The Big Bang is commonly thought of as the start of it all: About 13.8 billion years ago, the observable universe went boom and expanded into being.
But what were things like before the Big Bang?
Short answer: We don't know. Long answer: It could have been a lot of things, each mind-bending in its own way.
So, scrap the image of a tiny speck of dense matter suddenly exploding outward into a void. For one thing, the universe at the Big Bang may not have been particularly small, Carroll said. Sure, everything in the observable universe today — a sphere with a diameter of about 93 billion light-years containing at least 2 trillion galaxies — was crammed into a space less than a centimeter across. But there could be plenty outside of the observable universe that Earthlings can't see because it's physically impossible for the light to have traveled that far in 13.8 billion years.
Thus, it's possible that the universe at the Big Bang was teeny-tiny or infinitely large, Carroll said, because there’s no way to look back in time at the stuff we can’t even see today. All we really know is that it was very, very dense and that it very quickly got less dense.
As a corollary, there really isn't anything outside the universe, because the universe is, by definition, everything. So, at the Big Bang, everything was denser and hotter than it is now, but there was no more an "outside" of it than there is today. As tempting as it is to take a godlike view and imagine you could stand in a void and look at the scrunched-up baby universe right before the Big Bang, that would be impossible, Carroll said. The universe didn't expand into space; space itself expanded.
"No matter where you are in the universe, if you trace yourself back 14 billion years, you come to this point where it was extremely hot, dense and rapidly expanding," he said.
No one knows exactly what was happening in the universe until 1 second after the Big Bang....
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Originally Posted by Mister D
