A Dark Matter Sheet Shapes the Motion of the Milky Way

<v Speaker 1>Welcome to Bedtime Astronomy. Explore the wonders of the cosmos
<v Speaker 1>with our soothing Bedtime Astronomie podcast. Each episode offers a
<v Speaker 1>gentle journey through the stars, planets, and beyond, perfect for
<v Speaker 1>unwinding after a long day. Let's travel through the mysteries
<v Speaker 1>of the universe as you drift off into a peaceful
<v Speaker 1>slumber under the night sky.
<v Speaker 2>Hello, and welcome back. We are so thrilled to have
<v Speaker 2>you with us for this because today, well today we're
<v Speaker 2>embarking on a journey that is going to fundamentally change
<v Speaker 2>how you picture, well everything.
<v Speaker 3>That's a big claim to start with.
<v Speaker 2>It is I know, and I don't meet everything like
<v Speaker 2>you know your job or your relationships. Though honestly this
<v Speaker 2>might make you feel so small, it changes that too, Yeah,
<v Speaker 2>I mean the actual physical architecture of the reality we
<v Speaker 2>all live in.
<v Speaker 3>It's true. We're talking about our home today, and not
<v Speaker 3>just Earth, not just our solar system, but our entire
<v Speaker 3>galactic home, the Milky.
<v Speaker 2>And more than that, the neighborhood it's in because we
<v Speaker 2>all have this mental image of the Milky Way, don't we.
<v Speaker 2>It's this beautiful, glowing spiral just floating.
<v Speaker 3>A lonely island and a sea.
<v Speaker 2>Is nothing exactly like a frisbee thrown into a pitch
<v Speaker 2>black room. It's just sort of there, yeah, drifting in
<v Speaker 2>the void. We think of space as this big empty
<v Speaker 2>bucket that we're just rattling around inside.
<v Speaker 3>And that's the picture you see in textbooks, in planetariums.
<v Speaker 3>It's this romantic idea of solitary galaxy. But the research
<v Speaker 3>we're exploring today, which was just published yesterday January twenty seventh,
<v Speaker 3>twenty twenty six, it completely shatters that image.
<v Speaker 2>It turns out we're not just floating in some random void.
<v Speaker 2>We have an address, a very very specific, massive, and
<v Speaker 2>almost completely invisible address. We're not just in space. We
<v Speaker 2>are inside something.
<v Speaker 3>We are and finding that address, understanding that specific location
<v Speaker 3>actually solves a mystery that's been well, it's been keeping
<v Speaker 3>astronomers up at night for about fifty years.
<v Speaker 2>Fifty years, I mean that's a long time to be
<v Speaker 2>confused about your own backyard. That's generations of scientists staring
<v Speaker 2>at the same data and just going, wait, this doesn't
<v Speaker 2>add up.
<v Speaker 3>In cosmology, fifty years is practically a cold case, and
<v Speaker 3>this particular cold case among astronomers it's known as the
<v Speaker 3>Mystery of the Two Quiet Neighbors.
<v Speaker 2>I love that name. Yeah, sounds like a suburban crime novel.
<v Speaker 2>The neighbors were quiet.
<v Speaker 3>To quiet In a cosmological sense, that's exactly what it is.
<v Speaker 3>It's a puzzle about motion, and it's a puzzle about gravity.
<v Speaker 3>The central question is why does everything around our galaxy
<v Speaker 3>move so perfectly, so exactly the way the Big Bang
<v Speaker 3>predicts it should, when by all rights the laws of
<v Speaker 3>gravity say it should be a complete mess.
<v Speaker 2>And the answer, which we are going to unpack today,
<v Speaker 2>involves building a virtual twin of our universe. It involves
<v Speaker 2>a massive, invisible sheet of matter that we are apparently
<v Speaker 2>surfing on, and these vast, almost terrifyingly empty spaces called
<v Speaker 2>cosmic voids.
<v Speaker 3>It sounds like science fiction, but this is hard data
<v Speaker 3>coming from the University of growning In published in Nature Astronomy,
<v Speaker 3>which is one of the most prestigious journals out there.
<v Speaker 2>This is the real deal, it is.
<v Speaker 3>And it's one of those discoveries that feels so satisfying.
<v Speaker 3>It doesn't just give us a new piece of trivia.
<v Speaker 3>It makes the entire picture finally click into place.
<v Speaker 2>It really does so for today's exploration, and we are
<v Speaker 2>calling this an exploration. I want everyone listening to just
<v Speaker 2>strap in. We're not just looking at data points today.
<v Speaker 2>We're going to try to visualize the invisible architecture of
<v Speaker 2>our little corner of the cosmos. We're going to map
<v Speaker 2>out our cosmic geography.
<v Speaker 3>I like that cosmic geography. Just like you need to
<v Speaker 3>know about mountains and oceans to understand history on Earth,
<v Speaker 3>you have to understand dark matow sheets and voids to
<v Speaker 3>understand the history of our galaxy.
<v Speaker 2>Okay, so let's set the stage to understand why this
<v Speaker 2>is such a huge deal. We first need to understand
<v Speaker 2>the before picture. Let's talk about the local group. It
<v Speaker 2>sounds like a neighborhood watch committee.
<v Speaker 3>It's sort of a for galaxies. The local group is
<v Speaker 3>our immediate family in the universe. Space is vast, but galaxies,
<v Speaker 3>you know, they tend to cluster together.
<v Speaker 2>We're not alone in the Milky Way. We're one of
<v Speaker 2>the big ones in this group.
<v Speaker 3>We're one of the two titans. We have a sibling,
<v Speaker 3>a big sister actually, and the Andromeda galaxy. She's a
<v Speaker 3>bit bigger than us, maybe a little more massive depending
<v Speaker 3>on how you measure it. But the two of us.
