What is at the Edge of the Universe?

When we think about objects, we usually picture them as having some size, some volume of space they take up. There is an inside, and an outside. Inside, we say the object is “there,” and outside of it, the object is “not there.”

But what about the Universe? Does it have an edge? Is there some place we could go that we could say we are not in the Universe anymore? Perhaps if we got into a space ship and went far enough, we might come to some kind of boundary that stops us from going any farther. Yet once there, the next natural question to ask would be, “what is beyond this wall?” If we dug at it with picks and shovels, could we chip away at it, and maybe break through? Wouldn’t that mean there is actually more of the Universe past it?

Before we go any further, what exactly is the Universe? Our intuitive understanding is that the Universe is all of space and time and everything in it, but there are places and times where this definition gets fuzzy. So let’s try specifying it further. The Universe, for the purpose of this discussion, is all of space and time that can be gotten to by some smooth path through space and time, and everything in it. This would mean that if we came to a boundary that was the edge of the Universe, there would be nothing beyond it. In fact, there would be no “beyond it” at all. It might be possible to push against the barrier and make a dent in it, but we would be creating new space rather than discovering what was already there. This is very hard to imagine, even for me. Interestingly, such a barrier is physically possible: it could be an event horizon, a surface that requires going faster than the speed of light to pass. If nothing, not even space itself, could get to the other side, then we would have ourselves an edge of the Universe.

Such an event horizon would cause the shape of the Universe to be curved in ways that would be observable in the distribution of galaxies and how their light travels as it comes toward us. We see no sign of such things. As far as we know, the Universe is infinite in size. Nevertheless, it has a finite age, 13.8 billion years. Light takes time to travel, so the farther away something is, the closer to the beginning of the Universe we see it. In fact, there is a surface so far away that it is basically the beginning of time compared with the age of the Universe.

The entire sky unfolded.

In its cradle days, the Universe was thousands of degrees, so hot that electrons and protons could not stick together to make atoms. As the Universe expanded, it cooled, until the protons and electrons merged to form hydrogen, and the light that had been bouncing around the plasma soup became free to fly across the universe unbounded. This light started out visible, but got redshifted as the space it traveled through expanded, until today, when it is in the microwave spectrum. We now call this light the cosmic microwave background. It comes from the same distance in all directions, making its source the surface of a sphere centered on us, which we call the surface of last scattering, which for some reason sounds incredibly poetic to me.

As far as we know, there was no time before the big bang. Time literally started then, so the phrase “before the big bang” is meaningless. It is theoretically possible that future space-based gravitational wave observatories will be able to detect gravitational waves from before the big bang, but so far we have no reason to believe such a time existed. Thus, there is a certain distance that cannot cannot be seen past, because light from beyond it has not had time to reach us in all of time. This distance marks the edge of what is called the observable universe. Indeed this is not far beyond the surface of last scattering, 13.8 billion light years away.

The first moment of the big bang is not really a boundary in space, but in time. However, space and time are united by the speed of light, so if we think from the perspective that the farther something is away from us the further back in time it is, we could say the edge of the observable universe is the edge of the Universe.

On the other hand, perhaps the Universe has a finite size, but no boundary. It was once thought that the Earth had an edge, and that you could fall off. However, we now know that the Earth is a sphere; if you travel far enough, no matter which way you go, you will eventually end up where you started. It is theoretically possible for the same thing to be true of the Universe; if you go far enough in any direction, you will end up back where you started. It would be like the old arcade games, where going off one side of the screen puts you on the other side. This can be difficult to imagine for the Universe, since the Universe has three dimensions whereas the surface of the Earth has only two, but there is nothing wrong with it in theory. However, such a geometry to the Universe would leave signs in the structure of the filaments and voids, which we do not see. As far as our current knowledge can tell, the Universe is infinitely big.

The tunnel leading to the left connects with the one leading to the right.
If there were no walls, the game would be finite in size, but unbounded.

An infinite Universe brings up a new question; what is it expanding into? If the galaxies are moving away from us, wouldn’t they run into the galaxies that are farther away? The key to understanding is to realize it is not the galaxies that are moving, but the space between galaxies which is stretching out. The farther galaxies are being carried away faster than the close galaxies, and the galaxies that are even farther are being carried away even faster, and so on and so on to infinity. Bizarre, but it is theoretically solid. The reason this concept seems so strange to us is because we do not have everyday experiences with infinitely large things.

