An observational determination of the proton to electron mass ratio in the early universe

Rodger I. Thompson, Jill Bechtold, John H. Black, Daniel Eisenstein, Xiaohui Fan, Robert C. Kennicutt, Carlos Martins, J. Xavier Prochaska, Yancey L. Shirley

Research output: Contribution to journalArticlepeer-review

49 Scopus citations


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 (μz - μ0) where μz 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 etal., Δμ/μ = (2.6 3.0) × 10-6, and inconsistent with the positive detection of a change in μ by Reinhold etal. 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.

Original languageEnglish (US)
Pages (from-to)1648-1662
Number of pages15
JournalAstrophysical Journal
Issue number2
StatePublished - 2009


  • Early universe

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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