# Improved constraints on violations of the Einstein equivalence principle in the electromagnetic sector with complementary cosmic probes

## TL;DR

This paper uses multiple cosmological observations to tighten constraints on potential violations of the Einstein equivalence principle caused by electromagnetic sector couplings, finding no violations up to redshift 3.

## Contribution

It provides improved, multi-probe constraints on Einstein equivalence principle violations in the electromagnetic sector using diverse cosmological data sets.

## Key findings

- Errors reduced by 20-40% depending on parametrization
- Constraints are weakly affected by galaxy cluster geometry
- No violation detected up to redshift ~3

## Abstract

Recent results have shown that a field non-minimally coupled to the electromagnetic Lagrangian can induce a violation of the Einstein equivalence principle. { This kind of coupling is present in a very wide class of gravitation theories.} In a cosmological context, this would break the validity of the cosmic distance duality relation as well as cause a time variation of the fine structure constant. Here, we improve constraints on this scenario by using four different observables: the luminosity distance of type Ia supernovae, the angular diameter distance of galaxy clusters, the gas mass fraction of galaxy clusters and the temperature of the cosmic microwave background at different redshifts. We consider four standard parametrizations adopted in the literature and show that, due to a high complementarity of the data, the errors are shrunk between 20\% and 40\% depending on the parametrization. We also show that our constraints are weakly affected by the geometry considered to describe the galaxy clusters. In short, no violation of the Einstein equivalence principle is detected up to redshifts $\sim$ 3.