Small nonvanishing cosmological constant from vacuum energy: Physically and observationally desirable

Increasing improvements in the independent determinations of the Hubble constant and the age of the universe now seem to indicate that we need a small nonvanishing cosmological constant to make the two independent observations consistent with each other. The cosmological constant can be physically interpreted as due to the vacuum energy of quantized fields. To make the cosmological observations consistent with each other we would need a vacuum energy density ρ_v~(10^-3 eV)⁴ today (in the cosmological units ħ=c=k=1). It is argued in this paper that such a vacuum energy density is natural in the context of phase transitions linked to massive neutrinos. In fact, the neutrino masses required to provide the right vacuum energy scale to remove the age versus Hubble constant discrepancy are consistent with those required to solve the solar neutrino problem by the MSW mechanism.