A precise and accurate determination of the cosmic microwave background
  temperature at z=0.89
  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. We determine Tcmb=5.08 pm 0.10 K at 68% confidence level. Our
measurement is consistent with the value Tcmb=5.14 K predicted by the standard
cosmological model with adiabatic expansion of the Universe. This is the most
precise determination of Tcmb at z>0 to date.
