toward
• 1:50:55
  the center, all of it is real and measurable today.
• 1:51:00
  But the universe does not hold still. What dark energy
• 1:51:04
  does on long time scales is not dramatic. It is patient.
• 1:51:09
  It does not shatter structures. It stretches the space between them, slowly, steadily,
• 1:51:16
  without interruption, until distances that will once meaningful become distances
• 1:51:22
  that are simply too large for gravity to cross. In
• 1:51:26
  a few billion years, something significant happens closer to home. First,
• 1:51:32
  the Milky Way and the Andromeda Galaxy, currently approaching each
• 1:51:37
  other at about one hundred ten kilometers per second, will collide.
• 1:51:42
  This is not a disaster in any human sense of
• 1:51:45
  the word. Galaxies are mostly empty space. The collision will
• 1:51:51
  play out over hundreds of millions of years, stars sliding
• 1:51:55
  past each other, gas clouds smashing together, new stars igniting
• 1:52:01
  from the compression. What emerges from the other side will
• 1:52:05
  be a single, large elliptical galaxy. Astronomers sometimes call it Milkomeda.
• 1:52:13
  It will be larger, rounder, and quieter than either of
• 1:52:17
  its predecessors, and it will still be moving toward the
• 1:52:21
  greater Tractor region. The merger does not change the bulk flow,
• 1:52:26
  the gravitational pull of the normal cluster, and the dark
• 1:52:29
  matter concentration at the heart of Lania Keia will still
• 1:52:33
  be there, still tugging. The combined remnant of our two
• 1:52:38
  galaxies will still be answering that pull, just as they
• 1:52:42
  do today, but the universe around Milkomeda will look different.
• 1:52:47
  By then, the nearest galaxy clusters beyond the local group
• 1:52:52
  will have receded significantly. The Virgo cluster, which today sits
• 1:52:58
  about sixty five million light years away and already contributes
• 1:53:03
  to the complex of gravitational forces shaping our motion, will
• 1:53:08
  be pulling away faster than any spacecraft could follow. The
• 1:53:13
  expansion of space does not care about gravity at those scales.
• 1:53:19
  Structures that are not tightly bound to each other will
• 1:53:22
  simply drift apart, the gap between them widening at an
• 1:53:27
  accelerating rate. There is nothing on any known physics that
• 1:53:31
  reverses this. Over tens of billions of years, the outer
• 1:53:36
  regions of Laniakia begin to dissolve. The supercluster is not
• 1:53:41
  gravitationally bound. This is the crucial detail. The Greater Tractor
• 1:53:47
  pulls hard enough to shape the motions of galaxies across
• 1:53:51
  hundreds of millions of light years today, but the structure
• 1:53:56
  it sits at the center of is not a closed system.
• 1:54:00
  It is a basin, and basins leak. The galaxies at
• 1:54:05
  the far edges of Laniakia, the ones whose in full
• 1:54:09
  velocity toward the Greater Tractor is only slightly larger than
• 1:54:13
  the recession velocity from cosmic expansion, will at some point
• 1:54:18
  tip the other way, first slowly, then decisively. They will
• 1:54:23
  stop falling inward and start being carried outward the flow
• 1:54:28
  lines that the twoenty fourteen Laniakia paper made so vivid.
• 1:54:34
  Those rivers of galaxies converging on a center will fray
• 1:54:38
  at the edges and then go silent. The greater tractor
• 1:54:42
  region itself will not disappear quickly. The normal cluster and
• 1:54:47
  the dark matter mass at the heart of the structure
• 1:54:51
  will remain gravitationally bound within their local volume. Galaxies that
• 1:54:57
  are already tightly locked together will will not be torn
• 1:55:00
  apart by dark energy on any time scale that astronomy
• 1:55:05
  currently considers, but the sphere of influence will contract. The
• 1:55:11
  radius out to which the greater tractor can meaningfully compete
• 1:55:15
  with cosmic expansion will shrink. Over time. Everything beyond that
• 1:55:21
  shrinke and boundary will simply be gone, carried over the
• 1:55:25
  horizon at faster than light recession speeds, unreachable and invisible
• 1:55:31
  on time scales of trillions of years, the picture becomes stark.
• 1:55:37
  The universe will consist of isolated gravitational islands Milkomeda, along
• 1:55:43
  with the handful of smaller galaxies in the local group
• 1:55:47
  that are tightly bound to it will persist, but the
• 1:55:51
  rest of the observable universe. Everything beyond the local group
• 1:55:56
  will have expanded beyond the cosmological horizon. No light from
• 1:56:01
  those structures will be able to reach us. No gravitational
• 1:56:05
  signal travels faster than light. If the source is beyond
• 1:56:10
  the horizon, its gravity effectively vanishes from our universe. As
• 1:56:15
  a practical consideration, any observers still present in that future
• 1:56:21
  will look out and see almost nothing. No galaxy clusters
• 1:56:25
  in the distance, no filaments, no sign that the cosmic
• 1:56:30
  web ever existed. The universe will look to them like
• 1:56:34
  an almost perfect void surrounding one small island of stars.