<v Speaker 3>We are the dominant gravitational players in this region.
<v Speaker 2>But it's not just the two of us, right, Yeah,
<v Speaker 2>there are others in the house.
<v Speaker 3>Oh absolutely. Surrounding us two giants are dozens of smaller
<v Speaker 3>what we call satellite galaxies, mostly dwarf galaxies.
<v Speaker 2>Nothing like the Toro moons for our galaxy.
<v Speaker 3>That's a perfect way to think of it. They're caught
<v Speaker 3>in our combined gravitational web buzzing around us. The large
<v Speaker 3>and small Magellanic clouds are the most famous ones you
<v Speaker 3>can see from the Southern Hemisphere. We're all bound together
<v Speaker 3>by gravity. We're a family unit.
<v Speaker 2>Okay, so we're gravitationally bound family. We're all holding hands. Now,
<v Speaker 2>let's zoom out a bit, because to get to the mystery,
<v Speaker 2>we have to understand the main rule that the rest
<v Speaker 2>of the universe follows. If we look past our family,
<v Speaker 2>what are all the other families doing?
<v Speaker 3>They're all moving away. That's the big rule expansion. Almost
<v Speaker 3>one hundred years ago, Edwin Hubble and George's Lemetra figured
<v Speaker 3>out that the universe is getting bigger.
<v Speaker 2>The Hubble of Metro law. We hear this all the time,
<v Speaker 2>but let's just really break it down for a second.
<v Speaker 2>What does it actually mean On a practical level.
<v Speaker 3>The observation itself is actually remarkably simple. If you look
<v Speaker 3>at any distant galaxy and you measure the light coming
<v Speaker 3>from it, you'll see that the light waves are stretched out,
<v Speaker 3>they're red shifted, they're red shifted. It's the Doppler effect
<v Speaker 3>for light, and that stretching tells us that the galaxy
<v Speaker 3>is moving away from us.
<v Speaker 2>Like the sound of a siren dropping in pitch as
<v Speaker 2>an ambulance drives past you.
<v Speaker 3>Exactly like that. But here's the crucial part. Hubble noticed
<v Speaker 3>there's a pattern. The further away a galaxy is the
<v Speaker 3>faster it's moving away from us. A galaxy that's twice
<v Speaker 3>as far away is receding twice as fast.
<v Speaker 2>So it's not that galaxies are flow yeing through space
<v Speaker 2>like bullets. Is that the space between everything is stretching.
<v Speaker 3>Precisely. The classic analogy is drawing dots on a balloon
<v Speaker 3>and then blowing it up. From the perspective of any
<v Speaker 3>one dot, all the other dots appear to be moving away.
<v Speaker 3>This is the bedrock evidence for the Big Bang. If
<v Speaker 3>you run that movie in reverse, everything crunches down to
<v Speaker 3>a single point.
<v Speaker 2>Okay, So the rule of the cosmos is expansion. If
<v Speaker 2>I look out my window with a telescope, I should
<v Speaker 2>see cosmic red tail lights everywhere. Everyone is driving away.
<v Speaker 2>That's the standard flow.
<v Speaker 3>That is the hubble flow. That is the general rule.
<v Speaker 3>But there's always a butt in physics, isn't there?
<v Speaker 2>There usually is.
<v Speaker 3>And there is a very very famous exception to this rule.
<v Speaker 3>Sitting right on our doorstep Andromeda again, Andromeda again. If
<v Speaker 3>you measure the light from Andromeda, it is not red shifted,
<v Speaker 3>it's blue shifted.
<v Speaker 2>Which means it's coming toward us. It's not driving away,
<v Speaker 2>it's flooring it in our direction.
<v Speaker 3>It is barreling toward the Milky Way at a speed
<v Speaker 3>of about one hundred kilometers per second.
<v Speaker 2>Okay, one hundred kilometers a little second. Can you put
<v Speaker 2>that in perspective for me? That number is just it's too.
<v Speaker 3>Big, Okay. Imagine traveling from New York City to Philadelphia.
<v Speaker 3>That's about one hundred and fifty kilometers. So Andromeda covers
<v Speaker 3>that distance in one and a half seconds.
<v Speaker 2>It's unthinkable. A rifle bullet travels at what maybe one
<v Speaker 2>kilometer per second if that?
<v Speaker 3>So this is an entire galaxy with hundreds of billions
<v Speaker 3>of stars moving one hundred times faster than a speeding bullet,
<v Speaker 3>and it's aimed right at us.
<v Speaker 2>So we're due for a collision we are.
<v Speaker 3>In about four and a half billion years, our two
<v Speaker 3>galaxies will merge. It'll be a spectacular cosmic event. The
<v Speaker 3>night sky will be completely transformed as these two spirals
<v Speaker 3>tear each other apart and reform into one giant elliptical galaxy.
<v Speaker 2>But why why is this happening, If the whole universe,
<v Speaker 2>if the fabric of space time is stretching, why are
<v Speaker 2>we getting closer to Andromeda?
<v Speaker 3>Because on a local scale, gravity is stronger than the expansion.
<v Speaker 3>The Milky Way and Andromeda are just so massive, and
<v Speaker 3>we're relatively close to each other, only about two and
<v Speaker 3>a half million light.
<v Speaker 2>Years apart, only two and a half million light years.
<v Speaker 3>In cosmic terms, that's next door. Our mutual gravitational pull
<v Speaker 3>is so strong that it completely overpowers the gentle stretching
<v Speaker 3>of the universe between us.
<v Speaker 2>Okay, I get that. So locally we're a bound system.
