Probing proton–electron mass ratio variations using high-precision $$H_2$$ Lyman lines

Abstract We probe potential variations of the proton-to-electron mass ratio ( $$\mu $$ μ ) using high-resolution H $$_2$$ 2 Lyman transitions in the white dwarf GD 133, observed with the Hubble Space Telescope Cosmic Origins Spectrograph (HST/COS). White dwarf atmospheres provide a powerful laboratory for testing fundamental physics in strong gravitational fields. Applying a self-consistent spectral analysis, we obtain a tight constraint of $$\Delta \mu /\mu = (0.02 \pm 0.09)$$ Δ μ / μ = ( 0.02 ± 0.09 ) in a gravitational potential approximately $$10^{4}$$ 10 4 times stronger than that of Earth. The measured uncertainty implies that any variation of $$\mu $$ μ is tightly bounded in this strong-field fields, placing meaningful limits on models predicting couplings between fundamental constants and gravity.