The standard model of cosmology, the theory that explains the universe at large scales, is buckling under peculiar observations. Different measurements of the Hubble Constant, the expansion rate of the universe, produce two different numbers when they should just be finding one. There’s currently no leading explanation for this "tension" but physicists are working on it. A new solution has put forward the idea there are a lot more particles out there and they make up an invisible "mirror world" through our universe.

As reported in the journal Physical Review Letters, the team worked out a way to play with the model of the universe and reconcile observations by applying a uniform scaling of the gravitational free-fall rates and photon-electron scattering rate. This fixes the discrepancy without messing with anything else in the universe, but with a caveat.

“Basically, we point out that a lot of the observations we do in cosmology have an inherent symmetry under rescaling the universe as a whole. This might provide a way to understand why there appears to be a discrepancy between different measurements of the Universe’s expansion rate,” lead author Francis-Yan Cyr-Racine, assistant professor in the Department of Physics and Astronomy at the University of New Mexico, said in a statement.

The caveat is the existence of mirror matter. The mirror matter idea in its modern form is a few decades old. Symmetry for fundamental particles comes in multiple forms. There’s time reversal, which guarantees that energy is conserved. There’s charge symmetry that underpins the existence of antimatter. And then there’s spatial symmetry, rotation, translation, and reflection.

Elementary particles don’t respect the mirror reflection symmetry also known as P-symmetry or parity. They do respect a combined CPT (Charge, Parity, Time) symmetry but the break in the parity had some physicists considering if there was a mirror version of all known particles.

While no evidence currently suggests that that is the case, this new work found that if this mirror matter existed, then you can do the scaling of those rates and resolve the tension in the cosmological observations.

“In practice, this scaling symmetry could only be realized by including a mirror world in the model — a parallel universe with new particles that are all copies of known particles,” said Cyr-Racine. “The mirror world idea first arose in the 1990s but has not previously been recognized as a potential solution to the Hubble constant problem.

“This might seem crazy at face value, but such mirror worlds have a large physics literature in a completely different context since they can help solve important problems in particle physics,” Cyr-Racine added. “Our work allows us to link, for the first time, this large literature to an important problem in cosmology.”

The model is not the perfect solution just yet, however. The team plans to continue to work on it and look for a way to constrain two other characteristics that currently can’t be satisfied with the model: the abundance of helium and hydrogen isotope deuterium at the beginning of the universe.