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The $\textrm{A}^1\Pi-\textrm{X}^1\Sigma^+$ band system of carbon monoxide,which has been detected in six highly redshifted galaxies ($z=1.6-2.7$), isidentified as a novel probe method to search for possible variations of theproton-electron mass ratio ($\mu$) on cosmological time scales. Laboratorywavelengths of the spectral lines of the A-X ($v$,0) bands for $v=0-9$ havebeen determined at an accuracy of $\Delta\lambda/\lambda=1.5 \times 10^{-7}$through VUV Fourier-transform absorption spectroscopy, providing acomprehensive and accurate zero-redshift data set. For the (0,0) and (1,0)bands, two-photon Doppler-free laser spectroscopy has been applied at the $3\times 10^{-8}$ accuracy level, verifying the absorption data. Sensitivitycoefficients $K_{\mu}$ for a varying $\mu$ have been calculated for the CO A-Xbands, so that an operational method results to search for $\mu$-variation.

The CO A-X System for Constraining Cosmological Drift of the Proton-Electron Mass Ratio