WTF, Universe? The entirety of existence is getting bigger
Can infinity become more infinite? Yep, it sure can!
So far, we have established that distance and time are basically the same thing, on cosmological scales, which means the farther away we look with our telescopes, the further back in time we can see. This puts a limit to the amount of space we are capable of looking at — the “observable Universe” — because we can’t see past the distance/time where/when nothing existed yet, which we call, for completely snarky reasons, “The Big Bang.”
We then attempted, and failed, to convey a sense of just how fucking large the observable Universe is, which, it turns out, is nothing compared to the potential infinity of everything that’s out there beyond our observable horizon.
It is interesting to point out here that the “Big Bang” was initially a derisive term used by opponents of the idea that the Universe had a beginning. They advocated a competing theory, called the “Steady State” model, in which matter is continuously created in empty space. See, some liked their unexplained creation all at once, in a beginning of sorts, and some preferred it to be continuous, a little bit at a time. Both models had to explain data that showed that the Universe was sure as shit, no doubt about it, expanding.
The Embiggening of the Universe
Once upon a time, all the stuff in the Universe was so close together that light couldn’t get very far without bumping into electrons. Then, stuff was farther apart, so light could travel farther. Later, gravity clumped stuff together to create stars and galaxies and planets! Eventually, some humans on a planet they called Earth took some weird measurements that seemed to imply that the Universe is expanding.
Picturing the entire Universe is hard, so let’s think about raisin bread instead. Imagine that galaxies are like raisins suspended in bread dough. When the dough gets twice as big, all the raisins get farther away from each other. (The raisins themselves don’t get bigger because gravity holds them together. Little-known fact.)
In this example on the left, all the raisins get twice as far apart as they used to be from every other raisin in the same amount of time. Nearby raisins that were, let’s say, 5 cm apart, grow to be 10 cm apart, but far away raisins that were 10 cm grow to be 20 cm, and so on. This means nearby raisins appear to be moving slower than far away raisins. Their relative speed is proportional to their distance.
This is exactly the relation that was observed in the 1920s — the observed speed of galaxies relative to us depends on their distance to us, leading to the implication that our Universe itself is expanding.
This discovery is often cited as the first observational evidence that our Universe is weird as fuck. Further evidence came with the discovery of the Cosmic Microwave Background radiation, which I mentioned in episode 1, and which is definitely not pigeon shit. (Look, sometimes science means making sexy observations of galaxies on the top of a mountain, and sometimes it means ruling out any possible explanations of a weird signal you don’t quite understand by cleaning out the pigeon shit from your radio antenna.)
The microwave background comes from everywhere and has the exact temperature spectrum you would expect if the entire Universe was once in thermal equilibrium, as confirmed by later satellites that also contained no pigeon shit. This radiation is also partly responsible for the popcorn noise in old TVs — an awesome and soon to be useless factoid that no one will understand in the era of digital screens.
I could spend a lot more time explaining how we know the Universe is expanding… but what does that even mean? It turns out, the raisin bread model is a bit misleading, and not just because galaxies are not, in fact, raisins. Most importantly, there is no space or time outside of the raisin bread. The Universe is not a thing in space which is getting bigger in space. It is everything and everywhere. There is no “center” of the Universe, like there is a center of a mound of raisin bread. In fact, the Universe is probably infinite.
How can infinity get bigger, you ask? Though schoolchildren might think that “Nuh-uh times infinity plus one!” is larger than “Yuh-huh times infinity!”, these are actually both the same old infinity. Infinity gets bigger all the time. It’s practically the very nature of infinity to keep getting bigger, and never stop. So, if the Universe is infinite, such that “space” extends to infinity and there is an infinite amount of stuff in the Universe, then all expansion does is make that infinity of stuff spread out to be farther away from each other, more isolated, and more lonely. The infinity of “space” is becoming a bigger infinity, in some sense, but also the same infinity. Forever.
I have put “space” in “scare quotes” here because I don’t want you to get the wrong idea that “space” is a “thing.” Space is not a thing. Things exist in space, for a given amount of time. It is important that space is not a thing, because the recession velocity of galaxies can be faster than the speed of light for galaxies sufficiently far away from each other. But that’s not a problem because no information is being transmitted at superluminal speeds. These and other misconceptions have led some astronomers to postulate that the idea of expanding space is the “Root of all Evil.” Astronomers can be quite the melodramatic bunch.
Next time, I hope you will keep in mind that space is not a thing when I explain how this non-thing can warp, curve, and ripple. After that, I’ll show how by measuring the global curvature of space, or more precisely, space-time, we can figure out whether it is finite or potentially infinite.