What's at the Edge of the Universe?

What's at the Edge of the Universe?
Illustration: Chelsea Beck (Gizmodo)
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It is a routine emotion in 2019 to urgently wish, four or five times in a day, to be launched not simply into space but to the very edge of the universe, as far as it is possible to get from the fever dream of bad weather, busted trains and potentially cancerous thigh lesions that constitute life on Earth. But what would be waiting for you, up at the cosmological border? Is it even a border, or is what we’re dealing with here more like a kind of inconceivably vast ceiling? Is there even a border/ceiling up there at all? For this week’s Giz Asks, we talked with a number of cosmology-oriented physicists to find out.

Sean Carroll

Research Professor, Physics, Caltech, whose research focuses on quantum mechanics, gravitation, cosmology, statistical mechanics, and foundations of physics, among other things

There is no edge to the universe, as far as we know. There’s an edge to the observable universe—we can only see so far out. That’s because light travels at a finite speed (one light-year per year), so as we look at distant things we’re also looking backward in time. Eventually we see what was happening almost 14 billion years ago, the remnant radiation from the Big Bang. That’s the Cosmic Microwave Background, which surrounds us from all sides. But it’s not really a physical “edge” in any useful sense.

Because we can only see so far, we’re not sure what things are like beyond our observable universe. The universe we do see is fairly uniform on large scales, and maybe that continues literally forever. Alternatively, the universe could wrap around like a (three-dimensional version of a) sphere or torus. If that were true, the universe would be finite in total size, but still wouldn’t have an edge, just like a circle doesn’t have a beginning or ending.

It’s also possible that the universe isn’t uniform past what we can see, and conditions are wildly different from place to place. That possibility is the cosmological multiverse. We don’t know if there is a multiverse in this sense, but since we can’t actually see one way or another, it’s wise to keep an open mind.

Jo Dunkley

Professor, Physics and Astrophysical Sciences, Princeton University, whose research is in cosmology and studying the origins and evolution of the Universe

More of the same!

Okay, so we don’t actually think there is an edge to the universe. We think it either continues on infinitely far in all directions, or maybe it is wrapped up on itself so that it isn’t infinitely big, but still has no edges. The surface of a donut is like that: it doesn’t have an edge. It’s possible the whole universe is like that too (but in three dimensions—the surface of a donut is just two-dimensional). That means you could set off in any direction into space on a rocket ship, and if you traveled for long enough you would come back to where you started. No edges.

But there is also a thing we call the observable universe, which is the part of space that we can actually see. The edge of that is the place beyond which light hasn’t had time to reach us since the beginning of the universe. That’s only the edge of what we can see, and beyond that is probably more of the same stuff that we can see around us: super-clusters of galaxies, each enormous galaxy containing billions of stars and planets.

Jessie Shelton

Assistant Professor, Physics and Astronomy, University of Illinois Urbana-Champaign, whose research focuses on astrophysics and cosmology

That depends on what you mean by the edge of the universe. Because the speed of light is finite, as we look farther and farther out in space, we look farther and farther back in time — even when we look at the galaxy next door, Andromeda, we see not what’s happening now, but what was happening two and a half millions of years ago when Andromeda’s stars emitted the light that our telescopes are only now detecting. The oldest light we can see has come from the farthest away, so in one sense, the edge of the universe is whatever we can see in the most ancient light that reaches us. In our universe, this is the cosmic microwave background — a faint, lingering afterglow of the Big Bang, marking when the universe cooled down enough to let atoms form. This is called the surface of last scattering, since it marks the place where photons stopped ping-ponging around between electrons in a hot, ionized plasma and started streaming out through transparent space, all the way across billions of lightyears down to us on Earth. So you could say that the edge of the universe is the surface of last scattering.

What’s at the edge of the universe right now? Well, we don’t know — we can’t, we’d have to wait for the light being emitted there now to get here many, many billions of years in the future, and since the universe is expanding faster and faster, it probably won’t be able to make it here at all — but we can make a guess. On the largest scales, our universe looks pretty much the same in any direction we look. So odds are, if you were at the edge of our observable universe today, you would see a universe that looked more or less the same as ours — galaxies, big and small, in all directions. Thus a very good guess for what’s at the edge of the universe now is simply, more universe: more galaxies, more planets, maybe even more living things asking the same question.

Michael Troxel

Assistant Professor, Physic, Duke University whose researches focuses on observational and theoretical cosmology

Despite the Universe likely being infinite in size, there is actually more than one practical ‘edge’.

