Was There a 'Before' the Big Bang? Jim Al-Khalili on Time

Jim Al-Khalili Asks What Came Before the Big Bang

Jim Al-Khalili wants you to sit with an uncomfortable question. Not the kind you can wave away with a shrug or a cup of coffee. The kind that quietly undermines everything you assumed about the universe having a starting line. In his new book, "On Time: The Physics That Makes the Universe Tick," the author and theoretical physicist takes readers through the cosmological theories that dare to suggest the Big Bang was not the beginning of time itself.

It is a question that sounds almost childish at first. What was there before? But the answer, it turns out, depends entirely on which cosmological model you happen to find convincing. And there are several.

The Case for Eternal Inflation

The standard story goes like this: the Big Bang marked the beginning of time, followed by a brief period of rapid inflation that stretched space outward. Clean. Simple. Comforting in its finality. But Al-Khalili points to an alternative known as eternal inflation, which flips the script entirely.

In this scenario, our universe was born as a small bubble inside an infinite space that has been inflating forever. The Big Bang, rather than being the starting gun, was simply the moment our particular region of that infinite space stopped inflating and began expanding at a slower, more familiar pace. In other words, the Big Bang was not the beginning.

The theory carries a striking implication. If eternal inflation is correct, other big bangs are happening elsewhere, spawning other universes. Possibly an infinite number of them. All forever separated, all driven apart by the ever-expanding space between them. This totality has a name: the multiverse.

Some physicists argue that such an idea falls outside of proper science, since we do not have a way of testing it.

That objection matters. A theory that cannot be tested sits uncomfortably close to philosophy. Al-Khalili does not pretend otherwise.

Penrose, Steinhardt, and the Cyclic Universe

Roger Penrose offers a different path. His conformal cyclic cosmology proposes that our universe is one in an infinite series of epochs, each beginning with a big bang and ending in thermal equilibrium, a state where the universe is filled only with evenly spread-out thermal radiation. The transition between one epoch and the next is not a Big Crunch, as you might expect. Instead, a geometrical idea called conformal mapping connects the cold, spread-out end of one universe with the hot, dense birth of the next.

Al-Khalili notes that this theory has drawn criticism. The conservation of information problem, similar to the black hole information paradox, creates difficulties. So does the challenge of reconciling the high entropy at the end of an old epoch with the low entropy at the start of a new one.

Then there's the ekpyrotic universe model. Its name comes from ekpyrosis. That's Greek for conflagration. Paul Steinhardt, one of the theory's originators, coined the term, and the idea emerges from string theory, describing our three-dimensional space as a 3D brane floating in a higher-dimensional space. The Big Bang, in this telling, was a collision between our brane and a neighbouring one. That collision generated tremendous heat and radiation. And it set off the expansion of our space. Eventually, the two branes will be pulled back together and collide again in a new Big Bang.

A Mirror Universe on the Other Side

Neil Turok, another originator of the ekpyrotic idea, proposes something different. His mirror universe theory does away with inflation and suggests that on the other temporal side of the Big Bang, another universe evolves in a direction we would consider to be towards the past.

Here is the neat part. Both sides of the Big Bang are completely separated, with each seeing time point in the direction of increasing entropy. If intelligent life exists in that mirror universe and has hit upon the same idea, they would think our side was the one before the Big Bang, just as we think theirs is.

The mirror universe theory also explains why there is more matter than antimatter in our universe. The mirror universe would have it the other way round. What they call normal matter is our antimatter. It solves Loschmidt's paradox too. That's the longstanding conflict. It's between time symmetry and the second law of thermodynamics, and this theory, which flips the balance of matter and antimatter in a way that mirrors our own reality, offers a way to resolve a tension that physicists have struggled with for well over a century.

Where the Evidence Needs to Come From

Al-Khalili does not shy away from the hard question. How seriously should we take these theories? Are they even proper science?

Their proponents know they require empirical evidence. Without observational evidence, these ideas remain theoretical curiosities.

That is the honest position. Jim Al-Khalili is not selling certainty. He is offering a tour through the possibilities, and the tour is fascinating precisely because the answers are not settled.

That beginning might have had a before. We don't know yet. Someday, maybe, we'll get an answer to that question, though right now nobody can say when or how that answer might arrive, and so the wondering continues. For now, it has something else. Something better, maybe. A set of competing ideas. Each one has its own logic. And each one is waiting for the universe to give up a clue.

• Eternal inflation: our universe is one bubble among infinitely many, and the Big Bang ended inflation rather than starting time.
• Conformal cyclic cosmology: an infinite series of epochs connected by conformal mapping, proposed by Roger Penrose.
• Ekpyrotic universe: a collision between 3D branes in higher-dimensional space, named by Paul Steinhardt from the Greek word for conflagration.
• Mirror universe: a parallel cosmos on the other side of the Big Bang, proposed by Neil Turok, where time runs opposite and matter and antimatter are swapped.

What unites all of them is a single, stubborn requirement. Evidence. Without it, the question of what came before the Big Bang stays where it has always been, in the space between mathematics and imagination.