<v Speaker 2>Our little family unit is holding hands so tightly that
<v Speaker 2>the expanding current of the universe can't pull us apart exactly.
<v Speaker 3>We are gravitationally detached from the hubbleflow. We're our own
<v Speaker 3>little island of gravitational stability.
<v Speaker 2>Okay, So gravity pulls things together, expansion pushes them apart,
<v Speaker 2>a cosmic tug of war.
<v Speaker 3>I'm with you, now Here is the mystery. Here is
<v Speaker 3>the puzzle that has bothered astronomers for five decades. To
<v Speaker 3>see it, we need to look just outside the local.
<v Speaker 2>Group, the neighbors just over the fence.
<v Speaker 3>Right, Not our little satellite galaxies, not Andromeda. We're talking
<v Speaker 3>about the next set of galaxies out. They're close enough
<v Speaker 3>that they should still feel the immense gravity of our local.
<v Speaker 2>Group, because we're a giant gravitational sinkhole. Right. Yeah, the
<v Speaker 2>combined mass of the Milky Way and Andromedy is huge.
<v Speaker 2>If I'm a galaxy just drifting pass, I should feel
<v Speaker 2>that pull.
<v Speaker 3>You absolutely should. Newton's laws of gravity don't just stop
<v Speaker 3>at some arbitrary boundary. So we would expect our gravity
<v Speaker 3>to be tugging on these nearby galaxies. It should be
<v Speaker 3>slowing down their retreat, or it should be pulling them
<v Speaker 3>off course, making their movements messy, chaotic.
<v Speaker 2>It's like if you're trying to run away from someone
<v Speaker 2>and they managed to grab the back of your shirt.
<v Speaker 2>You might still get away, but you're gonna stumble. Your
<v Speaker 2>path won't be a smooth straight line.
<v Speaker 3>That is a perfect analogy, and in astronomy we call
<v Speaker 3>those stumbles peculiar velocities. Peculiar just means a velocity that's
<v Speaker 3>particular to the local gravity, not part of the general
<v Speaker 3>cosmic flow. We expect the galaxies right next to us
<v Speaker 3>to have very high peculiar velocities. We expect to see
<v Speaker 3>a total mess.
<v Speaker 2>But I'm guessing, based on the setup here, that we
<v Speaker 2>do not see a mess.
<v Speaker 3>We do not, that's the whole problem. For fifty years,
<v Speaker 3>observations have shown that these nearby galaxies are moving away
<v Speaker 3>from us almost perfectly. They follow the hubble of Matri
<v Speaker 3>law to the letter. Their motion is incredibly smooth. It's
<v Speaker 3>suspiciously quiet.
<v Speaker 2>So they're acting as if we, this entire massive local group,
<v Speaker 2>aren't even here.
<v Speaker 3>Precisely, it's as if our gravity just magically switches off
<v Speaker 3>at the property line. And physically that makes absolutely no sense.
<v Speaker 3>Imagine you place a heavy bowling ball on a trampoline.
<v Speaker 3>The fabric of the traanpoline curves down right, it creates
<v Speaker 3>a deep well.
<v Speaker 2>Of course, that's the classic visualization of gravity warping space time.
<v Speaker 3>Now, imagine you roll some marbles past that bowling ball.
<v Speaker 3>What should happen.
<v Speaker 2>They should curve inward, they should be deflected by the
<v Speaker 2>dip in the trampoline, Their paths should change exactly.
<v Speaker 3>But what we see in the cosmos is the equivalent
<v Speaker 3>of those marbles rolling in a perfectly straight line, as
<v Speaker 3>if the bowling ball and its huge gravitational well weren't
<v Speaker 3>there at all. They just cruise past completely ignoring the
<v Speaker 3>massive objects sitting right next to them.
<v Speaker 2>That would drive me insane. It defies basic physics. Mass
<v Speaker 2>he eats gravity, Gravity affects motion, end of story. Why
<v Speaker 2>aren't we affecting them?
<v Speaker 3>It drove astronomers crazy. It's been called the deviation problem.
<v Speaker 3>Why are the motions in our local universe so cold,
<v Speaker 3>so quiet, so orderly, when we have this enormous concentration
<v Speaker 3>of mass right here that should be stirring everything up.
<v Speaker 3>It's a huge anomaly.
<v Speaker 2>It sounds like a glitch in the matrix, like someone
<v Speaker 2>forgot to turn on the physics engine for our particular sector.
<v Speaker 3>It's funny you mentioned a simulation, because to finally solve
<v Speaker 3>this fifty year old puzzle, that's exactly where these researchers
<v Speaker 3>had to go. They had to build a simulation.
<v Speaker 2>So let's talk about the team that cracked the case. Yeah,
<v Speaker 2>we have the lead researcher, Ewood Wempe who was a
<v Speaker 2>PhD student at the time, and professor Amina Helmy, both
<v Speaker 2>from the Captime Institute in Groningen.
<v Speaker 3>That's right, and they had collaborators from Germany, France and Sweden.
<v Speaker 3>It was a big international effort to solve this very
<v Speaker 3>local problem.
<v Speaker 2>And their big idea was that just looking through telescopes
<v Speaker 2>wasn't cutting it anymore. People had been doing that for decades.
<v Speaker 2>They needed a new approach. They needed to build a
<v Speaker 2>universe from the ground up.
<v Speaker 3>They did. They set out to create what they call
<v Speaker 3>a virtual twin of our local universe.
<v Speaker 2>A virtual twin. I love that term. It sounds like
<v Speaker 2>something from engineering, where they build a digital replica of
<v Speaker 2>a jet engine to test it before building the real one.
