What is on the edge of the universe?

In 2019, this is a common emotion - to wish forfour or five times a day to go not to space, but to the very end of the world, as far as possible to get rid of bad glamor or bad weather, delayed trains or tight trousers, such ordinary things on Earth. But what will be waiting for you on this cosmological border? What is it all about - the edge of the world, the edge of the Universe - what will we see there? Is it a border or infinity in general?

Let's ask the scientists.

On the edge of the world

Sean Carroll, Professor of Physics, Caltech

“As far as we know, the universe has no limits. The observable universe has an edge - the limit of what we can see. This is due to the fact that the light moves at a finite speed (one light year per year), so when we look at distant things, we look back in time. At the very end, we see what has been happening for almost 14 billion years, the residual radiation of the Big Bang. This is a cosmic microwave background that surrounds us from all sides. But this is not a physical "frontier", if so to judge.

Since we can only see so farwe do not know what things look like outside our observable universe. That universe, which we see, is quite homogeneous on a large scale and, probably, it will literally go on like this forever. Alternatively, the universe could fold into a sphere or a torus. If this is the case, the universe will be limited in overall size, but it will not have a border anyway, just as a circle has no beginning or end.

It is also possible that the universe is heterogeneous inbeyond what we can see, and what conditions are very different from place to place. This opportunity is represented by the cosmological multiverse. We do not know whether the multiverse exists in principle, but since we do not see either one or the other, it would be reasonable to remain impartial. ”

Joe Dunkley, Professor of Physics and Astrophysical Sciences at Princeton University

“Yes, all the same!

Okay, we really don't think thatthe universe has a border or an edge. We think that it either continues infinitely in all directions, or turns around itself, so that it is not infinitely large, but still has no edges. Imagine the surface of a donut: it has no boundaries. Maybe the whole universe is like this (but in three dimensions - there are only two dimensions at the donut surface). This means that you can go on a space ship in any direction, and if you travel long enough, return to where you started from. There is no edge.

But there is also what we call observablethe universe, which is part of the space that we can really see. The edge of this place is there, from where the light did not have enough time to get to us from the beginning of the universe. We can only see such an edge, and behind it there will probably be all the same that we see around: superclusters of galaxies, in each of which billions of stars and planets.

Last scatter surface

Jesse Shelton, Assistant Professor, Department of Physics and Astronomy, University of Illinois at Urbana-Champaign

"It all depends on what you mean bythe edge of the universe. Since the speed of light is limited, the farther and farther into space we look, the farther and farther back in time we look - even when we look at the neighboring Andromeda galaxy, we see not what is happening now, but what happened two and a half million years ago when the stars of Andromeda radiated light that fell into our telescopes just now. The oldest light that we can see came from the farthest depths, therefore, in a sense, the edge of the universe is the most ancient light that reached us. In our universe, this cosmic microwave background is the barely noticeable, long afterglow of the Big Bang, which marks the moment when the Universe has cooled enough to allow atoms to form. This is called the last scattering surface, because it marks the place where photons stopped jumping between electrons in a hot, ionized plasma and started flowing out through transparent space, billions of light years away in our direction. Thus, it can be said that the edge of the universe is the surface of the last scattering.

What is on the edge of the universe right now? Well, we do not know - and cannot find out, we would have to wait until the light emitted there and coming to us flies many billions of years in the future, but as the universe expands faster and faster, we are unlikely to see a new edge of the universe . We can only guess. On a large scale, our universe looks for the most part the same, wherever you look. Chances are great that if you were on the edge of the observable universe today, you would see a universe that is, like it or not, our own: galaxies, larger and smaller, in all directions. I think that the edge of the universe right now is simply more of a universe: more galaxies, more planets, more sentient beings asking the same question. ”

Michael Troxel, Assistant Professor of Physics at Duke University

“Despite the fact that the Universe is probably infinite in size, in fact there is more than one practical“ edge ”.

We think the universe is in fact infinite- and it has no boundaries. If the Universe were “flat” (like a sheet of paper), as our tests showed up to a percent, or “open” (like a saddle), then it is really endless. If it is “closed” like a basketball, then it is not infinite. However, if you go far enough in one direction, you will end up where you started from: imagine that you are moving on the surface of a ball. As the hobbit named Bilbo once said: “The road runs on and on ...”. Again and again.

