# Testing the weak equivalence principle by differential measurements of fundamental constants in the Magellanic Clouds

## TL;DR

This study uses differential measurements of fundamental constants in the Magellanic Clouds to test for violations of the Weak Equivalence Principle caused by non-standard fields linked to dark matter and dark energy.

## Contribution

It introduces a novel method of probing fundamental constant variations in dwarf galaxies dominated by dark matter using Herschel observations.

## Key findings

- No significant variation detected in the average measurements.
- A higher resolution observation shows a potential offset, but its cause remains uncertain.
- Constraints on fundamental constant variations are established at the 10^-7 level.

## Abstract

Non-standard fields are assumed to be responsible for phenomena attributed to dark energy and dark matter. Being coupled to ordinary matter, these fields modify the masses and/or charges of the elementary particles, thereby violating the Weak Equivalence Principle. Thus, values of fundamental constants such as the proton-to-electron mass ratio, mu, and/or the fine structure constant, alpha, measured in different environment conditions can be used as probes for this coupling. Here we perform differential measurements of F = mu*alpha^2 to test a non-standard coupling in the Magellanic Clouds - dwarf galaxies where the overall mass budget is dominated by dark matter. The analysis is based on [CI] and CO lines observed with the Herschel Space Observatory. Since these lines have different sensitivities to changes in mu and alpha, the combined alpha and mu variations can be evaluated through the radial velocity offsets, Delta V, between the CO and [CI] lines. Averaging over nine positions in the Magellanic Clouds, we obtain <Delta V> = -0.02+/-0.07 km/s, leading to |Delta F/F| < 2*10^-7 (1sigma), where Delta F/F = (F_obs-F_lab)/F_lab}. However, for one position observed with five times higher spectral resolution we find Delta V = -0.05+/-0.02 km/s, resulting in Delta F/F = (-1.7+/-0.7)*10^-7. Whether this offset is due to changes in the fundamental constants, due to chemical segregation in the emitting gas or merely due to Doppler noise requires further investigations.