Cosmological Effects of Scalar-Photon Couplings: Dark Energy and Varying-alpha Models

TL;DR

This paper investigates how scalar-photon couplings influence cosmological observations, affecting constraints on dark energy and fundamental constants, and emphasizes the importance of combining temperature measurements with distance data for improved parameter constraints.

Contribution

It introduces a framework to analyze scalar-photon couplings' impact on cosmological parameters and highlights the necessity of joint observational strategies to break degeneracies.

Findings

Scalar-photon couplings weaken current cosmological constraints.

Temperature measurements at different redshifts help constrain photon number non-conservation.

Euclid can provide useful dark energy constraints even with photon coupling considerations.

Abstract

We study cosmological models involving scalar fields coupled to radiation and discuss their effect on the redshift evolution of the cosmic microwave background temperature, focusing on links with varying fundamental constants and dynamical dark energy. We quantify how allowing for the coupling of scalar fields to photons, and its important effect on luminosity distances, weakens current and future constraints on cosmological parameters. In particular, for evolving dark energy models, joint constraints on the dark energy equation of state combining BAO radial distance and SN luminosity distance determinations, will be strongly dominated by BAO. Thus, to fully exploit future SN data one must also independently constrain photon number non-conservation arising from the possible coupling of SN photons to the dark energy scalar field. We discuss how observational determinations of the background temperature at different redshifts can, in combination with distance measures data, set tight constraints on interactions between scalar fields and photons, thus breaking this degeneracy. We also discuss prospects for future improvements, particularly in the context of Euclid and the E-ELT and show that Euclid can, even on its own, provide useful dark energy constraints while allowing for photon number non-conservation.