Mr. Moderate

Anybody who pays much attention to science knows that the universe is big. But I’ve found that very few people, even very few scientists, have any idea of just how big the universe is. So, I’m going to attempt to explain what the inflationary theory of cosmology says about the size of our universe.

If you study cosmology at all, you will probably understand that visible light travels at a fixed speed, so the distance that light travels in a year is a fixed distance we call “one light year.” The visible universe is defined as that part of the universe which mankind can see by using telescopes. Whether you use the Hubble Space Telescope (HST) or a more-exotic instrument like the Cosmic Background Radiation Explorer (COBE), the distance you can see from the Earth is limited because of the way the universe was formed about 13.5 billion years ago.

Because of the fixed speed of light, when we look further and further away from the Earth, we are simultaneously looking further and further back in time. When we get to looking about 13.5 billion years away from the Earth, we can’t see any further away because at about 13.5 billion years ago the universe went from being opaque (light could not travel through it) to transparent (light could suddenly go, and keep going, until it finally arrives at our telescopes). What we see out there about 13.5 billion light years away is the Cosmic Background Radiation, which is the residue of the so-called “Big Bang” explosion that formed our universe. Since it is not possible to look “earlier” in time, it is also not possible to look “further” away in distance, because the two factors are directly related to each other.

So, we can imagine that the visible universe is a ping-pong ball, and the Earth is at the center. In that case, the radius of the ping-pong ball would be about 13.5 billion light years, meaning that the ping-pong ball is about 27 billion light years across. Some versions of inflation theory say that the universe is something roughly like “at least” the size of 10,000 such ping-pong balls. Others speculate that the “minimum size” of the universe is more like a few dozen such ping-pong balls. Any of these ideas are bigger than most of us can imagine. It also boggles the mind because we know what Einstein said about the speed of light being the fastest speed it is possible to obtain within our universe.

Inflation theory resolves this paradox by supposing that the very fabric of space and time expanded (called the metric expansion of space) in such a way that things moved apart without actually moving physically. Of course, it could be the case that they were moving physically too, at the same time, but the inflationary motion was by far the greatest part of how things moved apart. So, things that were very close together just an instant or two after the “Big Bang” happened could end up so far apart that they will never be able to contact each other again during the entire life of our universe because even traveling at the speed of light could not bring them together again before they are exterminated by the ending of the universe (glossing over the question of just how the universe will eventually meet its fate; this is true no matter what the case is for the particular fate of the universe).

Frankly, there are a lot of questions about inflationary theory with respect to the formation of our universe. And there is no good way to observe parts of the universe that are outside of our own little “ping-pong ball”-sized portion of the universe. We have nothing but some scribbled mathematical formulas to assert what the actual size of our universe is. But we have even less reason than that to disbelieve the current “best guess” as to that true size. Even the alternative theories of how the universe was formed, such as string theory, still must postulate some mechanism that would make a very-huge universe of the above size or larger. This is true because what we are able to observe is inconsistent with any other model of the size of the universe.