Dark Matter Constraints from a Cosmic Index of Refraction

TL;DR

This paper proposes a novel method to detect dark matter by analyzing its effect on the cosmic index of refraction, using gamma-ray burst observations to set new constraints on dark matter properties.

Contribution

It introduces a new approach to constrain dark matter electromagnetic interactions through frequency-dependent light propagation effects.

Findings

Set a new limit on the electric-charge-to-mass ratio of dark matter

Demonstrated the feasibility of using gamma-ray burst time delays for dark matter detection

Provided theoretical analysis of the index of refraction expansion related to dark matter

Abstract

The dark-matter candidates of particle physics invariably possess electromagnetic interactions, if only via quantum fluctuations. Taken en masse, dark matter can thus engender an index of refraction which deviates from its vacuum value. Its presence is signaled through frequency-dependent effects in the propagation and attenuation of light. We discuss theoretical constraints on the expansion of the index of refraction with frequency, the physical interpretation of the terms, and the particular observations needed to isolate its coefficients. This, with the advent of new opportunities to view gamma-ray bursts at cosmological distance scales, gives us a new probe of dark matter and a new possibility for its direct detection. As a first application we use the time delay determined from radio afterglow observations of distant gamma-ray bursts to realize a direct limit on the electric-charge-to-mass ratio of dark matter of |varepsilon|/M < 1 x 10^{-5} eV^{-1} at 95% CL.