Cosmological dynamics and observational constraints of an interacting early scalar field coupled to radiation

TL;DR

This paper investigates a scalar field interacting with radiation in the universe, deriving analytical models, constraining them with observational data, and finding that the interaction is minimal but could slightly address the Hubble tension.

Contribution

It extends previous models by including non-relativistic matter and provides a comprehensive observational analysis of the interacting scalar field scenario.

Findings

Interaction parameter is consistent with zero.

Small deviations can alleviate the Hubble tension.

Model remains close to LCDM at late times.

Abstract

We study the cosmic evolution of an interacting scalar field radiation model, in which a minimally coupled scalar field exchanges energy with the radiation sector through an exponential coupling. Extending previous formulations, a non-relativistic matter component is included explicitly, which allows a self consistent description of cosmological dynamics from the radiation-dominated era to late-time acceleration. Analytical expressions for the background expansion are derived and characterized using kinematic diagnostics. We constrain the model using observational Hubble data, Type Ia Supernovae, baryon acoustic oscillations (including DESI DR2), and compressed cosmic microwave background distance information, performing a Bayesian MCMC analysis. The interaction parameter is found to be consistent with zero, though small deviations from standard radiation scaling are allowed. These deviations can partially alleviate the Hubble tension by modifying the sound horizon, but this is accompanied by correlated shifts in the matter density. The reconstructed expansion history remains close to LCDM at late times. Model comparison suggest that the interacting scenario is statistically competitive but not decisively preferred by current background data.