HOT Big Bang cosmology predicts that the temperature of the cosmic microwave background radiation will increase linearly with increasing redshift to early in the history of the Universe. The local background temperature (2.7 K) is known very accurately from direct measurements1–3, but other techniques must be used to estimate it at non-zero redshifts. One way is to determine the excitation of atomic transitions in absorbing clouds along the lines-of-sight to distant quasars4. When the transitions are in equilibrium with the microwave background radiation, the radiation will populate the fine-structure levels of the ground states of certain atoms, and the relative populations of the levels can be used to calculate its temperature. Here we report the detection 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 + 170. The population ratio yields a temperature of 7.4 ± 0.8 K, assuming that no other significant sources of excitation are present. This agrees with the theoretical prediction of 7.58 K.
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Songaila, A., Cowie, L., Vogt, S. et al. Measurement of the microwave background temperature at a redshift of 1.776. Nature 371, 43–45 (1994). https://doi.org/10.1038/371043a0
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