An Observational Determination of the Proton to Electron Mass Ratio in the Early Universe

R. I. Thompson, J. Bechtold, J. H. Black, D. J. Eisenstein, X. Fan, R. C Kennicutt, C. J. A. P. Martins, J. X. Prochaska, Y. L. Shirley

Abstract
In an effort to resolve the discrepancy between two measurements of the fundamental constant μ, the proton to electron mass ratio, at early times in the universe we reanalyze the same data used in the earlier studies. Our analysis of the molecular hydrogen absorption lines in archival Very Large Telescope/Ultraviolet and Visible Echelle Spectrometer (UVES) spectra of the damped Lyman alpha systems in the quasi-stellar objects Q0347-383 and Q0405-443 yields a combined measurement of a Δμ/μ value of (-7 ± 8) × 10^-6, consistent with no change in the value of μ over a time span of 11.5 Gyr. Here, we define Δμ as (μ_z - μ₀) where μ_z is the value of μ at a redshift of z and μ₀ is the present-day value. Our null result is consistent with the recent measurements of King et al., Δμ/μ = (2.6 ± 3.0) × 10^-6, and inconsistent with the positive detection of a change in μ by Reinhold et al. Both of the previous studies and this study are based on the same data but with differing analysis methods. Improvements in the wavelength calibration over the UVES pipeline calibration is a key element in both of the null results. This leads to the conclusion that the fundamental constant μ is unchanged to an accuracy of 10^-5 over the last 80% of the age of the universe, well into the matter dominated epoch. This limit provides constraints on models of dark energy that invoke rolling scalar fields and also limits the parameter space of supersymmetric or string theory models of physics. New instruments, both planned and under construction, will provide opportunities to greatly improve the accuracy of these measurements.

Keywords
early universe

Notes
Based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under program IDs 68.A-0106 and 70.A-0017.

The Astrophysical Journal
Volume 703, Page 1648
September 2009

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