  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.
