Francis-Yan Cyr-Racine and Lloyd Knox talk about their work with Fei Ge pointing out a symmetry present in most cosmological observables.

The symmetry involves rescaling (almost) *all* the densities and temperatures in the universe thus leaving any dimensionless observables unchanged. When exploited it might pave a way to solving the Hubble tension as it allows one to change H0 without changing predictions for other crucial cosmological measurements (most of which are e.g. temperature, density, etc *contrasts* not absolute measurements).

The first major road-block is that the CMB temperature is an absolute measurement and a very precise one and thus one can’t just casually change it in one’s model.

One way to implement this is therefore to introduce a mirror dark sector. This allows the measured photons to be unchanged, but the overall density of radiation to be rescaled. The mirror world must be very similar to our world so that the exact rescaling happens (i.e. there must be mirror electrons, mirror protons, mirror neutrinos, etc too).

There’s is one remaining (potentially fatal) flaw, which is that one must also rescale the photon mean free path near to CMB formation as it affect observables and doesn’t change just be adding a dark sector. One way to approach a solution there would be to modify the primordial helium abundance, as this would change the density of free electrons at the time of CMB formation. However, this runs into trouble with observations and would need a model of nucleosynthesis that allows for this change…

… so this isn’t a complete solution at this point, but definitely something to keep an eye on.

Maybe the dark sector affects the helium abundance, maybe it changes the free electron density, maybe something else can exploit this symmetry and not run into this problem!?

The model has just *one* free parameter though and, at least phenomenologically, is able to completely solve the Hubble tension – albeit with the creation of a primordial helium tension!

Curiously, the best fit temperature of the dark photons is very close to the temperature of the cosmic neutrino background, suggesting that *maybe* the production of the mirror world is related to neutrino physics somehow.(!?)

Lloyd: https://www.lloydknox.com/
Francis-Yan: http://darkuniverse.unm.edu/
The paper: https://arxiv.org/abs/2107.13000