CO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>A</mml:mi><mml:mspace width="0.16em" /><mml:mo>−</mml:mo><mml:mspace width="0.16em" /><mml:mi>X</mml:mi></mml:mrow></mml:math>system for constraining cosmological drift of the proton-electron mass ratio

The {A}^{1}\ensuremath{\Pi}\ensuremath{-}{X}^{1}{\ensuremath{\Sigma}}^{+}band system of carbon monoxide, which has been detected in six highly redshifted galaxies (z=1.6-2.7), is identified as a probe method to search for possible variations of the proton-electron mass ratio (\ensuremath{\mu} on cosmological time scales. Laboratory wavelengths of the spectral lines of the A\ensuremath{-}X(v0) bands for v=0-9 have been...