In 2019, this is a common emotion - to wish to go four or five times a day, not just to space, but to the very end of the world, as far as possible, in order to get rid of bad obsession or bad weather, a delayed train or tight trousers, so ordinary on the earth of things. But what will await you on this cosmological frontier? 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 at California Institute of Technology:
“As far as we know, the universe has no boundaries. The observable universe has an edge - the limit of what we can see. This is because light travels at a finite speed (one light year per year), so when we look at distant things, we are looking 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 the cosmic microwave background that surrounds us from all directions. But this is not a physical "border", if you really judge that.
Since we can only see this far, we don't know what things are like outside of our observable universe. The universe that we see is quite homogeneous on a large scale and, perhaps, it will literally always continue this way. Alternatively, the universe could fold into a sphere or torus. If so, the universe will be limited in overall size, but it will still have no boundary, just like a circle has no beginning or end.
It is also possible that the universe is not homogeneous beyond what we can see, and that conditions vary greatly from place to place. This possibility is presented by the cosmological multiverse. We do not know if the multiverse exists in principle, but since we do not see either one or the other, it would be reasonable to remain impartial."
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Joe Dunkley, professor of physics and astrophysical sciences at Princeton University:
“Yes, everything is the same!
Okay, we don't really think the universe has a border or an edge. We think that it either continues infinitely in all directions, or wraps around itself, so that it is not infinitely large, but still has no edges. Imagine a donut surface: it has no boundaries. Maybe the whole universe is like that (but in three dimensions - the surface of a donut has only two dimensions). This means you can travel in a spaceship in any direction, and if you travel long enough, you will return to where you started. There is no edge.
But there is also what we call the observable universe, which is the portion of space that we can actually see. The edge of this place is where the light did not have enough time to reach us since the beginning of the universe. We can only see such an edge, and behind it, probably, will be everything that we see around: superclusters of galaxies, each of which contains billions of stars and planets."
Last scattering surface
Jesse Shelton, Assistant Professor in the Department of Physics and Astronomy at the University of Illinois at Urbana-Champaign:
“It all depends on what you mean by the edge of the universe. Since the speed of light is limited, the further and further into space we look, the further and further back in time we look - even when we look at the neighboring galaxy Andromeda, we see not what is happening now, but what happened two and a half million years ago when the stars of Andromeda were emitting light that has only now entered our telescopes. The oldest light we can see came from the deepest depths, so, in a sense, the edge of the universe is the oldest light that has reached us. In our universe, this is the cosmic microwave background - the faint, prolonged afterglow of the Big Bang that marks the moment when the universe has cooled down enough to allow atoms to form. This is called the surface of the last scattering,as it marks the spot where photons stopped bouncing between electrons in hot, ionized plasma and began flowing out through transparent space, billions of light years in our direction. Thus, we can say that the edge of the universe is the surface of the last scattering.
What's at the edge of the universe right now? Well, we don't know - and we can't find out, we would have to wait until the light emitted there now and coming towards 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 mostly the same wherever you look. Chances are good that if you were on the edge of the observable universe today, you would see a universe that is more or less similar to our own: galaxies, larger and smaller, in all directions. I think that the edge of the universe now is simply even more of the universe: more galaxies, more planets, more living things asking the same question."
Michael Troxel, Associate Professor of Physics at Duke University:
“While the universe is probably infinite in size, there is actually more than one practical 'edge'.
We think that the universe is actually infinite - and it has no boundaries. If the Universe were “flat” (like a sheet of paper), as our tests showed up to a percentage point, or “open” (like a saddle), then it really is infinite. 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: imagine you are moving on the surface of a ball. As a hobbit named Bilbo once said: "The road runs forward and forward …". Again and again.
The Universe has an "edge" for us - even two. This is due to a part of general 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 applies 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 from us, it takes time for light from that object to reach us (distance divided by the speed of light). The trick is that as space expands as light travels towards us, the distance the light must travel also increases over time as it travels towards us.
So, the first thing you might ask is, what is the farthest distance we could observe light from an object if it had been 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 about 63,241 times the distance between the Earth and the Sun), and is called the cosmological horizon. The question can be posed somewhat differently. If we sent a message at the speed of light, at what distance could we receive it? This is even more interesting because the rate of expansion of the universe in the future increases.
It turns out that even if this message flies forever, it can only reach those who are now at a distance of 16 billion light years from us. This is called the "horizon of cosmic events." However, the most distant planet that we could observe is 25 thousand light years away, 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 a galaxy is about 13.3 billion light years, so we cannot see what is at the edge of the universe. Nobody knows what's on both sides."
Abigail Weiregg, Associate Professor, Institute of Cosmological Physics. Kavila at the University of Chicago:
“Using telescopes on Earth, we look at light emanating from distant places in the universe. The further away the light source is, the longer it takes for this light to get here. Therefore, when you look at distant places, you look at what those places were like when the light you saw was born - and not at what these places look like today. You can keep looking further and further, which will correspond to moving further and further back in time, until you see something that existed several millennia after the Big Bang. Before that, the universe was so hot and dense (long before there were stars and galaxies!) That any light in the universe could never catch on, it cannot be seen with modern telescopes. This is the edge of the "observable universe" - the horizon - because you cannot see anything beyond it. Time goes by, this horizon is changing. 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, the speed of light and the expansion of the universe.
What does the place that corresponds to the earth's horizon look like? We do not know, because we can see this place as it was immediately after the Big Bang, and not as it is 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 the part of the universe that we can see from Earth today, and that the universe itself has no "edge" per se. It's just an expanding space-time."
The universe has no boundaries
Arthur Kosovsky, professor of physics at the University of Pittsburgh:
“One of the most fundamental properties of the universe is its age, which, according to various measurements, we now define 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 now (let's call this "distance to the horizon" or "Hubble distance"). Since nothing can travel faster than the speed of light, the Hubble distance will be the farthest distance we can ever observe in principle (unless we find some way to get around the theory of relativity).
We have a light source coming towards us from almost Hubble distance: cosmic microwave background radiation. We know that the universe does not have an "edge" at the distance to the microwave source, which is almost the entire 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 may exist is much further away than we could ever observe. Perhaps this is not true: it is possible that the edge of the universe is located immediately beyond the distance of the Hubble from us, and beyond it - the sea monsters. But since the entire universe we observe is relatively the same and homogeneous everywhere, such a turn would be very strange.
I'm afraid we will never have a good answer to this question. The universe may not have an edge at all, and if it does, it will be far enough away that we will never see it. It remains for us to comprehend only that part of the Universe that we can really observe."
Ilya Khel