The First Neutron Star Collision We've Ever Seen

“….Finally, there’s the “Goldilocks” case, or the case 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, but never quite turns around to recollapse.

As it turns out, we live almost in the Goldilocks case, with just a tiny bit of dark energy thrown in the mix, making the expansion rate just slightly larger, and meaning that eventually all the matter that isn’t gravitationally bound together already will be driven apart into the abyss of deep space.

What’s remarkable is that the amount of fine-tuning that needed to occur so that the Universe’s expansion rate and matter-and-energy density matched so well so that we didn’t either recollapse immediately or fail to form even the basic building-blocks of matter is something like one part in 1024, which is kind of like taking two human beings, counting the number of electrons in them, and finding that they’re identical to within one electron. In fact, if we went back to a time when the Universe was just one nanosecond old (since the Big Bang), we can quantify how finely-tuned the density and the expansion rate needed to be.

The level to which the expansion rate and the overall energy density must balance is insanely precise; a tiny change back then would have led to a Universe vastly different than the one we presently observe. And yet, this finely-tuned situation very much describes the Universe we have, which didn’t collapse immediately and which didn’t expand too rapidly…”