MEASUREMENT OF THE MICROWAVE BACKGROUND TEMPERATURE AT A REDSHIFT OF 1.776

HOT Big Bang cosmology predicts that the temperature of the cosmic mic rowave background radiation will increase linearly with increasing red shift to early in the history of the Universe. The local background te mperature (2.7 K) is known very accurately from direct measurements(1- 3), but other techniques must be used to estimate it at non-zero redsh ifts. One way is to determine the excitation of atomic transitions in absorbing clouds along the lines-of-sight to distant quasars(4). When the transitions are in equilibrium with the microwave background radia tion, the radiation will populate the fine-structure levels of the gro und states of certain atoms, and the relative populations of the level s can be used to calculate its temperature. Here we report the detecti on of absorption from the first fine-structure level of neutral carbon atoms in a cloud at a redshift of 1.776, towards the quasar Q1331 + 1 70. The population ratio yields a temperature of 7.4 +/- 0.8 K, assumi ng that no other significant sources of excitation are present. This a grees with the theoretical prediction of 7.58 K.