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