Abstract We probe potential variations of the proton-to-electron mass ratio (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 / = (0. 02 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 μ is tightly bounded in this strong-field fields, placing meaningful limits on models predicting couplings between fundamental constants and gravity.
T. D. Le (2026) studied this question.