The H0 tension: did a QCD meV axion emerge?

Abstract

I discuss the possibility that the difference in the measured Hubble constant Ho between the current, Late, and the z ~1100, Early, epochs is due to the emergence in between of a new particle. I connect that difference with a change in the effective cosmological constant observed by LambdaCDM, which is induced by the energy density of the vacuum of the field of the new particle. Then I try to provide the main characteristics of the particle, boson vs fermion and mass, using the measured change in Ho and a Lorentz invariant regularization of the energy density of vacuum, which relates it to such characteristics. The result indicates that a boson of mass in the range of few meV emerged. A QCD post-inflation cosmological axion in this mass range is allowed by recent analysis and its detection may be attempted according to recent experimental proposals.