Too Short for a Blog Post, Too Long for a Tweet XCI

Here's an excerpt from a book I recently read, "The Big Picture: On the Origins of Life, Meaning, and the Universe Itself," by Sean Carroll:

When we talk about the “Big Bang model,” we have to be careful to distinguish that from “the Big Bang” itself. The former is an extraordinarily successful theory of the evolution of the observable universe; the latter is a hypothetical moment that we know almost nothing about. The Big Bang model is simply the idea that approximately 14 billion years ago the matter in the universe was extremely hot, densely packed, and spread almost uniformly through space, which was expanding very rapidly. As space expanded, matter diluted and cooled, and stars and galaxies condensed out of the smooth plasma under the relentless pull of gravity. Unfortunately, the plasma was so hot and dense at early times that it was essentially opaque. The cosmic microwave background reveals what the universe looked like when it first became transparent, but before that, we cannot directly see. 

The Big Bang itself, as predicted by general relativity, is a moment in time, not a location in space. It would not be an explosion of matter into an empty, preexisting void; it would be the beginning of the entire universe, with matter smoothly distributed all throughout space, all at once. It would be the moment prior to which there were no moments: no space, no time. 

It’s also, most likely, not real. The Big Bang is a prediction of general relativity, but singularities where the density is infinitely big are exactly where we expect general relativity to break down—they are outside the theory’s domain of applicability. At the very least, quantum mechanics should become crucially important under such conditions, and general relativity is a purely classical theory. 

So the Big Bang doesn’t actually mark the beginning of our universe; it marks the end of our theoretical understanding. We have a very good idea, on the basis of observational data, what happened soon after the Bang. The microwave background radiation tells us to a very high degree of precision what things were like a few hundred thousand years afterward, and the abundance of light elements tells us what the universe was doing when it was a nuclear fusion reactor, just a few minutes afterward. But the Bang itself is a mystery. We shouldn’t think of it as “the singularity at the beginning of time”; it’s a label for a moment in time that we currently don’t understand.
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