<v Speaker 3>And it implies a kind of precision, that is, it's
<v Speaker 3>incredibly rovert. In cosmology, usually when we simulate the universe,
<v Speaker 3>we're dealing with big statistical averages. We create a huge
<v Speaker 3>generic box of the universe. We throw in the right
<v Speaker 3>ingredients and we let it evolve, and.
<v Speaker 2>You get oh universe.
<v Speaker 3>Yeah, but not our universe exactly. You get something that
<v Speaker 3>statistically looks like ours. It has the right number of galaxies,
<v Speaker 3>the right kind of clustering, but it's not our specific reality.
<v Speaker 3>It doesn't have the milky aa right here in Andromeda,
<v Speaker 3>right there, moving in just the right way.
<v Speaker 2>It's like having a stock photo of a family. It
<v Speaker 2>looks like a family, but it's not your family. The
<v Speaker 2>stock photo can't tell you why the floorboard in your
<v Speaker 2>specific living room squeaks.
<v Speaker 3>That's a great way to put it. This team needed
<v Speaker 3>a simulation that reproduced our specific reality down to the details.
<v Speaker 3>They needed to recreate the cosmic crime scene exactly as
<v Speaker 3>it is today.
<v Speaker 2>How do you even begin to do that? The universe
<v Speaker 2>is so complex. You can't just type create milky Way
<v Speaker 2>into a computer and hit enter.
<v Speaker 3>No, it's much more subtle than that. They used a
<v Speaker 3>really clever technique that works both forwards and backwards in time.
<v Speaker 3>They started at the very beginning. They started with the
<v Speaker 3>baby universe, using data from the cosmic microwave background.
<v Speaker 2>The CMB just remind us quickly what that is. Again.
<v Speaker 3>The CMB is the afterglow of the Big Bang. It's
<v Speaker 3>the oldest light in the universe, from about three hundred
<v Speaker 3>and eighty thousand years after the beginning. And what it
<v Speaker 3>gives us is a map. It's a map of how
<v Speaker 3>matter was distributed when the universe was just an infant.
<v Speaker 3>It wasn't perfectly smooth. There were tiny, tiny variations. Some
<v Speaker 3>spots were a fraction of a degree warmer, meaning slightly denser,
<v Speaker 3>and some were cooler.
<v Speaker 2>And those slightly denser spots are the seeds that eventually
<v Speaker 2>grow into galaxies and galaxy clusters.
<v Speaker 3>They're the seeds of everything. Gravity takes over, pulling more
<v Speaker 3>and more matter into those denser regions, and over thirteen
<v Speaker 3>point eight billion years, they blossom into the cosmic structure
<v Speaker 3>as we see today. So the researchers took that initial map,
<v Speaker 3>that primordial seed, and they plugged it into a supercomputer.
<v Speaker 2>And then they just hit fast forward.
<v Speaker 3>Essentially. Yes, they applied all the known laws of physics, gravity,
<v Speaker 3>dark matter interactions, the expansion of space, and they let
<v Speaker 3>this baby universe grow up. But here's the thing. The
<v Speaker 3>universe is chaotic. A tiny imperceptible change in those initial
<v Speaker 3>conditions can lead to a completely different universe thirteen point
<v Speaker 3>eight billion years later.
<v Speaker 2>A butterfly effect on a cosmic schedus exactly.
<v Speaker 3>So, they couldn't just run the simulation once. They had
<v Speaker 3>to run many, many variations, tweaking the initial conditions ever
<v Speaker 3>so slightly. They were searching for a very specific outcome.
<v Speaker 3>They needed to find a simulation that ended up with
<v Speaker 3>a Milky Way and an Andromeda that had the corre
<v Speaker 3>masses were in the correct positions, and we're moving toward
<v Speaker 3>each other at the correct speed that one hundred kilometers
<v Speaker 3>per second.
<v Speaker 2>Just that part alone seems impossible. It feels like trying
<v Speaker 2>to win the cosmic lottery.
<v Speaker 3>It's incredibly difficult. But they went even further. This is
<v Speaker 3>the part that just blows my mind. They constrained the
<v Speaker 3>simulation so that it also had to match the observed
<v Speaker 3>positions and velocities of thirty one specific galaxies just outside.
<v Speaker 2>Our local group thirty one. Wow. So they weren't just
<v Speaker 2>trying to get the two main characters right. They had
<v Speaker 2>to get the entire supporting cast in the right place.
<v Speaker 2>Doing the right thing.
<v Speaker 3>It's like trying to simulate a hurricane and demanding that
<v Speaker 3>it not only hits the right city, but that thirty
<v Speaker 3>one specific trees in that city bend in exactly the
<v Speaker 3>right way at the right time. It is an extraordinary
<v Speaker 3>level of constraint.
<v Speaker 2>It's looking for a needle in a haystack of possible universes.
<v Speaker 3>It is. They had to sift through countless virtual universes
<v Speaker 3>to find the one that perfectly matched our own. But
<v Speaker 3>they did it. They actually found it. They successfully created
<v Speaker 3>a virtual twin.
<v Speaker 2>And once you have that twin, that's where the magic happens. Yeah,
<v Speaker 2>because you can look under the hood. You can see
<v Speaker 2>all the parts that are invisible in the real world.
<v Speaker 3>That's the power of it. In the real universe, we
<v Speaker 3>have a huge problem. Most of the matter, about eighty
<v Speaker 3>five percent of it is dark matter. It's invisible. We
<v Speaker 3>know it's there from its gravitational effects, but we can't
<v Speaker 3>see it, we can't touch it. We are essentially stumbling
<v Speaker 3>around in the dark trying to map a landscape we
<v Speaker 3>can't see. But in the simulation, in the simulation, you
<v Speaker 3>know where every single particle of dark matter is. You
<v Speaker 3>put it there. You can just flip the switch and say, okay, computer,
<v Speaker 3>make the dark matter visible.
