Probing Fundamental Constant Evolution With Neutral Atomic Gas Lines

We have detected narrow H i 21 cm and C i absorption at z ∼ 1.4–1.6 toward Q0458−020 and Q2337−011, and use these lines to test for possible changes in the fine structure constant α, the proton–electron mass ratio μ, and the proton gyromagnetic ratio g p . A comparison between the H i 21 cm and C i line redshifts yields Δ X / X = (+6.8 ± 1.0) × 10 −6 over 0 < 〈 z 〉 < 1.46, where X = g p α 2 /μ, and the errors are purely statistical, from the Gaussian fits. The simple line profiles and the high sensitivity of the spectra imply that statistical errors in this comparison are an order of magnitude lower than in previous studies. Further, the C i lines arise in cold neutral gas that also gives rise to H i 21 cm absorption, and both background quasars are core-dominated, reducing the likelihood of systematic errors due to local velocity offsets between the hyperfine and resonance lines. The dominant source of systematic error lies in the absolute wavelength calibration of the optical spectra, which appears uncertain to ∼2 km s −1 , yielding a maximum error in Δ X / X of ∼6.7 × 10 −6 . Including this, we obtain Δ X / X = (+6.8 ± 1.0(statistical) ± 6.7(max. systematic)) × 10 −6 over 0 < 〈 z 〉 < 1.46. Using literature constraints on Δμ/μ, this is inconsistent with claims of a smaller value of α from the many-multiplet method, unless fractional changes in g p are larger than those in α and μ.