Probing Scalar-Photon Coupling in the Early Universe: Implications for CMB Temperature and Anisotropies

Abstract

The Hubble tension, as a persistent discrepancy between early-time and late-time measurements of the Hubble constant, motivates explorations of new physics in the early Universe. In a recent early dark energy (EDE) model, we introduced a scalar field interacting with the radiation sector at early-time before recombination. We showed that such a scalar-photon coupling can lead to an accelerated expansion phase in which the energy density of scalar component dilutes faster than radiation does, a crucial feature for a successful EDE model. In the present work, we extend our analysis to investigate how this scalar-photon coupling affects the CMB temperature-redshift law and CMB anisotropies. We demonstrate that the temperature-redshift law deviates from the standard relation $T(z)\propto (1+z)$ due to the scalar-photon coupling. This deviation is controlled by a model parameter $\epsilon$, which quantifies the rate of energy transfer between the scalar field and radiation. We also argue that a positive value of $\epsilon$ shifts the acoustic peaks to larger scales, which potentially alleviates the Hubble tension. These findings suggest that scalar-photon coupling is a testable mechanism for reconciling different cosmological observations.