<v Speaker 2>And when they flip that switch, when they finally looked
<v Speaker 2>at the distribution of the invisible stuff in their perfect
<v Speaker 2>virtual twin, they saw the answer to the fifty year
<v Speaker 2>old mystery.
<v Speaker 3>They did, and it was not what anyone expected.
<v Speaker 2>This is the big reveal. What did they see? What
<v Speaker 2>does our galactic neighborhood actually look like?
<v Speaker 3>They saw that we are not living in a random,
<v Speaker 3>spherical blob of matter. We are not in a cloud,
<v Speaker 3>the local Group, the Milky Way, Andromeda, all of us.
<v Speaker 3>We are embedded inside a vast, large scale sheet of
<v Speaker 3>dark matter.
<v Speaker 2>A sheet like a piece of paper.
<v Speaker 3>A very very thick piece of paper. Think of it
<v Speaker 3>as a flatten structure, a pancake or a wall, a
<v Speaker 3>flat extended plane of mass.
<v Speaker 2>And how big are we talking? What's the scale of this,
<v Speaker 2>this cosmic pancake.
<v Speaker 3>It extends for tens of millions of light years. It's
<v Speaker 3>an absolutely colossal structure.
<v Speaker 2>So if you could zoom way way out from our
<v Speaker 2>galaxy and put on a pair of dark matter goggles,
<v Speaker 2>you wouldn't see a ball. You'd see this giant flat sheet.
<v Speaker 3>Yes, and the Milky Way and Andromeda were like two
<v Speaker 3>blueberries baked into the middle of this pancake. We are
<v Speaker 3>sitting right inside this enormous flat layer of invisible mass.
<v Speaker 2>Okay, so we're in a pancake. But pancakes have a
<v Speaker 2>top and bottom. Their's space above and below them. What's
<v Speaker 2>above and below our dark matter sheet voids voids. That
<v Speaker 2>sounds empty.
<v Speaker 3>It is the technical term is also the literal description.
<v Speaker 3>Mumulations showed that our sheet is sandwiched between two enormous
<v Speaker 3>regions of emptiness, vast expanses where there are almost no
<v Speaker 3>galaxies and very very little matter at all.
<v Speaker 2>So let me try to visualize this. We have this dense,
<v Speaker 2>flat plane of dark matter and galaxies that we live inside,
<v Speaker 2>and if you were to travel up or down relative
<v Speaker 2>to that plane, you would just hit nothing for millions
<v Speaker 2>of light years.
<v Speaker 3>For millions and millions of light years, you'd enter what
<v Speaker 3>we call the local voids. It's like living on a
<v Speaker 3>very thin, very large island in the middle of a
<v Speaker 3>gigantic empty ocean that extends in both directions.
<v Speaker 2>Wow, that is simultaneous claustrophobic and agoraphobic. We're sort of
<v Speaker 2>trapped in this thin wall of stuff between two endless
<v Speaker 2>empty rooms.
<v Speaker 3>It's a very striking image and it fits into our
<v Speaker 3>broader understanding of the cosmic web. We know the universe's
<v Speaker 3>large scale structure is made of filaments, walls, and voids.
<v Speaker 3>This research tells us our specific address. We live in
<v Speaker 3>a wall. We are part of the dry wall of
<v Speaker 3>the universe.
<v Speaker 2>And this specific architecture, the sheet in these voids. This
<v Speaker 2>is the key to solving that gravity paradox we talked about.
<v Speaker 3>It is the absolute key. The simulation showed that this
<v Speaker 3>is the only geometry that works. If you have a
<v Speaker 3>random blob of dark matter, you get the messy motions
<v Speaker 3>everyone expected. But if you have this specific flattened sheet structure,
<v Speaker 3>you get the unnervingly quiet motions that we actually observe.
<v Speaker 2>Okay, let's connect the dots. We have the sheet, we
<v Speaker 2>have the mystery of the quiet neighbors, and the bowling
<v Speaker 2>ball that doesn't seem to affect the marbles. How does
<v Speaker 2>the sheet explain why those nearby galaxies are ignoring our gravity?
<v Speaker 3>It all comes down to a gravitational tug of war
<v Speaker 3>that basically ends in a perfect stalemate.
<v Speaker 2>Okay, walk me through the physics of that.
<v Speaker 3>Right, So take one of those nearby galaxies. It's sitting
<v Speaker 3>just outside our local group. The local group itself is
<v Speaker 3>incredibly massive, so we are pulling on that galaxy. We're
<v Speaker 3>pulling it in toward us.
<v Speaker 2>Okay, that's the grab the back of the shirt part
<v Speaker 2>of the analogy. We're slowing it down exactly.
<v Speaker 3>But now we know we're not just an isolated clump.
<v Speaker 3>We are part of this huge sheet that it extends
<v Speaker 3>far beyond that galaxy. There is a massive amount of
<v Speaker 3>dark matter further out in the sheet on the other
<v Speaker 3>side of that galaxy.
<v Speaker 2>Ah. Okay, so there's mass behind it.
<v Speaker 3>Too, a huge amount of mass, so that galaxy is
<v Speaker 3>essentially sandwiched. We the local group, are pulling it in
<v Speaker 3>one direction and the rest of the massive sheet is
<v Speaker 3>pulling it in the other direction. It's being pulled out.
<v Speaker 2>So we pull it toward us, and the rest of
<v Speaker 2>the sheet pulls it away from us.
<v Speaker 3>Precisely, And the simulation showed that those two opposing forces
<v Speaker 3>almost perfectly cancel each other out. They balance, they mask
<v Speaker 3>each other's effects. The net gravitational force on that galaxy
<v Speaker 3>from within the plane of the sheet is close to zero,
<v Speaker 3>so the galaxy is free to just coast.
