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The mirror symmetry of the universe may be in trouble. Based on our current understanding of the large-scale structure of the cosmos and what we know about how gravity works, if we look at how galaxies are distributed throughout the universe and then compare it with its mirror image, the two should be basically indistinguishable. But two separate analyses of our best map of galaxies have now found that this principle, called parity symmetry or just parity, doesn鈥檛 seem to hold true.

Each analysis examined galaxies from the Sloan Digital Sky Survey using a technique that takes advantage of the fact that a tetrahedron, or triangular pyramid, is the simplest 3D shape that can be distinguished from its mirror image. The researchers compared all of the possible聽tetrahedrons that could be made for a given sample of galaxies by placing a galaxy on each vertex.

鈥淚t鈥檚 just like how you can鈥檛 rotate your right hand and make it impossible to tell the difference from your left hand,鈥� says at the University of Florida, who performed one analysis along with , also at the University of Florida, and at Lawrence Berkeley National Laboratory in California. 鈥淚t鈥檚 the same with these pyramids of galaxies.鈥�

To determine whether parity was violated, the researchers assigned a primary vertex for each of the galaxy-assigned tetrahedrons and then split them into two groups: those where, looking down from the primary vertex, the sides increase in length when you move clockwise, and those where they increase in length when you move counterclockwise.

If our universe obeys parity, the two groups of tetrahedrons should be roughly the same size. Neither analysis found that to be the case. One, by at Princeton University, found parity violation at a level of 2.9 sigma, meaning there was only a 0.4 per cent chance that a pattern like this would show up as a statistical fluke due to random noise. The other, by Slepian and his colleagues, split the galaxies into two groups based on distance. They found even stronger parity violation in both 鈥� one at a level of 3.1 sigma and in the other at a level of 7.1 sigma.

鈥淭his observation is completely shocking,鈥� says at Brown University in Rhode Island. 鈥淚f it was just one group, I might have been more sceptical, but with two groups, it鈥檚 a lot harder to shake.鈥�

鈥淎n analysis like this lands with a lot of suspicion on everyone鈥檚 desk because it鈥檚 potentially making a claim about the physics of the early universe, which is really, really difficult to measure,鈥� says at Lawrence Berkeley National Laboratory. 鈥淭here鈥檚 nothing obviously wrong with the analyses, but I think the jury will be out on whether this result is true or not 鈥� the good news is that it鈥檚 very, very testable.鈥�

The claims about the early universe come from the fact that the distribution of galaxies takes up so much space that it would be difficult for a force to be so strong that it could affect the symmetry of the entire universe. But just after the big bang, the cosmos was much smaller and everything was far closer together, so the seeds of this asymmetry could have been planted then.

That could upend our understanding of not just the first moments after the big bang but also how the universe behaves now. 鈥淚f what we find turns out to be really genuinely from the actual early universe, it would mean there鈥檚 a new interaction between particles that previously has not been part of our understanding of physics 鈥� basically a new force of nature,鈥� says Slepian.

Whatever this exotic new physics is, the researchers hope that it will help explain one of the biggest mysteries of the universe: why the universe is made of mostly matter instead of equal parts matter and antimatter. 鈥淚f we want to generate more matter than antimatter, which we do, we do need something to violate parity,鈥� says Philcox. While there are some ideas for the reasons behind this parity violation, including new fields and particles in the early universe, the new analyses don鈥檛 point to any one of them in particular.

Before physicists can start narrowing down their hypotheses in earnest, these results need to be checked, all the researchers agree. One way to do that is to repeat the analyses on other, larger maps of the universe that are expected to be released in the next few years. But to be really certain, other methods will have to be used as well.

鈥淚f there is really a parity-breaking mechanism at cosmological scales, it would probably not only pop out in the large-scale structures of galaxies, but maybe also in the cosmic microwave background and even gravitational waves,鈥� says Hou. If future work confirms this large-scale asymmetry in the universe, then it is off to the races to try to find out what strange physics caused it.

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