What if the Big Bang wasn’t truly the beginning?
A team of cosmologists is using the power of supercomputers to push past the limits of Einstein’s equations and explore mysteries once thought unsolvable. By applying numerical relativity—simulations that can model extreme conditions—they hope to uncover clues about what came before the Big Bang, whether the cosmos is part of a cycle of rebirths, or even if our universe once collided with another.
Simulating the Unsolvable: A New Path Before the Big Bang
It is often said that asking what came before the Big Bang is “unscientific” or even “meaningless.” Yet a recent paper published in Living Reviews in Relativity offers a different perspective. Written by FQxI cosmologist Eugene Lim (King’s College London, UK), astrophysicist Katy Clough (Queen Mary University of London, UK), and Josu Aurrekoetxea (Oxford University, UK), the study suggests that complex computer simulations could provide a way forward.
By using numerical methods to approximate Einstein’s equations of gravity under extreme conditions, the researchers argue that cosmologists may finally be able to investigate questions that have long seemed out of reach. These include what may have happened before the Big Bang, whether multiple universes exist, if our universe ever collided with another, or whether reality passes through repeated cycles of expansion and collapse.
Einstein’s equations of general relativity describe how gravity shapes the behavior of matter and energy in the universe. However, when traced back to the earliest moments of the cosmos, they break down. At that point, the equations predict a singularity, a state of infinite temperature and density where the known laws of physics no longer apply. In such conditions, cosmologists cannot rely on their usual assumptions to solve the equations. The same problem appears when trying to describe other extreme environments, such as the centers of black holes.
“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 undergoes “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.”
Reference: “Cosmology using numerical relativity” by Josu C. Aurrekoetxea, Katy Clough and Eugene A. Lim, 23 June 2025, Living Reviews in Relativity.
DOI: 10.1007/s41114-025-00058-z
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