Cosmic Transients, Einstein's Equivalence Principle and Dark Matter Halos

TL;DR

This paper uses cosmic transients and dark matter halo models to set the most stringent limits on Einstein's Equivalence Principle violations, considering dark matter and alternative gravity scenarios.

Contribution

It introduces a novel approach to constrain EEP violations by incorporating dark matter halos and non-minimal gravitational couplings in the analysis.

Findings

Upper limit on PPN gamma difference: Δγ < 1.06 × 10⁻²⁸

Most stringent EEP violation limit to date

Similar bounds obtained with visible matter and non-minimal coupling

Abstract

Cosmic transients, such as gamma-ray bursts and fast radio bursts, have been used to constrain the Einstein's Equivalence Principle (EEP) through the parametrized-post-Newtonian (PPN) formalism. In this approach, the time delay of photons with different energies from these cosmic transients are used to obtain upper bounds on the difference of the PPN $\gamma$ parameter. In this work we assume that an important contribution to the time delay is due to the dark matter halo of the Milky Way and consider the dark matter mass distribution given by the Navarro--Frenk--White profile. We obtain the upper limit on the difference of the PPN parameter $\gamma$ for the polarized gamma-ray emission of GRB 110721A, $\Delta \gamma < 1.06 \times 10^{-28}$, the most stringent limit to date on the EEP. In addition, we show that a very similar upper bound is obtained if, instead of having a dark matter component, a visible matter density profile and a non-minimal gravitational coupling between curvature and matter are present.