After the Big Bang, could our Universe have become a black hole?

The structure of our Universe could have been vastly different plus, or minus, a proton.
By Lliane Hunter | Mar 28, 2018
Why didn't our Universe collapse, gravitationally, into a black hole moments after the Big Bang? Writing for Forbes Magazine, astrophysicist and author, Ethan Siegel, discusses how general relativity answers the profound questions that flow from the very first moments of the Universe. In the paper, he posits that from the moment of the Big Bang onwards, the Universe would have had only three possible outcomes dependent on the matter and energy present within it, and the initial expansion rate.

As a backdrop, he re-introduces Einstein's theory of general relativity (still the most successful theory of gravity to-date) to explain these possible outcomes. General relativity is the idea that spacetime (the "fabric" of the universe) is curved, and the very thing that determines the curvature of spacetime is the presence of energy in all of its forms, including mass. Additionally, general relativity explains that not only does the presence of matter and energy determine the curvature of spacetime, but the properties and evolution of everything in space determines the evolution of that spacetime itself.

Thus, the evolution of our Universe could have progressed in one of three ways:

The expansion rate could have been insufficiently large for the amount of matter and energy present within it, meaning that the Universe would have expanded for a brief time, reach a maximum size, and then (because of the weight of the matter and energy present) recollapsed into a singularity known as the "Big Crunch."

Alternatively, the expansion rate could have been too large for the amount of matter and energy present within it. In this case, all the matter and energy would be driven apart at a rate too rapid for gravitation to bring all the components of the Universe back together, which would cause the Universe to expand too quickly to ever form galaxies, planets, stars, or even atoms.

The third outcome, the "Goldilocks" case, is where the Universe is right on the bubble between recollapsing (which it would do if it had just one more proton) and expanding into oblivion (which it would do if it had one fewer proton), and instead just asymptotes to a state where the expansion rate drops to zero, never quite turning around to recollapse. According to Siegel, we live almost in the Goldilocks case, with a small amount of dark energy thrown in the mix to make the expansion rate slightly larger than the amount of matter and energy present in our Universe. This means that eventually all the matter that isn't gravitationally bound together, will be driven apart into the abyss of deep space.

Significantlyfor our Universe to exist in its current statethe level to which the expansion rate and the overall energy density had to balance is insanely precise (approximately one part in 1024). A tiny change would have led to a Universe vastly different than what we presently observe. The Universe didn't collapse into a black hole because of the remarkably balanced conditions under which it was born, he concludes.

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