The H0 tension: did a QCD meV axion emerge?

Understand

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.