The Universe Has No Center—Here's Why | Leonard Susskind

[00:00:00]Why the universe has no center point to where the universe began. Go ahead, point to the spot where the Big Bang happened. You're pointing in the wrong direction. I don't care which direction you're pointing, you're wrong. Because the Big Bang didn't happen somewhere out there. It didn't happen at a specific location. And and the universe, this vast structure of galaxies and dark matter and radiation and empty space, does not have a center. There is no middle, there is no edge. There is no special place where it all started. I know that sounds impossible. I know it contradicts everything your brain is telling you right now. And so if the universe expanded from a tiny point, surely that point is the center, right?

[00:00:42]Surely everything's flying away from some origin, some cosmic ground zero.

[00:00:46]That's what the word explosion implies.

[00:00:48]That's what the phrase Big Bang makes you picture. A bomb going off in empty space, debris flying outward from a single spot. And that picture is completely, fundamentally spectacularly wrong. And I'm Leonard Susskind, and I've spent over 50 years thinking about the structure of the universe. And I can tell you with absolute certainty that the absence of a center is not a minor technical detail. And so it's one of the most profound and disturbing facts about the cosmos. It changes what the universe is. It changes what space is. And and once you understand it, you will never look at the night sky the same way again. Let me start by explaining why your intuition fails here. And when you hear that the universe is expanding, your brain does something natural. It imagines a balloon inflating, or a bomb exploding, or a crowd of people running away from a building. And so in all of these cases, there's a center, the middle of the balloon, the location of the bomb, the building everyone is running from. These are spatial events happening inside a pre-existing space.

[00:01:52]And the debris moves through space away away a point in space. But the expansion of the universe is not like any of these things. The universe is not expanding into anything. Hence, there is no pre-existing space that the universe is filling up. Space itself is what's expanding. Every point in the universe is moving away from every other point.

[00:02:14]Hence, not because things are flying through space, but because the space between things is growing. This is a crucial distinction, and most people miss it entirely. Hence, let me give you an analogy that's slightly better than the balloon, though no analogy is perfect. Imagine an infinite sheet of rubber, not a balloon with a surface.

[00:02:32]Hence, an infinite flat sheet extending forever in all directions. Now, imagine that this rubber sheet is being stretched uniformly in every direction.

[00:02:41]Hence, every point on the sheet moves away from every other point. If you're an ant standing on the sheet, you look around and you see every other ant moving away from you. Hence, the farther away an ant is, the faster it's receding. You might conclude that you're at the center, that everything is expanding away from you. But, here's the thing. Hence, every single ant on that infinite sheet sees exactly the same thing. Every ant thinks it's at the center.

[00:03:07]Every ant sees all other ants receding.

[00:03:10]No ant is special. Hence, there is no center of the stretching because the sheet is infinite, and the stretching is uniform. That's what the expansion of the universe is like. Hence, except it's three-dimensional space, not a two-dimensional sheet. And as far as we can tell, it might actually be infinite.

[00:03:26]Hence, now, let me tell you what the Big Bang actually was, because this is where people get most confused. The Big Bang was not an explosion in space. The Big Bang was an event in time. Hence, it was the moment when the universe was in an extremely hot, extremely dense state, not at a single point in space, but everywhere. Hence, the entire universe, all of it, every region of space that exists today was hot and dense 13.8 billion years ago. If the universe is infinite now, it was infinite then, too.

[00:03:57]An infinite space can be uniformly hot and dense. There's no contradiction. It just means that every cubic centimeter of that infinite space was packed with energy. Think about that. The Big Bang didn't happen at a particular location.

[00:04:11]It happened at a particular time. It happened everywhere simultaneously.

[00:04:16]Hence, every point in the universe can trace its history back 13.8 billion years to a state of extreme heat and density. No point is closer to where it happened because it happened at all points. Hence, this is what Einstein's general relativity tells us. This is what the Friedmann equations describe.

[00:04:36]This is what the cosmic microwave background confirms. Hence, the universe has no center because the Big Bang had no location. Let me show you the evidence because this is not just theoretical speculation. We can actually see that the universe has no center.

