Probing Variations of Newton's Constant in Strong Gravitational Fields

Abstract

The constancy of Newton's gravitational constant G is a fundamental prediction underlying general relativity and a critical assumption in theories of gravity beyond it. We report a high-precision test of a possible temporal variation of G in a strong gravitational field, using high-resolution ultraviolet spectra of Ni V absorption lines in the white dwarf G191-B2B obtained with the Hubble Space Telescope Imaging Spectrograph (HST/STIS). By comparing the measured line centroids with laboratory wavelengths and isolating the gravitational-redshift contribution, we obtain dG/G = (-0.014 +/- 0.016) x 10^-15 yr^-1 at a strong-field potential phi approximately 10^4, corresponding to an absorption redshift z_abs approximately 8.47 x 10^-5. The result is consistent with a constant G and places a stringent constraint on its secular variation in the strong-field regime. Our measurement demonstrates the potential of white-dwarf stars as a sensitive probe of temporal variations in the gravitational coupling.