Constraints on running vacuum models with the baryon-to-photon ratio

TL;DR

This paper investigates how running vacuum models affect the baryon-to-photon ratio, concluding that coupling of vacuum to photons or baryons is highly constrained and likely negligible, thus challenging certain RVM scenarios.

Contribution

It provides constraints on the coupling of running vacuum to photons and baryons, ruling out significant interactions based on observational limits.

Findings

Coupling of running vacuum to photons or baryons must be extremely small.

Time evolution of baryon-to-photon ratio is tightly constrained by observations.

Coupling scenarios are effectively ruled out as unnatural.

Abstract

We study the influence of running vacuum on the baryon-to-photon ratio in running vacuum models (RVMs). When there exists a non-minimal coupling between photons and other matter in the expanding universe, the energy-momentum tensor of photons is no longer conserved, but the energy of photons could remain conserved. We discuss the conditions for the energy conservation of photons in RVMs. The photon number density and baryon number density, from the epoch of photon decoupling to the present day, are obtained in the context of RVMs by assuming that photons and baryons can be coupled to running vacuum, respectively. Both cases lead to a time-evolving baryon-to-photon ratio. However the evolution of the baryon-to-photon ratio is strictly constrained by observations. It is found that if the dynamic term of running vacuum is indeed coupled to photons or baryons, the coefficient of the dynamic term must be extremely small, which is unnatural. Therefore, our study basically rules out the possibility that running vacuum is coupled to photons or baryons in RVMs.