Searching for variations in the fine-structure constant and the proton-to-electron mass ratio using quasar absorption lines

Abstract

Quasar absorption lines provide a precise test of the assumed constancy of the fundamental constants of physics over cosmological times and distances. We have used quasar absorption lines to investigate potential changes in the fine-structure constant, alpha=e^2/(4*pi*epsilon_0*hbar*c),and the proton-to-electron mass ratio, mu = m_p/m_e.

The many-multiplet method allows one to use optical fine-structure transitions to constrain (delta alpha)/alpha at better than the 10^(−5) level. We present a new analysis of 154 quasar absorbers with 0.2 < z < 3.7 in VLT/UVES spectra. From these absorbers we find 2.2sigma evidence for angular variations in alpha under a dipole+monopole model. Combined with previous Keck/HIRES observations, we find 4.1sigma evidence for angular (and therefore spatial) variations in alpha, with maximal increase of alpha occurring in the direction RA = (17.3 ± 1.0) hr, dec. = (−61 ± 10)deg. Under a model where the observed effect is proportional to the lookback-time distance the significance increases to 4.2sigma. Importantly, dipole models fitted to the VLT and Keck samples independently yield consistent estimates of the dipole direction, which suggests that the effect is not caused by telescope systematics. Similarly, dipole models fitted to z < 1.6 and z > 1.6 sub-samples also point in a consistent direction. The observed dipole effect is stable under iterative trimming of potentially outlying (delta alpha)/alpha values, implying that the result is not being generated by a subset of the data. We consider a number of systematic effects, including potential wavelength scale distortions and evolution in the abundance of Mg isotopes, and show that they are unable to explain the observed dipole effect. If these results are correct, they directly demonstrate the incompleteness of the Standard Model and violation of the Einstein Equivalence Principle.

Optical spectra of z > 2 molecular hydrogen absorbers can probe evolution in mu. We have used spectra of the quasars Q0405−443, Q0347−383 and Q0528−250 from VLT/UVES to investigate the absorbers at z = 2.595, 3.025 and 2.811 in these spectra respectively. We find that (delta mu)/mu = (10.1 ± 6.6) × 10^(−6), (8.2 ± 7.5) × 10^(−6) and (−1.4 ± 3.9) × 10^(−6) in these absorbers respectively. A second spectrum of Q0528−250 provides an additional constraint of (delta mu)/mu = (0.2 ± 3.2_stat ± 1.9_sys) × 10^(−6). The weighted mean of these values yields (delta mu)/mu = (1.7 ± 2.4) × 10^(−6), the most precise constraint on evolution in mu at z > 1.

We also demonstrate the application of Markov Chain Monte Carlo methods to determining (delta alpha)/alpha from quasar spectra.