The universe might meet its end in a big freeze, a big crunch, or a big rip. But whether time ends with the demise of the cosmos depends on whether it is even real after all
TO GRAPPLE WITH questions of time’s end, let’s first look the other way, to its beginning. Some 13.8 billion years ago, our universe, once thought to be eternal and unchanging, began. Space and time popped up spontaneously, out of nothing, in the big bang. Except we don’t know for sure that is what happened. It is an extrapolation based on the equations of general relativity – Albert Einstein’s theory of gravity – and our observations of a universe that, over billions of years, has been expanding and cooling from something denser and hotter.
Our observations take us back to some 380,000 years after a putative beginning, when the universe had cooled enough for the first atoms to form, sending radiation pinging through the cosmos. We hear that radiation today as the hiss of the cosmic microwave background.
With Einstein’s theories, we can go back further, to the first microsecond, when the entire observable cosmos was about the size of our solar system. Beyond that, we are all at sea, with neither theory nor observations to help us. Wind Einstein’s equations back enough, and you end up in a “singularity” of infinite temperature and density that seems to be a physical nonsense. Did time – did everything – really begin then?
The answer to that might depend on where you think the universe is going. “There are a couple possibilities there,” says Katie Mack at North Carolina State University, author of The End of Everything (Astrophysically speaking). “The expansion could continue forever, or it could stop and reverse.”
That second, “big crunch”, scenario has generally been more favoured. On its own, the …