Axionic quantum fluctuations, dark energy, and the Hubble tension

The cosmological constant is now a fundamental ingredient of the standard ΛCDM model, and its value is constrained by concordance with empirical data. Despite its importance in modern cosmology, we still do not understand its origin. A naive calculation of the contribution of the quantum vacuum fluctuations to vacuum energy (considering it to be the source of the cosmological constant) yields predictions 120 orders of magnitude larger than observations. This poses one of the most celebrated unsolved problems in physics and cosmology. This work discusses a model of quantum-thermal fluctuations of the cosmic microwave background with a Planck factor. Fluctuations of a bosonic field are studied, and we show that they could match the vacuum energy density if they correspond to an axionic field with a particle rest mass in the range of a fraction of a meV. This mass range is in agreement with present bounds on the mass of the Peccei–Quinn axions arising from the spontaneous symmetry breaking that explains CP conservation in strong interactions, as well as estimations of the mass of axions in the galactic halo and experiments on Shapiro step anomalies using Josephson junctions. We also show that this model can clarify the Hubble tension debate, i.e., the statistically significant discrepancy between measurements of the Hubble parameter based upon the cosmic microwave background and those using low redshift observations.