What happened before the Big Bang?

(Press-News.org) We’re often told it is “unscientific” or “meaningless” to ask what happened before the big bang. But a new paper by FQxI cosmologist Eugene Lim, of King's College London, UK, and astrophysicists Katy Clough, of Queen Mary University of London, UK, and Josu Aurrekoetxea, at Oxford University, UK, published in Living Reviews in Relativity, in June 2025, proposes a way forward: using complex computer simulations to numerically (rather than exactly) solve Einstein’s equations for gravity in extreme situations. The team argues that numerical relativity should be applied increasingly in cosmology to probe some of the universe’s biggest questions–including what happened before the big bang, whether we live in a multiverse, if our universe has collided with a neighboring cosmos, or whether our universe cycled through a series of bangs and crunches.

Einstein’s equations of general relativity describe gravity and the motion of cosmic objects. But wind the clock back far enough and you’ll typically encounter a singularity–a state of infinite density and temperature–where the laws of physics collapse. Cosmologists simply cannot solve Einstein’s equations in such extreme environments–their normal simplifying assumptions no longer hold. And the same impasse applies to objects involving singularities or extreme gravity, such as black holes.

One issue might be what cosmologists take for granted. They normally assume that the universe is ‘isotropic’ and ‘homogeneous’–looking the same in every direction to every observer. This is a very good approximation for the universe we see around us, and one that makes it possible to easily solve Einstein’s equations in most cosmic scenarios. But is this a good approximation for the universe during the big bang?

“You can search around the lamppost, but you can’t go far beyond the lamppost, where it’s dark–you just can’t solve those equations,” explains Lim. “Numerical relativity allows you to explore regions away from the lamppost.”

Beyond the Lamppost

Numerical relativity was first suggested in the 1960s and 1970s to try to work out what kinds of gravitational waves (ripples in the fabric of spacetime) would be emitted if black holes collided and merged. This is an extreme scenario for which it is impossible to solve Einstein’s equations with paper and pen alone–sophisticated computer code and numerical approximations are required. Its development received renewed focus when the LIGO experiment was proposed in the 80s, although the problem was only solved in this way in 2005, raising hopes that the method could also be successfully applied to other puzzles.

“You can search around the lamppost, but you can’t go far beyond the lamppost, where it’s dark–you just can’t solve those equations. Numerical relativity allows you to explore regions away from the lamppost,” says Eugene Lim.

One longstanding puzzle that Lim is particularly excited about is cosmic inflation, a period of extremely rapid expansion in the early universe. Inflation was initially proposed to explain why the universe looks the way it does today, stretching out an initially small patch, so that the universe looks similar across a vast expanse. “If you don't have inflation, a lot of things fall apart,” explains Lim. But while inflation helps explain the state of the universe today, nobody has been able to explain how or why the baby universe had this sudden short-lived growth spurt.

The trouble is, to probe this using Einstein’s equations, cosmologists have to assume that the universe was homogeneous and isotropic in the first place–something which inflation was meant to explain. If you instead assume it started out in another state, then “you don't have the symmetry to write down your equations easily,” explains Lim.

But numerical relativity could help us get around this problem–allowing radically different starting conditions. It isn’t a simple puzzle to solve, though, as there’s an infinite number of ways spacetime could have been before inflation. Lim is therefore hoping to use numerical relativity to test the predictions coming from more fundamental theories that generate inflation, such as string theory.

Cosmic Strings, Colliding Universes

There are other exciting prospects, too. Physicists could use numerical relativity to try to work out what kind of gravitational waves could be generated by hypothetical objects called cosmic strings–long, thin “scars” in spacetime–potentially helping to confirm their existence. They might also be able to predict signatures, or “bruises,” on the sky from our universe colliding with neighboring universes (if they even exist), which could help us verify the multiverse theory.

Excitingly, numerical relativity could also help reveal whether there was a universe before the big bang. Perhaps the cosmos is cyclic and goes though “bounces” from old universes into new ones–experiencing repeated rebirths, big bangs and big crunches. That’s a very hard problem to solve analytically. “Bouncing universes are an excellent example, because they reach strong gravity where you can’t rely on your symmetries,” says Lim. “Several groups are already working on them–it used to be that nobody was.”

Numerical relativity simulations are so complex that they require supercomputers to run. As the technology of these machines improves, we might expect significant improvement in our understanding of the universe. Lim is hoping the team’s new paper, which outlines the methods and benefits of numerical relativity, can ultimately help get researchers across different areas up to speed.

“We hope to actually develop that overlap between cosmology and numerical relativity so that numerical relativists who are interested in using their techniques to explore cosmological problems can go ahead and do it,” Lim says, adding, "and cosmologists who are interested in solving some of the questions they cannot solve, can use numerical relativity.”

Journal reference, Living Reviews in Relativity: Cosmology using numerical relativity

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