[00:04:52]Hence, the most powerful piece of evidence is the cosmic microwave background radiation, the CMB. This is the afterglow of the Big Bang. Hence, about 380,000 years after the Big Bang, the universe cooled enough for atoms to form. Hence, before that, the universe was a plasma, a super free electrons and protons, so hot and dense that light couldn't travel through it. Photons would scatter off free electrons almost immediately. Hence, the universe was opaque, like being inside a fog. Then, at about 3,000° K, the electrons combined with protons to form hydrogen atoms. Suddenly, the universe became transparent. Hence, light could travel freely. Hence, and the photons that were released at that moment have been traveling through space ever since, cooling as the universe expanded, stretching from visible light to microwave wavelengths. Hence, today, those photons fill the entire universe.

[00:05:49]They come from every direction and they are almost perfectly uniform. The temperature of the CMB is 2.725 Kelvin and it's the same in every direction to better than one part in 100,000.

[00:06:03]This uniformity is extraordinary.

[00:06:06]It means that 380,000 years after the Big Bang, the universe looked almost exactly the same everywhere. There was no center, there was no edge, there was no special direction. It's just a smooth hot plasma filling all of space uniformly.

[00:06:20]If the Big Bang had happened at a particular location, we would expect to see a gradient in the CMB.

[00:06:26]The radiation would be hotter in the direction of the explosion and cooler away from it. We would see a pattern, a bull's-eye, a signature of a center. We see nothing of the sort. The CMB is isotropic, the same in all directions.

[00:06:42]This is exactly what you'd expect if the Big Bang happened everywhere and the universe has no center. Now, there are tiny fluctuations in the CMB.

[00:06:51]Temperature variations of about one part in 100,000. And these fluctuations are critically important because they're the seeds of all the structure we see today.

[00:07:01]Galaxies, galaxy clusters, cosmic filaments, voids, all of it grew from those tiny density fluctuations in the early universe. But these fluctuations are random. They're distributed uniformly across the sky with no preferred direction, no preferred location. Statistical analysis of the CMB confirms this.

[00:07:22]The fluctuations are consistent with a Gaussian random field, exactly what inflationary cosmology predicts for a universe with no center and no edge. But let me go deeper because the geometry of the universe is where this gets really interesting and really strange. In general relativity, the geometry of space is determined by its content.

[00:07:44]Hence, matter and energy curve space-time. The overall geometry of the universe depends on its total density.

[00:07:51]There are three possibilities. Hence, if the density is above a critical value, space is positively curved. Think of the surface of a sphere. It's finite but has no boundary and no center. Hence, you can walk forever on a sphere and never reach an edge. Every point on the sphere is equivalent. If the density is below the critical value, space is negatively curved. Hence, think of a saddle shape extending infinitely. Again, no center.

[00:08:19]If the density is exactly the critical value, space is flat, Euclidean geometry extending infinitely in all directions, no center. Hence, our measurements, primarily from the CMB but also from galaxy surveys and supernova observations, tell us that the universe is very close to flat. Hence, the density is within about half a percent of the critical value. Hence, this means space is either exactly flat extending forever or very nearly flat with an extremely slight curvature that makes it so large we can't detect the curvature.

[00:08:54]Either way, no center. Hence, here's what bothers people.

[00:08:58]If the universe has no center, where are we? What's our position in the cosmos?

[00:09:03]The answer is that we're at a perfectly ordinary location. Hence, the Copernican principle, named after Nicolaus Copernicus, who showed that Earth is not the center of the solar system, extends to the entire universe. Earth is not at the center of the solar system. Hence, the Sun is not at the center of the Milky Way. The Milky Way is not at the center of any larger structure. Hence, and no galaxy, no cluster, no supercluster is at the center of the universe because there is no center.

[00:09:33]This is deeply unsettling to human psychology. Hence, we evolved to think in terms of centers and edges, our world has a center, the place where we stand.

