A precise and accurate determination of the cosmic microwave background temperature at z=0.89

Understand

According to the Big Bang theory and as a consequence of adiabatic expansion of the Universe, the temperature of the cosmic microwave background (CMB) increases linearly with redshift.

• This relation is, however, poorly explored, and detection of any deviation would directly lead to (astro-)physics beyond the standard model.
• We aim at measuring the temperature of the CMB with an accuracy of a few percent at z=0.89 toward the molecular absorber in the galaxy lensing the quasar PKS1830-211.
• We adopt a Monte-Carlo Markov Chain approach, coupled with predictions from the non-LTE radiative transfer code RADEX, to solve the excitation of a set of various molecular species directly from their spectra.