<v Speaker 2>So it just follows the general expansion of the universe
<v Speaker 2>as if there were no major gravitational forces acting on
<v Speaker 2>it at all.
<v Speaker 3>Because the forces are there, but they're pulling equally from
<v Speaker 3>both sides. It's not that there's no gravity, it's that
<v Speaker 3>there's balanced gravity.
<v Speaker 2>That is that is so elegant, it's brilliant. It's not
<v Speaker 2>a violation of physics. It's a constant sequence of a
<v Speaker 2>very specific geometric arrangement of mass.
<v Speaker 3>It's a dynamic equilibrium. Imagine standing exactly in the middle
<v Speaker 3>of a narrow hallway with two giant fans at either end,
<v Speaker 3>both blowing at you with the exact same force. You
<v Speaker 3>might not move at all, but that doesn't mean there
<v Speaker 3>are no forces acting on you. The forces are huge,
<v Speaker 3>but they sum to zero.
<v Speaker 2>Okay, that makes perfect sense. But you mentioned the voids
<v Speaker 2>were also part of the solution. The empty spaces above
<v Speaker 2>and below the sheet, How do they fit into this
<v Speaker 2>illusion of calmness.
<v Speaker 3>They are just as important. This is the second piece
<v Speaker 3>of the puzzle. Think about where those messy, chaotic, peculiar
<v Speaker 3>velocities usually come from in a galaxy cluster.
<v Speaker 2>They come from things falling into the cluster from all directions.
<v Speaker 3>Right, Exactly, if you have a big spherical cloud of matter,
<v Speaker 3>you expect other things to be falling toward it, from
<v Speaker 3>the sides, from the top, from the bottom, from every
<v Speaker 3>possible angle. It should be raining matter.
<v Speaker 2>You'd have galaxies coming in from all.
<v Speaker 3>Over the place, and that infall would look very messy.
<v Speaker 3>In our data, we'd see galaxies with peculiar velocities coming
<v Speaker 3>from every direction. It would be a very hot or
<v Speaker 3>dynamic system.
<v Speaker 2>But we don't have matter above and below us.
<v Speaker 3>We have voids, and you can't have rain if there
<v Speaker 3>are no.
<v Speaker 2>Clouds, there's nothing to fall.
<v Speaker 3>Decisely, the directions from which you would expect to see
<v Speaker 3>messi infall from above and below the plane of our
<v Speaker 3>sheet are empty. There are no galaxies there to be
<v Speaker 3>seen falling in.
<v Speaker 2>So the data looks clean because the parts that would
<v Speaker 2>make it messy are just empty. There's no data there
<v Speaker 2>to begin with.
<v Speaker 3>Exactly, the observations we make are naturally biased toward the
<v Speaker 3>galaxies that lie within the sheet, and those are the
<v Speaker 3>very galaxies that are caught in that perfect gravitational tug
<v Speaker 3>of war. The whole system is constructed in a way
<v Speaker 3>that hides its own gravitational influence.
<v Speaker 2>That is an incredibly clever solution to a fifty year
<v Speaker 2>old problem. It's not that the physics is wrong, it's
<v Speaker 2>that our local geography is filtering what we're able to see.
<v Speaker 3>Yes, the researchers concluded that this flat mass distribution is
<v Speaker 3>the only shape that simultaneously explains the massive local group,
<v Speaker 3>the collision course of Andromeda, and the quiet, orderly flow
<v Speaker 3>of the neighboring galaxies. It all clicks into place if
<v Speaker 3>and only if we live in a sheet.
<v Speaker 2>This has to feel like a huge validation for a
<v Speaker 2>lot of people. I was reading some of the quotes
<v Speaker 2>from the paper and Professor Amina Helmy, one of the leads,
<v Speaker 2>seem really excited by this.
<v Speaker 3>She was. She said something along the lines of how
<v Speaker 3>exciting it was that they could solve a decade's old
<v Speaker 3>problem purely based on the motions of galaxies. It's a
<v Speaker 3>real triumph for the standard model of cosmology.
<v Speaker 2>It feels like a dodged a bullet in a way.
<v Speaker 3>It absolutely did. You know, in science, whenever your observations
<v Speaker 3>don't match your theory, you face a choice. Either your
<v Speaker 3>theory is wrong or your understanding of a specific situation
<v Speaker 3>is incomplete.
<v Speaker 2>And it's always more dramatic to say the theory is wrong.
<v Speaker 2>You see those headlines all the time, Einstein was wrong,
<v Speaker 2>or dark matter is a hoax.
<v Speaker 3>It makes for great clickbait, And to be fair, there
<v Speaker 3>are some real tensions and cosmology right now. We have
<v Speaker 3>the Hubble tension, where different methods of measuring the universe's
<v Speaker 3>expansion rate give different answers, So people are actively looking
<v Speaker 3>for cracks in the standard model.
<v Speaker 2>So when this quiet neighbors problem persisted, some people must
<v Speaker 2>have started to wonder, well, maybe gravity works differently on
<v Speaker 2>these scales, or maybe dark matter isn't real exactly.
<v Speaker 3>People were proposing alternative theories like modified Newtonian dynamics or
<v Speaker 3>mond But this research pulls us back from that ledge.
<v Speaker 3>It shows that the standard theory, cold dark matter, the
<v Speaker 3>Big Bang, general relativity, it all works perfectly. We just
<v Speaker 3>didn't understand our local context. We didn't know we were
<v Speaker 3>living in a sheet.
<v Speaker 2>We were trying to solve the puzzle without ever looking
<v Speaker 2>at the map of the crime scene.