We think the Universe is actually infinite—it has no edge to it. If the Universe is ‘flat’ (like a sheet of paper), as we’ve tested it to be at better than a percent precision, or ‘open’ (like a saddle), then it really is infinite. If it is ‘closed’, which is kind of like a basketball, then it isn’t infinite. However, if you go far enough in one direction, you’ll eventually end up back where you started—just think about moving along the surface of the ball. As a hobbit named Bilbo once said, “The Road goes ever on and on/Out from the door where it began” (over and over...).

The Universe still has an ‘edge’ for us, though—two, really. This is due to a part of General Relativity that says that all things (including light) in the Universe have a speed limit—around 670 million miles per hour—and that speed limit is the same everywhere. Our measurements also tell us that the Universe is expanding in every direction, and not just expanding, but expanding faster and faster over time. What this means is that when we observe an object very far from us, the light from that object takes some time to reach us (the distance divided by the speed of light). The tricky thing is that because space is expanding while that light travels to us, the distance the light has to travel is also increasing over time on its way to us.

So the first thing you could ask is what’s the farthest distance we could observe light from an object if it were emitted at the very beginning of the Universe (which is about 13.7 billion years old). This turns out to be about 47 billion light-years away (a light-year is about 63,241 times the distance between the Earth and the Sun), and is called the ‘comoving horizon’. You can also ask the question slightly differently. If we sent a message at the speed of light, what’s the furthest distance away that someone from another planet could ever receive it? This is even more interesting, because the expansion rate of the Universe gets faster in the future (instead of slowing down in the past).

It turns out that even if the message travelled forever, it would only ever be able to reach someone that was 16 billion light-years from us now. This is called the ‘cosmic event horizon’. The furthest planet that we’ve been able to observe is only about 25 thousand light-years away, though, so we could still eventually say hi to everyone we know might exist in the Universe so far. The furthest distance our current telescopes may have identified a galaxy from us is only about 13.3 billion light-years, though, so we can’t see what is at either of these ‘edges’ right now. So no one knows what’s at either edge!

Abigail Vieregg

Assistant Professor at the Kavil Institute for Cosmological Physics at the University of Chicago

Using telescopes on Earth, we look at light coming from distant places in the universe. The farther away the source of the light is, the longer it takes for that light to get here. So, when you look at far away places, you’re looking at what those places were like when the light you saw was created—not at what those places are like today. You can keep looking farther and farther away, corresponding to farther and farther back in time, until you hit a place corresponding to a few hundred thousand years after the Big Bang. Before that, the universe was so hot and dense (well before there were stars and galaxies!) that any light in the universe just rattled around, and we can’t see it with our telescopes today. This place is edge of the “observable universe”—sometimes called the horizon—because we can’t see beyond it. As time goes on, this horizon changes. If you could look out from another planet somewhere else in the universe, presumably you would see something very similar to what we see here from Earth: your own horizon, limited by the time that has elapsed since the Big Bang, the speed of light, and the how the universe has expanded.

What does the place that corresponds to Earth’s horizon today look like today? We can’t know, since we can only view that place as it was just after the Big Bang, not as it is today. However, all the measurements indicate that all of the universe we can see, including the edge of the observable universe, looks approximately like our local universe does today: with stars, galaxies, and clusters of galaxies and lots of empty space.

We also think that the universe is much much bigger than the part of the universe we happen to be able to see here from Earth today, and there is no “edge” to the universe itself. It is just spacetime, expanding.

Arthur B. Kosowsky

Professor, Physics, University of Pittsburgh, whose research focuses on cosmology and related issues of theoretical physics

One of the most fundamental properties of the universe is its age, which from a variety of measurements we now determine to be 13.7 billion years. Because we also know that light propagates at a constant speed, this means that a light ray that started out at a very early time has traveled a particular distance by today (called the “horizon distance” or the “Hubble distance”). Since nothing propagates faster than the speed of light, the Hubble distance is the furthest distance we can ever observe in principle (unless we discover some way around the theory of relativity!).

We have a source of light coming to us from almost the Hubble distance: the cosmic microwave background radiation. We know that there is no “edge” to the universe out to the distance of the microwave background origin, which is almost the entire Hubble distance away from us. So we typically make the assumption that the universe is much larger than our own observable Hubble volume, and any actual edge that might exist is much further away than we can ever observe. This could conceivably not be correct: maybe the universe has an edge just beyond the Hubble distance from us, and beyond that is sea monsters. But since all of the universe we can observe looks relatively similar and uniform, this would be an extremely strange state of affairs.

So I’m afraid that we will never have a good answer to the question: the universe may not have an edge at all, and if it does have an edge, that edge is far enough away that light from the edge has not yet had enough time to get to us in the entire history of the universe. We have to settle for understanding the part of the universe we can actually observe.

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