Axion baryogenesis puts a new spin on the Hubble tension

TL;DR

This paper proposes a rotating axion model that transitions from matter-like to kination-like behavior around recombination, helping to resolve the Hubble tension and simultaneously explaining baryon asymmetry with testable predictions for future colliders.

Contribution

It introduces a UV-complete rotating axion model that alleviates the Hubble tension and accounts for baryogenesis, linking cosmological observations with particle physics predictions.

Findings

The model reduces the Hubble tension without increasing the S8 tension.

It predicts heavy neutral leptons accessible at future colliders.

The axion transition occurs around the epoch of recombination, affecting cosmological parameters.

Abstract

We show that a rotating axion field that makes a transition from a matter-like equation of state to a kination-like equation of state around the epoch of recombination can significantly ameliorate the Hubble tension, i.e., the discrepancy between the determinations of the present-day expansion rate $H_0$ from observations of the cosmic microwave background on one hand and Type Ia supernovae on the other. We consider a specific, UV-complete model of such a rotating axion and find that it can relax the Hubble tension without exacerbating tensions in determinations of other cosmological parameters, in particular the amplitude of matter fluctuations $S_8$. We subsequently demonstrate how this rotating axion model can also generate the baryon asymmetry of our universe, by introducing a coupling of the axion field to right-handed neutrinos. This baryogenesis model predicts heavy neutral leptons that are most naturally within reach of future lepton colliders, but in finely-tuned regions of parameter space may also be accessible at the high-luminosity LHC and the beam dump experiment SHiP.