If you knew, from first principles, what the laws of physics were everywhere and at all times in our Universe, that still wouldn’t be enough for you to come up with the prediction that the Universe as we see it ought to exist. Because while the laws of physics set the rules for how a system evolves over time, it still needs a set of initial conditions to get started. This week’s Ask Ethan comes courtesy of a submission from Andreas Lauser, who asks:
While I don’t have many doubts that the theory of the Big Bang (™) is correct (or as you would probably say, a pretty good approximation of what happened), there is a thing I have been wondering when it comes to this part of cosmology for a while: Is there any explanation why the whole Universe did not become a black hole immediately? I suppose that its close to initial density was quite a bit above the Schwarzschild limit.
We’ve taken this topic on before, but you deserve more detail and a better answer than I gave last time. Let’s go back to the birth of our most successful theory of gravity — general relativity — some 100 years ago.
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