Constraint on a cosmological variation in the proton-to-electron mass ratio from electronic CO absorption

TL;DR

This study investigates potential variations in the proton-to-electron mass ratio over cosmic time by analyzing CO absorption lines at high redshift, providing constraints consistent with no variation over 11.4 billion years.

Contribution

First detection of CO absorption at high redshift used to constrain proton-electron mass ratio variation with updated molecular data and comprehensive fitting methods.

Findings

No significant variation in proton-to-electron mass ratio detected.

Constraints on variation are at the level of a few parts in 10^6.

Results are consistent with a constant fundamental constant over 11.4 billion years.

Abstract

Carbon monoxide (CO) absorption in the sub-damped Lyman-$\alpha$ absorber at redshift $z_{abs} \simeq 2.69$, toward the background quasar SDSS J123714.60+064759.5 (J1237+0647), was investigated for the first time in order to search for a possible variation of the proton-to-electron mass ratio, $\mu$, over a cosmological time-scale. The observations were performed with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 40 per 2.5 kms$^{-1}$ per pixel at $\sim 5000$ \AA. Thirteen CO vibrational bands in this absorber are detected: the A$^{1}\Pi$ - X$^{1}\Sigma^{+}$ ($\nu'$,0) for $\nu' = 0 - 8$, B$^{1}\Sigma^{+}$ - X$^{1}\Sigma^{+}$ (0,0), C$^{1}\Sigma^{+}$ - X$^{1}\Sigma^{+}$ (0,0), and E$^{1}\Pi$ - X$^{1}\Sigma^{+}$ (0,0) singlet-singlet bands and the d$^{3}\Delta$ - X$^{1}\Sigma^{+}$ (5,0) singlet-triplet band. An updated database including the most precise molecular inputs needed for a $\mu$-variation analysis is presented for rotational levels $J = 0 - 5$, consisting of transition wavelengths, oscillator strengths, natural lifetime damping parameters, and sensitivity coefficients to a variation of the proton-to-electron mass ratio. A comprehensive fitting method was used to fit all the CO bands at once and an independent constraint of $\Delta\mu/\mu = (0.7 \pm 1.6_{stat} \pm 0.5_{syst}) \times 10^{-5}$ was derived from CO only. A combined analysis using both molecular hydrogen and CO in the same J1237+0647 absorber returned a final constraint on the relative variation of $\Delta\mu/\mu = (-5.6 \pm 5.6_{stat} \pm 3.1_{syst}) \times 10^{-6}$, which is consistent with no variation over a look-back time of $\sim 11.4$ Gyrs.