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One of my favorite aspects about science, and astronomy in particular, is how it allows us to pursue some of the biggest questions we can think of.
Why is there something rather than nothing? How did the Universe come to be? What is its eventual fate?
A century or two ago these questions were the province of philosophy or religion. But now we have observations, evidence, and mathematical modeling that allow us to pursue the answers to these questions rigorously. They’re now in the domain of science.
Scientifically speaking, the idea that the Universe had an actual beginning is relatively new, only about a century old. Astronomers discovered that distant galaxies are moving away from us, and that implies the Universe was smaller in the past. Rewind the clock all the way, and you get to a moment where everything in the Universe—all matter, energy, even space itself—was crammed into one point. Let the clock move forward again, and you get a big bang.
We can understand pretty well what happened even a tiny fraction of a second after that moment, but the moment itself we don’t understand, and perhaps cannot understand. It’s a cloak, a shroud, where our mathematics and physics break down.
We call that moment the beginning of the Universe … but is it really?
Cosmologist (and my friend) Sean Carroll discusses this with Robert Lawrence Kuhn for a PBS TV show called Closer to Truth, and as usual does an excellent job describing what we know, and what we don’t know, about this moment.
As Sean points out, what we call the Big Bang is a placeholder, a way to hang a sign on something that, for the moment, we haven’t quite figured out. Everything after we have a decent grasp on, but at that moment we wind up dividing by 0 a lot. But cosmologists are working on it.
I want to point out something he said, to clarify it a bit. At about 1:20, he mentions that general relativity is wrong. I think he was being succinct to save time; I’m quite sure he doesn’t think it’s wrong, in the sense that it fails completely to explain how the Universe behaves.
Instead, he means it’s incomplete. General relativity makes a huge number of predictions of how things work in the Universe, and every single prediction we have tested has been shown to be true. GR (as those in the know call it) does an extraordinary job explaining things!
But. It turns out that quantum mechanics, which we also know works extremely well, makes different predictions about some things in the Universe, predictions that contradict what GR says. This is a problem.
But it doesn’t mean either theory is wrong, just that there’s something we’re missing. The best analogy is to Newton: He postulated a set of laws of motion that work extremely well, but it turns out they work only if you have low mass objects moving slowly with respect to one another. If the masses get large or velocities approach that of light, Newtonian mechanics breaks down. We need a more overarching set of rules … and those rules are described in general relativity! Newton’s laws weren’t wrong, just incomplete. GR does a better job explaining things.
So there’s likely a bigger theory that covers both quantum mechanics and general relativity, but is yet to be discovered. When someone figured that out it will be a big deal, and in fact may solve many of the problems Sean discusses about how the Universe began, and how it will end.
So, what about Sean’s idea that the Universe may not have had a beginning? Note how careful he is to say he doesn’t know (he’s a good scientist!), but he hopes it doesn’t, he hopes that there was something before our Universe. If that’s the case, we may need to expand what we think of as “the Universe.”
As is usually the case, the Universe knows what it’s doing. Our job is to figure out what it’s telling us about it.
Postscript: I talk about the Big Bang model in an episode of Crash Course Astronomy that may help you understand some of the topics Sean discussed:
I also talk about the eventual fate of the Universe as well in another episode.