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).
Other suggestions for a pre-Big Bang time include a mirror universe, and one that cycles endlessly through expansion and contraction.
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I will not say any more about this theory, in particular since it has been argued that it suffers from several flaws, such as the problem of conservation of information (similar to the so-called black hole information paradox) and the difficulty of reconciling the high entropy at the end of the old epoch with the low entropy of the new epoch.
A related hypothesis, which I admit I find aesthetically rather appealing, is known as the ekpyrotic universe model. The name is based on the Greek word ekpyrosis (ἐκπύρωσι), meaning ‘conflagration’, which refers to a cosmological model proposed by the Stoic philosophers of ancient Greece in which the universe cycles endlessly through fiery birth, expansion, and cooling, followed by contraction, reheating, and rebirth.
The name ‘ekpyrotic universe’ was devised by one of the modern theory’s originators, Paul Steinhardt. The idea comes out of the highly mathematical world of string theory. Basically, the whole of our three-dimensional space (which as we know is really a part of 4D spacetime) forms what is called a 3D brane (a higher-dimensional generalisation of a 2D membrane, or sheet). This 3D brane of ours is floating in a higher-dimensional (4D) space, and the Big Bang is when it collided with a neighbouring brane (in a sense this would be another universe). This collision creates tremendous heat and radiation (the Big Bang) and sets off the expansion of our space.
At the same time, it is drifting away from the other brane that it collided with, but will eventually reach maximum separation from it before they are pulled back together, colliding again in a new Big Bang. Here I simply wish to stress that if this theory is right then there may not have been a beginning of time, and certainly not at the Big Bang.
What if there were a mirror universe on the other temporal side of the Big Bang, evolving in a direction we would consider to be towards the past?
Another cosmological model, which also does away with the need for inflation, as well as suggesting that the Big Bang was not the beginning of time, has been proposed by another of the originators of the ekpyrotic universe idea, Neil Turok. But this idea, known as the ‘mirror universe’ theory, is rather different. It’s a nice idea, and it certainly solves Loschmidt’s paradox regarding the conflict between time symmetry and the second law of thermodynamics. Due to the time-symmetric underlying fundamental equations of physics, starting a system off at any arbitrary moment means that its entropy should increase equally into the past as it does into the future.
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But this just doesn’t make sense. If the system at all times before our chosen arbitrary moment has higher entropy, then starting from any of those earlier times should see the system decrease in entropy in order to reach its state at the original arbitrary moment. Therefore, it cannot be arbitrary; the system had to have been in a specially prepared state at that moment, the moment of the Big Bang. There would be no increase in entropy going backwards in time if that is when time started.
But what if there were a mirror universe on the other temporal side of the Big Bang, evolving in a direction we would consider to be towards the past? This is precisely what Turok is suggesting. What is neat (or convenient, depending on how much you like this theory) is that both sides of the Big Bang are completely separated from each other, with each seeing time pointing in the direction of increasing entropy. If such a mirror universe exists and contains intelligent life that has also hit upon the mirror universe idea, then they would think that our side was the one before the Big Bang in the same way we think that theirs is. As well as doing away with inflation, this idea also conveniently explains why there is more matter than antimatter in our universe because, guess what, the mirror universe would have it the other way round: What they call normal matter is our antimatter.
How seriously should we take these cosmological theories? Are they even proper science? Their proponents know they require some sort of empirical evidence to support their hypotheses, such as some imprint of a reality before the Big Bang frozen in our cosmic microwave background. For without observational evidence, these ideas will remain just theoretical curiosities.
Excerpted from On Time: The Physics That Makes the Universe Tick © 2026 by Jim Al-Khalili. Reprinted by permission of Princeton University Press.
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Princeton University Press
On Time: the Physics That Makes the Universe Tick
In this mind-altering book, Jim Al-Khalili tackles the enduring problems of time head-on, carefully examining them across the three central pillars of physics—quantum mechanics, general relativity, and thermodynamics.
Al-Khalili makes the persuasive case that time is real, not illusory, and that a master arrow of time is not merely a trick of our senses but is rather a fundamental feature of reality that was baked into the structure of our universe from the very beginning.