The evolution of the cosmic microwave background temperature. Measurements of T_CMB at high redshift from carbon monoxide excitation

Abstract

A milestone of modern cosmology was the prediction and serendipitous discovery of the cosmic microwave background (CMB), the radiation leftover after decoupling from matter in the early evolutionary stages of the Universe. A prediction of the standard hot Big-Bang model is the linear increase with redshift of the black-body temperature of the CMB (T_CMB). This radiation excites the rotational levels of some interstellar molecules, including carbon monoxide (CO), which can serve as cosmic thermometers. Using three new and two previously reported CO absorption-line systems detected in quasar spectra during a systematic survey carried out using VLT/UVES, we constrain the evolution of T_CMB to z ∼ 3. Combining our precise measurements with previous constraints, we obtain T_CMB(z) = (2.725 ± 0.002) × (1 + z)^1-β K with β = -0.007 ± 0.027, a more than two-fold improvement in precision. The measurements are consistent with the standard (i.e. adiabatic, β = 0) Big-Bang model and provide a strong constraint on the effective equation of state of decaying dark energy (i.e. w_eff = -0.996 ± 0.025).