A milestone of modern cosmology was the prediction and serendipitous discovery of the Cosmic Microwave Background ( CMB ) , the radiation left over 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 \sim 3 .
Combining our precise measurements with previous constraints , we obtain T _ { CMB } ( z ) = ( 2.725 \pm 0.002 ) \times ( 1 + z ) ^ { 1 - \beta } K with \beta = -0.007 \pm 0.027 , a more than two-fold improvement in precision .
The measurements are consistent with the standard ( i.e .
adiabatic , \beta = 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 \pm 0.025 ) .