• 1:56:41
  The Greater Tractor, the object whose gravitational fingerprint the seven
• 1:56:46
  Samurai found in nineteen eighty six, the object toward which
• 1:56:51
  our galaxy has been accelerating for billions of years, will
• 1:56:56
  by that point be entirely beyond any possible observation. Not
• 1:57:01
  because it collapsed, not because it exploded, simply because the
• 1:57:06
  space between it and Milkomeda will have grown so vast
• 1:57:11
  that no signal of any kind could bridge it. The
• 1:57:15
  bulk flow will have ended, the peculiar velocity will have
• 1:57:20
  decayed to zero against the background of an expansion that
• 1:57:24
  has swallowed everything. All the structure we have spent this
• 1:57:30
  entire series of sections describing the Norma Cluster, the Norma Wall,
• 1:57:36
  the Laniakia basin, the convergence of flow lines toward a
• 1:57:41
  gravitational center will be gone from any detectable universe, replaced
• 1:57:47
  by silence and an expanding void that contains no trace
• 1:57:51
  of what came before. Somewhere in that enormous stretch of
• 1:57:56
  future time, the last trace of the Greater Tractors gravitate
• 1:58:00
  influence will fall below any threshold that future observers could detect.
• 1:58:06
  The object that has been pulling our galaxy for billions
• 1:58:09
  of years will effectively cease to exist, not by collapsing
• 1:58:14
  or exploding, but simply by becoming too far away to matter.
• 1:58:20
  Step back from the distances and the dark matter and
• 1:58:24
  the mass figures that stopped meaning anything several sections ago.
• 1:58:30
  The Greater Tractor is really a story about how incomplete
• 1:58:34
  our map of the universe kept turning out to be.
• 1:58:37
  For most of recorded human history, the night sky was
• 1:58:41
  the universe. Every point of light was something in our
• 1:58:45
  own galaxy, or so astronomers believed. Well into the twentieth century.
• 1:58:51
  The debate about whether spiral nebulae were nearby gas clouds
• 1:58:56
  or distant galaxies in their own right. Was not settled
• 1:59:00
  until the nineteen twenties, when Edwin Hubble measured the distance
• 1:59:05
  to Andromeda and the answer came back staggering. Our galaxy
• 1:59:10
  was one among many. The universe was immensely larger than
• 1:59:15
  anyone had seriously proposed. That was the first time the
• 1:59:20
  map turned out to be wrong in a fundamental way.
• 1:59:24
  It would not be the last. By the middle of
• 1:59:26
  the twentieth century, astronomers had organized galaxies into clusters and
• 1:59:31
  clusters into superclusters. The picture seemed reasonably complete. The Milky
• 1:59:38
  Way belonged to the local group. The local group belonged
• 1:59:42
  to the Virgo supercluster. There was a structure, a hierarchy.
• 1:59:49
  It seemed like the kind of thing that could eventually
• 1:59:51
  be fully described. Then the Seven Samurai mapped the peculiar
• 1:59:57
  velocities of four hundred galaxies and found that none of
• 2:00:01
  it was sitting still the way anyone expected. Everything was flowing.
• 2:00:08
  Something enormous was doing the pulling. It was sitting in
• 2:00:13
  a part of the sky nobody could see clearly. The
• 2:00:18
  map turned out to be wrong again. What the Greater
• 2:00:21
  Tractor forced astronomers to confront was not just a missing object,
• 2:00:26
  It was a missing scale. The structures driving the bulk
• 2:00:31
  motion of our entire cosmic neighborhood were so large, so distant,
• 2:00:36
  and so thoroughly hidden that the tools and frameworks astronomers
• 2:00:41
  had built were not sufficient to find them. The zone
• 2:00:45
  of avoidance is not an exotic problem. It is our
• 2:00:49
  own galaxy blocking our view. The irony is almost mocking.
• 2:00:55
  The single most familiar structure in the night sky, the
• 2:00:59
  band of the Milky Way, that humans have stared at
• 2:01:02
  since before writing was invented, turns out to be a
• 2:01:07
  curtain drawn across the most gravitationally significant region in the
• 2:01:12
  local universe. There is still a great deal that remains unresolved.
