Was there a 'before' the Big Bang? Jim Al-Khalili on the mind-bogglingness of time

Time keeps getting away from us. It has a fraught relationship with our subjective experience — sometimes slowing down or dragging on; and at other times speeding up, with years seeming to fly by as we get older . Our brain cells are even known to distort our sense of time when we're tired .

In physics, time gets even more complicated , incorporated into theories of relativity — the experience of the clock's ticking being relative to the observer and their place in space — and entropy, which gives time its unidirectional flow. Meanwhile quantum mechanics indicates time can move both forwards and backwards , at least inside quantum systems.

In this excerpt from " On Time: The Physics That Makes the Universe Tick " (Princeton University Press, 2026), author and theoretical physicist Jim Al-Khalili looks at the possibility that time did not begin with the Big Bang — but if this outburst was just another moment, what was there before?

There is an alternative cosmological theory to the one that says the Big Bang marked the beginning of time itself, which was then followed by a brief period of rapid inflation of space. The alternative is known as eternal inflation. In this scenario, our universe was born as a small bubble in an infinite space that has been undergoing inflation forever.

So, rather than time beginning with our Big Bang, we should regard the Big Bang as an event that took place at some moment in time, in which our region of this infinite space suddenly stopped inflating and began instead to expand at a more sedate pace.

So instead of a brief period of inflation happening just after the Big Bang, in the eternal inflation theory it is the other way around, with the Big Bang marking the end of inflation in our part of space, creating our bubble universe outside of which inflation continues. The theory predicts there will be other big bangs happening elsewhere that give rise to other universes — possibly an infinite number of them — all forever separated from each other, and all being driven apart by the ever-expanding, rapidly inflating space outside of them. The totality of this infinite space is referred to as the multiverse, to distinguish it from the individual bubble universes.

Some physicists argue that such an idea falls outside of proper science, since we do not have a way of testing it. There are two other ideas worth mentioning on the matter of whether the Big Bang marked the beginning of time or not, both of which are alternatives to inflation theory. The first is that our universe is part of a repeating process called conformal cyclic cosmology'.

This idea, proposed by Roger Penrose , suggests that our universe is going through an infinite series of lives, or epochs, each of which starts with a big bang and ends with a universe in thermal equilibrium, filled only with evenly spread-out thermal radiation. The idea here is not, as you might expect, that the universe reaches some maximum expansion before collapsing down to a 'Big Crunch' that then marks the Big Bang of the next epoch, but rather because of a subtle geometrical idea called 'conformal mapping' that connects up the very dif­ferent end of one universe (cold and spread-out) with the birth of the next (hot and dense).