The cosmic microwave background radiation temperature at a redshift of 2.34

The existence of the cosmic microwave background radiation is a fundamental prediction of hot Big Bang cosmology, and its temperature should increase with increasing redshift. At the present time (redshift z = 0), the tempera ture has been determined with high precision to be T-CMBR(0) = 2.726 +/- 0. 010 K. In principle, the background temperature can be determined using mea surements of the relative populations of atomic fine-structure levels, whic h are excited by the background radiation. But all previous measurements ha ve achieved only upper limits, thus still formally permitting the radiation temperature to be constant with increasing redshift. Here we report the de tection of absorption lines from the first and second fine-structure levels of neutral carbon atoms in an isolated cloud of gas at z = 2.3371. We also detected absorption due to several rotational transitions of molecular hyd rogen, and fine-structure lines of singly ionized carbon. These constraints enable us to determine that the background radiation was indeed warmer in the past: we rnd that T-CMBR(z = 2.3371) is between 6.0 and 14 K. This is i n accord with the temperature of 9.1 K predicted by hot Big Bang cosmology.