By Don Lincoln, Ph.D., Fermi National Accelerator Laboratory
We know that the universe is 13.7 billion years old and the light that can be seen by us from the early moments of the universe is the radio hiss. That is the farthest we are able to see. So we can call it the visible universe. This term is used to differentiate between the universe we can see and the universe as a whole. But are we sure about the size and the shape of the universe that is visible to us?
The question here is how big is the universe that is visible to us. The answer to this question is trickier than you might think. That it took a little less than 14 billion years for the radio hiss of the big bang to arrive is known to us. Another thing we know is that the radio waves travel at the speed of light. So you would think that the universe visible to us is a sphere with a radius of 14 billion light-years. And that would be correct only if the universe was not expanding, but it surely is expanding.
The radio hiss that we discover, it turns out, was released only about 43 million light-years away. So, one may think that radio hiss would take 43 million light-years to reach here, but since the universe was expanding quickly and space was getting bigger and bigger and light had to travel much more distance. So, to travel just 43 million light-years it took light 13.7 billion light-years.
The things become more confusing when we see that the sphere that was 43 million light-years and had a radius of m-million light-years at the time of emission of radio hiss is now at a radius of around 46 billion light-years. We cannot see the sphere now as it is too far away. But those things can be seen which are presently at a distance of 46 billion light-years though it was much closer than when the light was released. You can now see how confusing it is.
This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.
The Size of the Universe
Perhaps the actual message is that the universe that we see has a finite size. It is a sphere that has the Earth as its center. But we have no reason to think that the universe is only as big as is visible to us. It could be much bigger than that. Maybe it is infinite in size. We can’t be sure. We can get some insight through the balloon idea. Although the surface of the balloon is curved if we keep inflating it, the surface becomes flatter and flatter. At some point, after inflating a little more, it becomes difficult to differentiate between the curved and the flat surface. So, perhaps the universe visible to us is just a tiny patch of a gigantic large curved universe.
If we consider another alternative, that again is mind-blowing. The universe could be infinite even before the big bang began. In that case, space is flat now and was always flat. The universe that we can see was just a tiny volume in the infinite universe that had an infinite number of other tiny volumes. The universe began expanding and has been doing so ever since.
There can be a better mental picture of the big bang. To start with, we can take a common one that is a singularity sitting in space. The whole matter and energy of the universe were concentrated in a single point. Then that point exploded like a grenade and since then the universe has been expanding. So, in this idea somewhere in the space there is a place where the universe began. And that is the actual center of the universe. But that is definitely incorrect and so it presents a wrong mental picture of the big bang. Then what is a better idea?
When we discuss the black hole we know why singularities can’t exist. So we can leave that idea. Rather, we can assume that the whole matter and energy of the universe that is visible to us were concentrated in a small volume that was not of zero sizes. The size of that small volume is not known to us. It could even be smaller than an atom or it could be bigger. This particular condition is ruled by the principals of quantum gravity. And as we know there is no theory of gravity so we have no idea of what was happening in detail.
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Space: Flat or Curved?
We are not sure if the space was flat or curved at this early time. If the universe was curved, maybe there were many such small volumes having deformed shapes, maybe a cluster of volumes on the surface of the sphere. And if space were flat, there could be an infinite number of such volumes in an infinite space. In any case, space started expanding very quickly. The expansion was so rapid that if initially space was curved the expansion was enough for us to be unable to see any residual curvature. And if it were flat, it stayed that way.
Now, what was a tiny volume has presently become an enormous volume that is 46 billion light-years in radius or 92 light-years across. We are not sure if that is just one of an infinite number of volumes. Also, we are not sure of how the universe actually began. It could be different. There are many other ideas regarding that. One of those is of many universes floating in a higher-dimensional space like ribbons, ramming into each other and heating each other up, and looking like the big bang. And some of those are simpler ideas.
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Common Questions about the the Size, the Shape, and the Idea of the Universe
Perhaps the universe that we see has a finite size. It is a sphere that has the Earth as its center. But we have no reason to think that the universe is only as big as is visible to us. It could be much bigger than that. Maybe it is infinite in size.
The universe consists of all living things, planets, stars, galaxies, dust clouds, and light.
The astronomers use the big bang theory to explain the origin of the universe. According to the big bang theory, the universe began as a single point and expanded to reach its current size. There is a possibility that the universe is still expanding.