<v Speaker 3>Right We were treating the local group as an isolated island,
<v Speaker 3>when in reality it's part of a huge continent. Once
<v Speaker 3>you factor in the geography of the continent, the mystery vanishes,
<v Speaker 3>the math works.
<v Speaker 2>It brings observation and theory back into perfect harmony. We
<v Speaker 2>don't need new physics, we just needed a better map.
<v Speaker 3>And speaking of maps, the lead author, Ewould Wempy, called
<v Speaker 3>this the first assessment of the distribution and velocity of
<v Speaker 3>dark matter in the regions surrounding the Milky Way and Andromeda.
<v Speaker 2>That's a huge milestone. Think about the early explorers in
<v Speaker 2>the fifteenth and sixteenth centuries painstakingly mapping the coastlines of
<v Speaker 2>new continents for the first time. That's what this is.
<v Speaker 2>But for the invisible landscape we inhabit, it's.
<v Speaker 3>A fundamental shift. We are finally moving from just cataloging
<v Speaker 3>points of light in the sky to understanding the underlying
<v Speaker 3>terrain that those lights are sitting on, and it.
<v Speaker 2>Really contextualizes our place in the cosmic web. We've always
<v Speaker 2>heard that the universe on the largest scales looks like
<v Speaker 2>a web or a sponge.
<v Speaker 3>Yes, a vast network of filaments, sheets, and voids, But that.
<v Speaker 2>Was always an abstract statistical idea. Now we know specifically
<v Speaker 2>which part of the web we're in.
<v Speaker 3>We do Our address just got a lot more specific.
<v Speaker 3>We're in the local group, which is located inside the
<v Speaker 3>local sheet of dark matter, which is bounded by the
<v Speaker 3>local void.
<v Speaker 2>It gives you a real sense of place, doesn't it. It
<v Speaker 2>grounds you. Instead of just being in space, we're in
<v Speaker 2>the sheet. It sounds like a faction from a sci
<v Speaker 2>fi novel, it does, but it has really profound implications
<v Speaker 2>for how we interpret everything else we see in the universe.
<v Speaker 2>Right this so wet question, beyond being incredibly cool, Why
<v Speaker 2>does knowing we live in a sheet matter to you,
<v Speaker 2>the person listening right now?
<v Speaker 3>Well, it matters because our location fundamentally biases our view
<v Speaker 3>of everything else. How So, think about it. We've just
<v Speaker 3>established we live in a flat pancake, and there's empty
<v Speaker 3>space above and below us. We can't step outside of
<v Speaker 3>the structure to get a clear view. All of our
<v Speaker 3>observations of the entire universe are made from within this sheet.
<v Speaker 2>I see where you're going with this. It's like living
<v Speaker 2>in a deep valley versus living on a high mountain peak.
<v Speaker 2>Your view of the surrounding landscape is completely different depending
<v Speaker 2>on your location exactly.
<v Speaker 3>Or imagine trying to survey a forest, your view is
<v Speaker 3>very different if you look along a dense row of
<v Speaker 3>trees versus looking through a clearing. Our sheet is the
<v Speaker 3>dense row of trees, and the voids are the clearings.
<v Speaker 2>So when we look out at the most distant galaxies
<v Speaker 2>billions of light years away, the light from those galaxies
<v Speaker 2>has to travel through our local environment to get to us.
<v Speaker 3>Yes, and that environment is not uniform. If we look
<v Speaker 3>out along the plane of the sheet, we're looking through
<v Speaker 3>a huge amount of matter. We're looking through the pancake.
<v Speaker 3>There's more dust, more gas, more galaxies, more gravitational lensing
<v Speaker 3>from all that dark matter.
<v Speaker 2>But if we look up or down out.
<v Speaker 3>Of the sheet, we're looking through the voids. We're looking
<v Speaker 3>through the clean part of the window.
<v Speaker 2>And that could affect our measurements. If we don't account
<v Speaker 2>for the fact that we're looking through a dirty part
<v Speaker 2>of the window in one direction and a clean part
<v Speaker 2>in another, we could misinterpret what we're seeing.
<v Speaker 3>It could affect everything. It could affect our measurements of
<v Speaker 3>the cosmic expansion rate, it could affect our counts of
<v Speaker 3>distant galaxies. It might even help explain some of those
<v Speaker 3>other tensions in cosmology. This research is so critical because
<v Speaker 3>it helps us calibrate our own point of view.
<v Speaker 2>We have to subtract the local effects to see the
<v Speaker 2>global picture.
<v Speaker 3>Clearly, we have to understand the quirks of our own
<v Speaker 3>front porch before we can make grand statements about the
<v Speaker 3>whole neighborhood. If we assume we live in a perfectly
<v Speaker 3>average random spot, our calculations will be slightly off. Now
<v Speaker 3>that we know we live in a sheet, we can
<v Speaker 3>correct for that bias.
<v Speaker 2>It's a real lessening cosmic humility, isn't it. We thought
<v Speaker 2>the Earth was the center, than the Sun, than the
<v Speaker 2>Milky Way. Now we're realizing that even our local bit
<v Speaker 2>of cosmic web has a specific, non average structure that
<v Speaker 2>shapes our view.
<v Speaker 3>The Copernican principle tells us we're not in a special place,
<v Speaker 3>and that's true in a grand sense. But this shows
<v Speaker 3>our location isn't generic either, It's specific, and that specificity matters.
<v Speaker 2>Empty space isn't just empty space.
<v Speaker 3>No, it has a geography, has a landscape. Dark matter
<v Speaker 3>forms these sheets and filaments, and galaxies flow along them
<v Speaker 3>like water following the contours of a riverbed.