[00:09:43]It has edges, the horizon, the boundaries of our territory. As we instinctively place ourselves at the center of our experience, and we project that onto the universe, but the universe doesn't care about our psychology. As it has its own geometry, and that geometry has no center. Let me talk about something that confuses even physics students, the observable universe. The observable universe is a sphere centered on us. As it has a radius of about 46.5 billion light-years. Wait, you might say, if the universe is 13.8 billion years old, how can the observable universe have a radius of 46.5 billion light-years? As shouldn't it be 13.8 billion light-years? No, because the universe has been expanding during the time light has been traveling. As the light that left a distant galaxy 13 billion years ago has been traveling through expanding space. The galaxy that emitted that light has been receding from us during the entire travel time.

[00:10:43]As by now, that galaxy is about 46.5 billion light-years away, even though the light took only 13 billion years to reach us. As the observable universe is a sphere centered on us simply because we are the observers. It's not a physical boundary, it's an informational boundary. As it's the region from which light has had time to reach us since the Big Bang. An alien on a planet in the Andromeda galaxy would have a different observable universe centered on them. As it would overlap enormously with ours, but it would extend 46.5 billion light-years in every direction from their location, not ours. As they would see galaxies we can't see, and we see galaxies they can't see. Neither of us is at the center of anything. We're just at the center of our own observational horizons. And so, now I need to address the expansion directly because this is where the lack of a center becomes most vivid and most counterintuitive. And so, Edwin Hubble discovered in 1929 that distant galaxies are moving away from us. The farther away a galaxy is, the faster it's receding.

[00:11:52]This is Hubble's law, and it's described by a simple equation. And so, the recession velocity equals the Hubble constant times the distance. This looks like we're at the center, everything is moving away from us, the farther away the faster. And that's exactly what you'd see if you were at the center of an explosion.

[00:12:10]But here's the key insight. Hubble's law looks exactly the same from every point in the universe. And so, if you were in the Andromeda galaxy, you'd see all other galaxies receding from you. If you were in a galaxy 10 billion light-years away, same thing. And so, every observer sees themselves at the apparent center of expansion. This is a mathematical consequence of uniform expansion. And so, if every distance is increasing by the same fraction per unit time, then every observer sees every other point receding with a velocity proportional to distance. It's automatic, it requires no center. And so, think about my rubber sheet analogy again. Every ant sees every other ant receding. The velocity of recession is proportional to distance. No ant is at the center because there is no center.

[00:12:57]And so, the stretching is uniform, the expansion is homogeneous.

[00:13:02]Let me tell you about something that makes this even more mind-bending, the cosmological principle. And so, this is one of the foundational assumptions of modern cosmology, and it's been confirmed by observation to extraordinary precision. And so, the cosmological principle states that on sufficiently large scales, the universe is homogeneous and isotropic.

[00:13:25]Homogeneous means it looks the same at every point. And so, isotropic means it looks the same in every direction.

[00:13:31]Together, these properties guarantee that there is no center and no preferred direction. On small scales, of course, the universe is lumpy. And there are stars, galaxies, clusters, voids, filaments. The distribution of matter is highly non-uniform on scales of millions of light years. And so, but zoom out to scales of hundreds of millions of light years or more, and the lumpiness averages out. The distribution of galaxies becomes smooth and uniform. And galaxy surveys like the Sloan Digital Sky Survey have mapped millions of galaxies and confirmed this. On scales above about 300 million light years, the universe is remarkably uniform. And so, no special structures, no edges, no walls, no center. But here's where it gets philosophically deep. If the universe has no center, what does that mean for us? And so, what does it mean for our significance in the cosmos? For most of human history, we believed we were at the center. The Earth was the center of the universe. The sun and stars revolved around us. And so, we were special. Then Copernicus moved us away from the center of the solar system.

[00:14:38]Then we learned the sun is an ordinary star in an ordinary galaxy. And so, then we learned our galaxy is one of hundreds of billions. And now we learn there isn't even a center to be displaced from. There's no throne at the middle of the cosmos that we've been kicked off of. And so, the throne never existed.

[00:14:57]Every point is as central as every other point, which means no point is central at all. Some people find this depressing. And so, the universe has no center, so we're nowhere special.