The Universe has an “edge” for us - even two. This is due to part of the general theory of relativity, which states that all things (including light) in the Universe have a speed limit of 299,792,458 m / s — and this speed limit is maintained everywhere. Our measurements also tell us that the Universe is expanding in all directions, and expanding faster and faster. This means that when we observe an object that is very far away from us, the light from this object needs time to reach us (distance divided by the speed of light). The trick is that as space expands as the light travels toward us, the distance the light must travel also increases over time on the way to us.

So the first thing you might ask is: what the farthest distance we could observe the light from an object if it were emitted at the very beginning of the Universe (which is about 13.7 billion years old). It turns out that this distance is 47 billion light years (a light year is approximately 63,241 times the distance between the Earth and the Sun), and is called the cosmological horizon. You can put the question a little differently. If we sent a message at the speed of light, at what distance could we get it? This is even more interesting, because the rate of expansion of the universe increases in the future.

It turns out that even if this message willto fly forever, it can only get to those who are now at a distance of 16 billion light years from us. This is called the "horizon of cosmic events." However, the farthest planet that we could observe is 25 thousand light years, so we could still greet everyone who lives in this Universe at the moment. But the farthest distance at which our current telescopes could distinguish the galaxy is about 13.3 billion light years, so we do not see what is on the edge of the universe. No one knows what is on both sides. ”

Abigail Wiregg, Associate Professor of the Institute of Cosmological Physics. Kavila University of Chicago

"Using telescopes on Earth, we look atlight coming from distant places of the universe. The farther the light source, the longer it takes for the light to fall here. Therefore, when you look at distant places, you look at what these places looked like when the light you saw was born - not how these places look today. You can continue to look farther and farther away, which will correspond to moving farther and farther back in time, until you see something that existed several millennia after the Big Bang. Prior to this, the universe was so hot and dense (long before the stars and galaxies appeared!) That any light in the universe could not be caught, it cannot be seen with modern telescopes. This is the edge of the "observable universe" - the horizon - because nothing can be seen behind it. As time goes on, this horizon changes. If you could look at the Universe from another planet, you would probably see the same thing that we see on Earth: your own horizon, limited by the time that has passed since the Big Bang, by the speed of light and the expansion of the universe.

What is the place that corresponds toearth horizon? We do not know, because we can see this place as it was immediately after the Big Bang, and not what it became today. But all measurements show that the entire visible universe, including the edge of the observable universe, looks about the same, just like our local universe today: with stars, galaxies, clusters of galaxies and a huge empty space.

We also think that the universe is much larger than of the universe that we can see today from Earth, and that the universe itself does not have an “edge” as such. It’s just expanding space-time. ”

The universe has no limits

Arthur Kosovsky, Professor of Physics, University of Pittsburgh

"One of the most fundamental properties of the universeis its age, which, according to various dimensions, we define today as 13.7 billion years. Since we also know that light travels at a constant speed, this means that a ray of light that appeared in early times has traveled a certain distance by today (let's call it “distance to the horizon” or “Hubble distance”). Since nothing can move faster than the speed of light, the Hubble distance will be the farthest distance we can ever see in principle (if we don’t find any way to circumvent the theory of relativity).

We have a light source coming to us almost fromHubble distances: cosmic microwave background radiation. We know that the universe does not have an “edge” at a distance from the source of microwave radiation, which is located almost at the whole Hubble distance from us. Therefore, we usually assume that the universe is much larger than our own observable Hubble volume, and that the real edge that can exist is much farther away than we could ever observe. Perhaps this is not true: it is possible that the edge of the universe is right behind the Hubble distance from us, and behind it is sea monsters. But since the entire universe we observe is relatively uniform and homogeneous everywhere, such a turn would be very strange.

I'm afraid we will never have a good answer tothis question. The universe may have no edge at all, and if it is, it will be far enough for us to never see it. It remains for us to comprehend only that part of the Universe that we can actually observe. ”

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