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

Understand

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.