<v Speaker 2>I really do love that idea of us living on
<v Speaker 2>a continent of dark matter. It makes the universe feel
<v Speaker 2>so much more.
<v Speaker 3>Tangible, and it explains why our corner of it is
<v Speaker 3>the way it is, the quietness of our neighborhood, the
<v Speaker 3>orderly motions, it's all dictated by this invisible continent we're
<v Speaker 3>writing on.
<v Speaker 2>So as we start to wrap up this incredible exploration,
<v Speaker 2>let's just do a quick recap of the journey we've
<v Speaker 2>been on.
<v Speaker 3>Let's do it.
<v Speaker 2>We started with a fifty year old puzzle, why are
<v Speaker 2>our galactic neighbors so quiet? Why do the galaxies just
<v Speaker 2>outside our local group move so smoothly, seemingly ignoring our
<v Speaker 2>massive gravitational pull.
<v Speaker 3>We then looked at how researchers and Groningen tackled this
<v Speaker 3>by building a virtual twin of our universe, an incredibly
<v Speaker 3>precise simulation that didn't just create a universe, but recreated
<v Speaker 3>our specific corner of it, down to the positions of
<v Speaker 3>thirty one specific galaxies.
<v Speaker 2>And when they made the invisible dark matter in that
<v Speaker 2>simulation visible, they discovered the secret architecture of our home.
<v Speaker 2>We are embedded in a massive flat sheet of dark
<v Speaker 2>matter tens of mins, millions of light years across.
<v Speaker 3>And that sheet is sandwiched between two enormous voids, two
<v Speaker 3>great empty spaces, and.
<v Speaker 2>That structure is the key that unlocks the whole mystery.
<v Speaker 2>The sheet creates a gravitational tug of war, balancing out
<v Speaker 2>our pull and letting nearby galaxies drift smoothly, and the
<v Speaker 2>voids mean there's nothing above or below us to fall
<v Speaker 2>in and mess up the data.
<v Speaker 3>Resulting in the clean, orderly, anomalously quiet flow we see today.
<v Speaker 3>It's not new physics. It's just cosmic geography.
<v Speaker 2>A beautiful, self consistent, and deeply satisfying solution.
<v Speaker 3>It really is.
<v Speaker 2>So I have one final provocative thought. I want to
<v Speaker 2>leave everyone with something to chew on.
<v Speaker 3>I'm ready laid on me.
<v Speaker 2>Okay, If we live in this quiet, stable sheet and
<v Speaker 2>there are these giant empty voids on either side, it
<v Speaker 2>really emphasizes the difference between something and nothing, and it
<v Speaker 2>makes me wonder, is life itself more likely to arise
<v Speaker 2>in structures like these sheets. Are we here because the
<v Speaker 2>sheet is so quiet and stable?
<v Speaker 3>That is a fascinating question. It gets right to the
<v Speaker 3>heart of the anthropic principle, the idea that the reason
<v Speaker 3>we observe the universe to have certain properties is because
<v Speaker 3>if it had different properties, we wouldn't be here to
<v Speaker 3>observe it exactly. Well. We know that the most extreme
<v Speaker 3>environments are probably not great for life. A giant galaxy cluster,
<v Speaker 3>for instance, where filaments of the cosmic web intersect. That's
<v Speaker 3>a really chaotic, violent place.
<v Speaker 2>Right, Galaxies are constantly crashing into each other, supermassive black
<v Speaker 2>holes or blazing with radiation. It seems like a tough
<v Speaker 2>place to build a stable solar system and have billions
<v Speaker 2>of years of peace and quiet.
<v Speaker 3>And the voids, on the other hand, are well, they're
<v Speaker 3>too empty, too lonely. It would be very difficult for
<v Speaker 3>enough matter to clump together to form stars and planets efficiently.
<v Speaker 3>You might not get enough of the heavy elements needed
<v Speaker 3>to build a rocky planet like Earth.
<v Speaker 2>So the sheet is the cosmic Goldilocks zone. Not too hot,
<v Speaker 2>not too cold, not too chaotic, not too empty.
<v Speaker 3>It's entirely possible. Perhaps this quietness that puzzled astronomers for
<v Speaker 3>fifty years isn't just a core of gravity. Maybe that stability,
<v Speaker 3>that lack of messy infall is a prerequisite for a
<v Speaker 3>galaxy like ours to gently nurture a solar system like
<v Speaker 3>ours over billions of years. Maybe you need a quiet
<v Speaker 3>street to build a home.
<v Speaker 2>So the mystery of the quiet neighbors isn't just about
<v Speaker 2>galactic dynamics. It might actually be a clue about cosmic habitability.
<v Speaker 2>We are here because it's quiet here.
<v Speaker 3>It might just be a quiet neighborhood is a good
<v Speaker 3>place to raise a civilization.
<v Speaker 2>I love that we didn't just end up here by chance.
<v Speaker 2>We moved to the cosmic suburbs for the good schools
<v Speaker 2>and the quiet streets and.
<v Speaker 3>The expansive, flat dark matter lawns.
<v Speaker 2>On that note, we're going to sign off. Thank you
<v Speaker 2>so much for joining us on this journey through our
<v Speaker 2>galactic neighborhood. I hope your sense of your place in
<v Speaker 2>the universe feels a little bit bigger and a little
<v Speaker 2>more specific than it did an hour ago.
<v Speaker 3>It was a true pleasure.
<v Speaker 2>The next time you look up at the night sky,
<v Speaker 2>don't just see the stars. Imagine the invisible sheet holding
<v Speaker 2>it all together. Imagine the great voids above them, and
<v Speaker 2>remember that the map is always always bigger than what
<v Speaker 2>you can see. Keep looking up. We'll see you next time.
<v Speaker 3>U