• 2:01:17
  The precise mass of the Greater Tractor depends heavily on
• 2:01:21
  how you define its boundaries and how much of the
• 2:01:24
  signal in galaxy peculiar velocities you attribute to it versus
• 2:01:29
  to the Shapleigh supercluster lying beyond it. The two thousand
• 2:01:35
  and five c Iza survey pushed the mass estimate down
• 2:01:39
  by a factor of ten from early predictions, but how
• 2:01:44
  much of that revision was genuine, and how much reflects
• 2:01:48
  the limits of X ray surveys that still cannot fully
• 2:01:51
  penetrate the zone of avoidance is not entirely clear. The
• 2:01:56
  split between the Greater Tractor's gravitational contribution and Shapley's roughly
• 2:02:03
  forty four percent and fifty six percent of our peculiar velocity, respectively,
• 2:02:10
  is itself an estimate that has shifted as surveys improved
• 2:02:15
  and may shift again. And behind the zone of avoidance,
• 2:02:19
  they're a re still structures we have only partially mapped.
• 2:02:24
  Radio and X ray observations have revealed pieces, the full
• 2:02:28
  three dimensional picture remains incomplete. The nature of dark matter
• 2:02:33
  is its own open question. Sitting at the center of everything,
• 2:02:39
  roughly eighty five percent of the mass responsible for pulling
• 2:02:42
  the Milky Way toward the Greater Tractor region is matter
• 2:02:46
  that has never been detected in any laboratory on Earth.
• 2:02:50
  Its existence is not seriously doubted by astronomers because the
• 2:02:55
  gravitational evidence for it is overwhelming. But what it actually is,
• 2:03:01
  what particle or particles, what properties beyond mass, remains unknown.
• 2:03:08
  The Greater Tractor is, in a very direct sense, mostly
• 2:03:12
  made of something we cannot identify. When you ask what
• 2:03:17
  the Greater Tractor is made of. The honest answer is
• 2:03:21
  we are not sure. Step back even further and the
• 2:03:25
  situation becomes something stranger than a scientific puzzle. Every person
• 2:03:30
  alive right now is on a planet orbiting a star
• 2:03:34
  inside a galaxy that is moving at six hundred kilometers
• 2:03:38
  per second toward a gravitational structure. It will never reach
• 2:03:43
  inside a supercluster that is itself not gravitationally bound, and
• 2:03:48
  will eventually be pulled apart by the expansion of the universe.
• 2:03:54
  None of this motion is felt, None of it is
• 2:03:57
  visible without instruments of considerable sophistication. The ground underfoot gives
• 2:04:04
  no indication. The sky overhead gives no indication. You would
• 2:04:10
  have to be an astronomer with access to galaxy redshift
• 2:04:14
  surveys and a working knowledge of the cosmic microwave background
• 2:04:19
  just to know it was happening. There is something worth
• 2:04:22
  sitting within that The forces shaping the largest features of
• 2:04:27
  our cosmic address are entirely imperceptible at human scale. The
• 2:04:33
  Greater Tractor pulls on our galaxy with a gravitational force
• 2:04:37
  that represents ten thousand times the mass of the Milky
• 2:04:41
  Way across a distance of one hundred fifty to two
• 2:04:45
  hundred fifty million light years. The result is a motion
• 2:04:50
  of six hundred kilometers per second that no instrument aboard
• 2:04:55
  any spacecraft has ever measured directly. Because it is a
• 2:05:01
  bulk motion, every part of us moves together, so nothing
• 2:05:06
  inside the system registers the speed. It is only visible
• 2:05:11
  from outside, using other galaxies as reference points. That is
• 2:05:17
  a peculiar situation. Astronomy has been for most of its history,
• 2:05:22
  the science of things that stay fixed while we watch them.
• 2:05:27
  The Greater Tractor is something different. It is a destination.
• 2:05:32
  It is a direction. It is the center of a
• 2:05:36
  gravitational basin so large that it contains our galaxy, all
• 2:05:41
  of our nearest neighbors, roughly one hundred thousand other galaxies
• 2:05:46
  draining toward the same low point. And yet it resists
• 2:05:51
  being pinned down. Its mass keeps being revised, its structure
• 2:05:56
  is still being mapped. The dark matter that makes up
• 2:05:59
  most of it has no agreed upon physical description, and
• 2:06:05
  the dark energy working against it guarantees that the journey
• 2:06:09
  toward it will never end in arrival. We cannot escape it,
• 2:06:15
  we cannot reach it, We cannot see most of it,
• 2:06:18
  we are not entirely sure what it is made of,
• 2:06:22
  and for now that is the most honest summary of
• 2:06:25
  where things stand.