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

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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, μ, and/or the fine structure constant, α, measured in different environment conditions can be used as probes for this coupling. Here we perform differential measurements of F = μα2 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 [C i] and CO lines observed with the Herschel Space Observatory. Since these lines have different sensitivities to changes in μ and α, the combined α and μ variations can be evaluated through the radial velocity offsets, ΔV, between the CO and [C i] lines. Averaging over nine positions in the Magellanic Clouds, we obtain 〈ΔV〉 = −0.02 ± 0.07 km s−1, leading to |ΔF/F| < 2 × 10−7 (1σ), where ΔF/F = (Fobs − Flab)/Flab. However, for one position observed with five times higher spectral resolution we find ΔV = −0.05 ± 0.02 km s−1, resulting in ΔF/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.