Quantum-vacuum and dark-entities in the Universe: experimental test of the vacuum inhomogeneity

A Test-experiment based on the Spontaneous Emission (SE) from optically excited Cr atoms in a ruby crystal is proposed. The system, to be carried into the skyes by a mission spacecraft, provides a unique and most direct measurement apparatus of the density ρvac of the inhomogeneous QED vacuum-field in the Universe. As such it may contribute to enlighten several puzzling cosmological processes as, for instance, the ones related to the very nature of QED vacuum, of dark-energy and of dark-matter. A measured large inhomogeneity of ρvac is expected to resolve the enigma of the cosmological constant and several aspects of the Hubble constant tension.