Consistent Constraints on the Equivalence Principle from localised Fast Radio Bursts

TL;DR

This paper uses localized Fast Radio Bursts to set the most stringent constraints yet on violations of the Weak Equivalence Principle at low energies, by analyzing time delay correlations and covariance matrices.

Contribution

It introduces a novel covariance matrix approach to constrain WEP violations using FRB data, improving the robustness and tightness of the bounds.

Findings

WEP violation parameter γ constrained to 1 in 10^13 at 68% confidence

Covariance matrix method accounts for electron, host galaxy, and Milky Way effects

Tightest constraints to date on WEP violation at low energies

Abstract

Fast Radio Bursts (FRBs) are short astrophysical transients of extragalactic origin. Their burst signal is dispersed by the free electrons in the large-scale-structure (LSS), leading to delayed arrival times at different frequencies. Another potential source of time delay is the well known Shapiro delay, which measures the space-space and time-time metric perturbations along the line-of-sight. If photons of different frequencies follow different trajectories, i.e. if the universality of free fall guaranteed by the weak equivalence principle (WEP) is violated, they would experience an additional relative delay. This quantity, however, is not an observable on the background level as it is not gauge independent, which has led to confusion in previous papers. Instead, an imprint can be seen in the correlation between the time delays of different pulses. In this paper, we derive robust and consistent constraints from twelve localised FRBs on the violation of the WEP in the energy range between 4.6 and 6 meV. In contrast to a number of previous studies, we consider our signal to be not in the model, but in the covariance matrix of the likelihood. To do so, we calculate the covariance of the time delays induced by the free electrons in the LSS, the WEP breaking terms, the Milky Way and host galaxy. By marginalising over both host galaxy contribution and the contribution from the free electrons, we find that the parametrised post-Newtonian parameter $\gamma$ characterising the WEP violation must be constant in this energy range to 1 in $10^{13}$ at 68$\;\%$ confidence. These are the tightest constraints to-date on $\Delta\gamma$ in this low energy range.