[00:15:07]But I think there's another way to look at it. Every point in the universe is equally valid. Every location is as fundamental as any other. And so, right now, wherever you are, are at a point in the universe that is exactly as central, exactly as significant, exactly as original as any other point that exists or has ever existed. And so, you're not at the outskirts, you're not at the periphery, you're not this place from some cosmic center, because there's no outskirt, no periphery, no center.

[00:15:38]You're here. And and here is as good as anywhere. Let me tell you about inflation, because it's the theory that best explains why the universe has no center and why it looks the way it does.

[00:15:49]And so, in the early 1980s, Alan Guth proposed that the very early universe underwent a period of exponential expansion. In a tiny fraction of a second, space expanded by a factor of at least 10 to the 26th, and maybe much more. This inflationary epoch solved several puzzles about the universe. And so, it explained why the CMB is so uniform, because regions that appear causally disconnected today were actually in contact before inflation stretched them apart. And so, it explained why the universe is so flat, because inflation drives the geometry toward flatness, regardless of the initial curvature. And and it explained where the density fluctuations came from. Quantum fluctuations in the inflaton field, stretched to cosmic scales by the expansion. And so, but inflation also has a profound implication for our question about centers. If inflation happened, the observable universe is just a tiny patch of a much, much larger universe. And so, imagine the universe as the surface of a balloon that's been inflated to an absurdly large size, so large that any patch you can see looks flat. And so, you can't detect the curvature because the balloon is so enormous. Our observable universe is like a postage stamp on the surface of that balloon.

[00:17:09]And so, we see a tiny flat patch and we think the universe might be infinite and flat. It might be, or it might be a finite but unimaginably large curved space, so large that the curvature is undetectable. And either way, we're a tiny patch of something vastly larger, and in that vastly larger space, there's certainly no center. Every patch looks like ours, every region has the same properties. The same density, the same temperature, the same physics. We're not special, our patch is not special. There is no center because there's no feature of the geometry that picks out any location as different from any other.

[00:17:48]Now, I want to address an objection that sharp thinkers sometimes raise.

[00:17:53]What about the cosmic microwave background? You said it looks the same in every direction, but there's a dipole anisotropy. The CMB is slightly hotter in one direction and slightly cooler in the opposite direction. Doesn't that define a special direction, a center?

[00:18:09]No. The dipole anisotropy is caused by our motion relative to the CMB rest frame. The Earth, the Sun, the Milky Way, we're all moving through space at about 370 km per second relative to the frame in which the CMB is isotropic.

[00:18:24]This motion causes a Doppler shift, making the CMB slightly hotter in the direction we're moving toward and slightly cooler in the direction we're moving away from.

[00:18:35]When you subtract this dipole, the CMB is isotropic to one part in 100,000. Our motion defines a velocity, not a position. It tells us how fast we're moving, not where we are relative to a center. Because there is no center. Let me bring in something from my own work because it connects to this in a fascinating way, the string landscape and the multiverse. If the inflationary multiverse picture is correct, and I've spent a lot of my career arguing that it might be, then our universe is just one bubble in an infinite sea of bubble universes. And each bubble nucleated from a different vacuum state in the string theory landscape, each one has potentially different physical constants, different particles, different forces. And so in this picture, asking where the center of our universe is becomes even more absurd. Our universe is a bubble in an infinite foam. There is no center of the foam. There is no edge. And there is no preferred location, just an infinite eternally inflating space spawning bubble universes forever. And within each bubble, the same story repeats. No center, no edge, no special point. And so it's homogeneity all the way up. But let me come back down to Earth.

