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

Dorian Araya, Felipe Herrera, Nelson Videla

TL;DR

The paper investigates a cosmological model where an early-universe scalar field exchanges energy with radiation through an exponential coupling, extended to include non-relativistic matter for a complete background evolution. By deriving analytical background solutions and statefinder diagnostics, the authors constrain the model with observational Hubble data, SNIa Pantheon+, BAO (DESI DR2), and compressed CMB distances using Bayesian MCMC under two priors on $\Omega_{m0}$. They find the interaction parameter $\epsilon$ is consistent with zero, though small deviations can modify the sound horizon and potentially ease the $H_0$ tension at the expense of correlated shifts in $\Omega_{m0}$; the late-time expansion remains close to $\Lambda$CDM$, and information criteria do not decisively favor the interacting scenario. The study demonstrates that early-time modifications from scalar–radiation coupling can address part of the Hubble discrepancy but highlight degeneracies with matter density, underscoring the need for perturbation and large-scale structure analyses for a conclusive assessment.

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.

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

TL;DR

The paper investigates a cosmological model where an early-universe scalar field exchanges energy with radiation through an exponential coupling, extended to include non-relativistic matter for a complete background evolution. By deriving analytical background solutions and statefinder diagnostics, the authors constrain the model with observational Hubble data, SNIa Pantheon+, BAO (DESI DR2), and compressed CMB distances using Bayesian MCMC under two priors on . They find the interaction parameter is consistent with zero, though small deviations can modify the sound horizon and potentially ease the tension at the expense of correlated shifts in ; the late-time expansion remains close to CDM$, and information criteria do not decisively favor the interacting scenario. The study demonstrates that early-time modifications from scalar–radiation coupling can address part of the Hubble discrepancy but highlight degeneracies with matter density, underscoring the need for perturbation and large-scale structure analyses for a conclusive assessment.

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.
Paper Structure (11 sections, 51 equations, 8 figures, 3 tables)

This paper contains 11 sections, 51 equations, 8 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Scalar–Radiation Interaction: Theoretical Setup
  3. Cosmological data
  4. Observational Hubble Data
  5. Type Ia Supernovae
  6. Baryonic Acoustic Oscillations
  7. Cosmic Microwave Background
  8. Comparison of models
  9. Results
  10. Bayesian analysis results
  11. Discussion and Conclusion

Figures (8)

  • Figure 1: 1D posterior distributions and 2D confidence contours for the free parameters characterizing the case CI of the interacting scalar–radiation model, using OHD, CMB, SNIa, BAO, and their Joint combination.
  • Figure 2: 1D posterior distributions and 2D confidence contours for the free parameters characterizing the case CII of the interacting scalar–radiation model, using OHD, CMB, SNIa, BAO, and their Joint combination.
  • Figure 3: Reconstruction of the Hubble rate $H(z)$ for the interacting scalar–radiation model case CI with the respective data points from OHD data. Left panel: reconstruction from the joint analysis compared to the best-fit $\Lambda$CDM model, where shaded regions indicate the $1\sigma$ (darker) and $3\sigma$ (lighter) confidence bands and their uncertainties. Right panel: Best-fit constraints from OHD, CMB, SNIa, BAO, and their joint combination.
  • Figure 4: Reconstruction of the Hubble parameter $H(z)$ for the interacting scalar-radiation model case CII with the respective data points from OHD data. Left panel: Best-fit curve from the Joint analysis compared to the best-fit from the $\Lambda$CDM model, where shaded regions indicate the $1\sigma$ (darker) and $3\sigma$ (lighter) confidence bands and their uncertainties. Right panel: Best-fit curves constraints from OHD, CMB, SNIa, BAO, and their joint combination.
  • Figure 5: Reconstruction of the deceleration parameter $q(z)$ for the interacting scalar-radiation model case CI Left panel: Best-fit curve from the Joint analysis compared to the best-fit from the $\Lambda$CDM model, where shaded regions indicate the $1\sigma$ (darker) and $3\sigma$ (lighter) confidence bands. Right panel: Best-fit constraints from OHD, CMB, SNIa, BAO and their Joint combination.
  • ...and 3 more figures