Search for a drifting proton--electron mass ratio from H${}_2$

Abstract

An overview is presented of the H${}_2$ quasar absorption method to search for a possible variation of the proton--electron mass ratio $\mu =m_p/m_e$ on a cosmological time scale. Details of the analysis of astronomical spectra, obtained with large 8--10 m class optical telescopes, equipped with high-resolution echelle grating based spectrographs, are explained. The methods and results of the laboratory molecular spectroscopy of H${}_2$, in particular the laser-based metrology studies for the determination of rest wavelengths of the Lyman and Werner band absorption lines, are reviewed. Theoretical physics scenarios delivering a rationale for a varying $\mu$ will be discussed briefly, as well as alternative spectroscopic approaches to probe variation of $\mu$, other than the H${}_2$ method. Also a recent approach to detect a dependence of the proton-to-electron mass ratio on environmental conditions, such as the presence of strong gravitational fields, will be highlighted. Currently some 56 H${}_2$ absorption systems are known and listed. Their usefulness to detect $\mu$-variation is discussed, in terms of column densities and brightness of background quasar sources, along with future observational strategies. The astronomical observations of ten quasar systems analyzed so far set a constraint on a varying proton-electron mass ratio of $|\Delta \mu /\mu |<5\times 10^{-6}$ (3-$\sigma$), which is a null result, holding for redshifts in the range $z=2.0-4.2$. This corresponds to look-back times of 10--12.4 billion years into cosmic history. Attempts to interpret the results from these 10 H${}_2$ absorbers in terms of a spatial variation of $\mu$ are currently hampered by the small sample size and their coincidental distribution in a relatively narrow band across the sky.

26 pages, 13 figures, accepted Review of Modern Physics

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