[00:19:54]Literally, because I want to talk about what this means for you, for your life, for how you think about your place in the cosmos. And so you live on a small rocky planet orbiting an ordinary star in an ordinary galaxy. There are roughly 200 billion stars in our galaxy, and roughly 2 trillion galaxies in the observable universe. And so the number of stars in the observable universe is something like 10 to the 24th, a trillion trillion. And every single one of those stars, and every planet around them, and every atom in every planet is in a location that is exactly as central to the universe as your location. And so because there is no center, you might think this makes you insignificant, a speck on a speck in a centerless expanse. But I think significance isn't about location. It never was. And so the medieval view that Earth was at the center of the universe didn't make human life more meaningful. It was just a geometric claim, and it was wrong. And so our significance comes from something else entirely. It comes from the fact that we are the only known part of the universe that is aware of the universe.

[00:21:04]And so we are matter that has organized itself to ask questions about its own origins. Stars don't wonder where the center is. Galaxies don't ask whether they're special, and stark matter doesn't contemplate its own existence.

[00:21:16]We do, and that not our location is what makes us remarkable.

[00:21:21]Here's what I find most beautiful about a universe with no center. It's democratic. And no civilization anywhere in the cosmos, if other civilizations exist, can claim to be at a more fundamental location than any other.

[00:21:35]Every observer everywhere sees the same large-scale universe. And it's the same expansion, the same CMB temperature, the same laws of physics, the same fundamental constants. The universe presents the same face to everyone, regardless of where they are.

[00:21:51]This is an extraordinary statement about the nature of reality. It means the laws of physics are truly universal, not in a metaphorical sense, but in a literal geometric sense. Hence, there is no privileged frame, no special location, no cosmic center where the laws are written and from which they radiate outward. The laws are everywhere. They are the same everywhere. Hence, and that universality, that deep symmetry of homogeneity and isotropy, is one of the most profound discoveries in the history of science. Could I be wrong? Hence, could the universe actually have a center that we haven't detected in science? We should always hold our conclusions tentatively, but the evidence is overwhelming.

[00:22:36]The CMB is isotropic. Hence, the galaxy distribution is homogeneous on large scales.

[00:22:42]General relativity solutions for a uniform universe have no center.

[00:22:47]Inflation predicts no center.

[00:22:49]Everything points in the same direction.

[00:22:51]Hence, the universe has no center. Now, there are exotic possibilities. Some cosmologists have explored models with non-trivial topology, a universe that wraps around on itself like a torus. And in such a universe space is finite but still has no center and no edge. Just like the surface of a donut has no center. You can walk forever and eventually return to where you started.

[00:23:16]And we've looked for signatures of this in the CMB repeating patterns that would indicate the universe wraps around. We haven't found any but we haven't ruled it out entirely. And even if the universe has exotic topology though it still wouldn't have a center. A torus has no center. A Klein bottle has no center. And none of the compact topologies that cosmologists consider have a center.

[00:23:41]The absence of a center is robust. It survives every model, every topology, every variation of the theory. And let me leave you with this. The next time you look up at the night sky I want you to resist the urge to think of yourself as being at any particular place in the universe. You're not at the center. And you're not at the edge.

[00:24:01]You're not at any definable position relative to the whole because the whole has no reference point. You're just here. And say conscious being made of atoms forged in the ancient stars standing on a rocky planet in a galaxy among trillions in a universe that extends in every direction with no beginning and no end in space. And the Big Bang was not a location. It was a moment. It happened everywhere. It happened where you're standing right now. And the ground beneath your feet was there at the Big Bang compressed to unimaginable density hotter than the center of any star 13.8 billion years ago. You're standing at the site of the Big Bang. And so is everyone else. So is every point in the universe. That's the deepest truth about cosmic geography.

[00:24:46]There is no center because everywhere is the center or nowhere is. And depending on how you want to think about it. And after 50 years of studying the structure of the universe of working through the mathematics of general relativity and quantum field theory, and inflationary cosmology, this is what I keep coming back and still we are not at a special place, we are at every place, we are the universe looking at itself from one of infinitely many equivalent vantage points, and and we are the only part of the universe that knows this. We are the cosmos understanding its own geometry.

[00:25:22]And that understanding, that awareness is more central than any location could ever be. And thank you for listening.

[00:25:30]Now, go outside and remember wherever you are, you're exactly at the center of everything and nowhere at all at the same time. That's not a contradiction.

[00:25:38]And that's the universe.