Revisiting the Cosmological Constant Problem within~Quantum Cosmology

The Cosmological Constant (CC) problem is discussed within the multiverse context. It is assumed that each member of the ensemble of universes has a characteristic scale $a$ that can be used as integration variable. An averaged characteristic scale $\bar{a}$ of the ensemble is estimated by using only members that satisfy the Einstein Field Equations (EFEs). The $\bar{a}$ is compatible with the Planck length when considering an ensemble of solutions to the EFEs. The multiverse ensemble is split in Planck-seed universes with vacuum energy density of order one and $a$-derivable universes. For $a$-derivable universe with a characteristic scale of the order of the observed Universe $a\approx 8\times10^{60}$, one has $\Lambda=\tilde{\Lambda}/a^{2}$ is in the range $10^{-121}$--$10^{-122}$, which is close in magnitude to the observed value $10^{-123}$. We point out that the smallness of $\Lambda$ can be viewed to be natural if its value is associated with the entropy of the Universe. This approach to the CC problem reconciles the Planck-scale huge vacuum energy--density predicted by QFT considerations, as valid for Planck-seed universes, with the observed small value of the CC as relevant to an $a$-derivable universe as observed. [Universe 2020,{\bf 6},108; doi:10.3390/universe6080108].