Improved constraints on possible variation of physical constants from H i 21 cm and molecular QSO absorption lines

QSO absorption spectra provide an extremely useful probe of possible cosmological variation in various physical constants. Comparison of H i 21 cm absorption with corresponding molecular (rotational) absorption spectra allows us to constrain variation in y ≡ α2gp where α is the fine structure constant and gp is the proton g-factor. We analyse spectra of two QSOs, PKS1413+135 and TXS 0218+357, and derive values of ∆y/y at absorption redshifts of z = 0.2467 and 0.6847 by simultaneous fitting of the H i 21 cm and molecular lines. We find ∆y/y = (−0.20 ± 0.44) × 10 and ∆y/y = (−0.16 ± 0.54)× 10 respectively, indicating an insignificantly smaller y in the past. We compare our results with other recent constraints from the same two QSOs (Drinkwater et al. 1998; Carilli et al. 2000) and with our recent optical constraints which indicated a smaller α at higher redshifts.