A model-independent test of speed of light variability with cosmological observations

TL;DR

This study tests whether the speed of light has varied over cosmological timescales using Type Ia Supernova and cosmic chronometer data, finding it consistent with constancy within 5% precision up to redshift 2.

Contribution

It introduces a model-independent cosmological method to measure the speed of light at high redshift, avoiding assumptions about the underlying cosmology.

Findings

Speed of light remains constant within 5% at redshifts around 1.36 and 1.58.

Two different reconstruction methods yield consistent results.

No evidence of speed of light variability detected in the studied redshift range.

Abstract

A powerful test of fundamental physics consists on probing the variability of fundamental constants in Nature. Although they have been measured on Earth laboratories and in our Solar neighbourhood with extremely high precision, it is crucial to carry out these tests at the distant Universe, as any significant variation of these quantities would immediately hint at new physics. We perform a cosmological measurement of the speed of light using the latest Type Ia Supernova and cosmic chronometer observations at the redshift range $0<z<2$. Our method relies on the numerical reconstruction of these data in order to circumvent {\it a priori} assumptions of the underlying cosmology. We confirm the constancy of the speed of light at such redshift range, reporting two $\sim 5$\% precision measurements of $c = (3.20 \pm 0.16) \; \times 10^5 \; \mathrm{km \; s}^{-1}$ in $z \simeq 1.58$, and $c = (2.67 \pm 0.14) \; \times 10^5 \; \mathrm{km \; s}^{-1}$ in $z \simeq 1.36$, depending on the reconstruction method, at a $1\sigma$ confidence level.