What if there is not an edge of space, but an edge of stuff? Might there be some distance we could travel, that when we got there we would see nothing beyond but an empty void for all eternity, behind us the brilliance of stars and galaxies uncountable, before us nothing but darkness? When astronomy was young, we though the bunch of stars around us making up the Milky Way galaxy was all there was, and assumed that outside of our galaxy there was a vast expanse of emptiness. Then along came Edwin Hubble, who measured the Andromeda Nebula to be far outside the Milky Way. This meant it was not a nebula at all, but another galaxy. For a time, galaxies were called “island universes,” until we decided that it was more convenient to speak of them as objects in the one Universe. But is there an edge to the galaxies? Galaxies clump into clusters, and galaxy clusters clump into filaments (sometimes called superclusters). Between filaments, there are vast empty regions called voids, which have no galaxies in them. It turns out that as far as we can see, no void is endless, and the galaxies continue on forever.

There actually is a kind of barrier in space right now. Einstein taught us that nothing can go faster than the speed of light, and Hubble taught us that the Universe is expanding. By “expanding,” we do not just mean that things are getting farther apart, although that is true, but that space itself is stretching out. Nothing can go faster than light through space, but there is no limit on how fast space can stretch. Therefore, there is a distance called the cosmic event horizon (not to be confused with the hypothetical event horizon at the edge of space we talked about above), where space is expanding away from us at the speed of light, and everything past it is expanding away faster than light. No matter how fast we go to chase this horizon, we will never be able to reach what has passed beyond it.

However, if we think about it, the cosmic event horizon does not feel like a boundary, because it never stops us from going farther. Indeed, it is always about 16 billion light years away from you in all directions, no matter where you are and how far or fast you have traveled. Everything is in the center of its own cosmic event horizon. Thus, if you were to get on a space ship and fly at the speed of light for 16 billion years, you would never be stopped by any kind of edge of space. However, Earth would pass out of your cosmic event horizon, and you would never be able to go home.

Following the cosmic event horizon to its logical conclusion, we find that sometime in the far distant future all of the galaxies that are not in our own gravitationally-bound cluster, the Local Group, will continue to expand away from us, until they have all passed the cosmic event horizon. Once this happens, the edge of the Local Group will be the edge of stuff, because no matter how far or fast we fly away from it, we will never reach anything else. Then, the age-old hypothesis of the island universe will become true.

Credit: NASA/CXC/M.Weiss

So if the Universe is infinite in all directions, with nothing stopping us from going in a straight line forever—excluding the odd bit of matter we may have to go around—there is really no edge to the Universe, right? The question does not make sense, because there is no place that would be outside the Universe. . . . Except there is still one little detail we have overlooked: the Universe has three dimensions. We have up-down, left-right, and forward-backward. Where exactly each direction points does not matter; what is important is that we can have three lines intersect at a single point and all be at 90 degree angles from each other. But what if there is a fourth dimension? Other than time, I mean; time makes everything more complicated.

A fourth dimension in space is extremely hard to visualize. We may wonder if there is any reason to think about it, since it seems impossible. However, we know of no reason why our Universe had to have three dimensions. As far as we can tell, it was more or less coincidental. There is a theory, beyond the edge of currently measurable physics, that suggests there are many universes with different numbers of dimensions. It is called String Theory.

String Theory deals with things called D-branes, which are objects with a certain number of dimensions, and to which the hypothetical 1-dimensional strings that make up all matter are attached. The only thing we know of that could be classified as a D-brane is the Universe itself. If String Theory is true, our universe is a mere 3D subspace of a 9- or 10D hyperspace, and all the matter we have ever observed is stuck to it.

Think of an infinitely large piece of paper. Imagine drawings that can move. These drawing can go up, down, left, and right, but no matter what they do they cannot get off the paper. Now think of us and the Universe. We can go up, down, left, right, forward, and backward. But if we could step even a single inch into a fourth dimension, we would find ourselves outside the Universe.

Whether or not String Theory is true, whether or not there is any higher-dimensional space or not, we have found the edge of the Universe. We won’t find it by looking out into space. No, the edge of the Universe is here. We are touching it right now. Bounded within these three dimensions, unable to reach even a millimeter into a fourth. This is the edge of the Universe.