Has the universe existed forever? Or was there something before it? To find out, we need a working theory of quantum gravity and a new conception of time
PAUSE. Rewind. Suddenly the outward rush of 200 billion galaxies slips into reverse. Instead of expanding at pace, the universe is now imploding like a deflating balloon: faster and faster, smaller and smaller, everything hurtling together until the entire cosmos is squeezed into an inconceivably hot, dense pinprick. Then pshhht! The screen goes dead.
According to the big bang theory – our best explanation for why space is expanding – everything exploded from nothing about 13.8 billion years ago. Cosmologists have been able to wind things back to within a tiny fraction of a second of this moment. But now they’re stuck.
The trouble is, our understanding of space-time, and gravity in particular, is built from Einstein’s equations of general relativity, whereas the extreme conditions of the very early universe can only be described by quantum mechanics. No one knows how to reconcile the two to take us further back. “The rules we have simply don’t work in that regime,” says Carlo Contaldi at Imperial College London. “Nothing makes sense any more.”
That’s a problem for our origin story. Did time begin with the big bang? Or was there an epoch before it?
Some insist that if we rewind the universe far enough, time just stops.But Lee Smolin of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, is having none of it.
“It’s a cute idea but there’s not much evidence for it,” he says. In fact, Smolin wants to see the idea that the universe has a starting point dropped entirely. We can only hope to explain why our universe is the way it is, he says, if there was something before the big bang. It’s about cause and effect; to arrive at satisfying explanations …