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The cosmic microwave background radiation temperature at a redshift of 2.34

Abstract

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 temperature has been determined with high precision to be TCMBR(0) = 2.726 ± 0.010 K. In principle, the background temperature can be determined using measurements of the relative populations of atomic fine-structure levels, which are excited by the background radiation. But all previous measurements have achieved only upper limits, thus still formally permitting the radiation temperature to be constant with increasing redshift. Here we report the detection 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 hydrogen, 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 find that TCMBR(z = 2.3371) is between 6.0 and 14 K. This is in accord with the temperature of 9.1 K predicted by hot Big Bang cosmology.

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Figure 1: A sample of H2 and C0 absorption lines at zabs = 2.33771.
Figure 2: A sample of heavy-element absorption lines at zabs = 2.33771.
Figure 3: The hydrogen density as a function of kinetic temperature in the zabs = 2.33771 cloud.
Figure 4: Cosmic microwave background temperature as a function of kinetic temperature of the gas.
Figure 5: Measurements of the cosmic microwave background radiation temperature at various redshifts.

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Acknowledgements

The observations presented here have been obtained using the Ultra-violet and Visible Echelle Spectrograph mounted on the 8.2-m KUEYEN telescope operated by the European Southern Observatory at Paranal, Chile. P.P. thanks A. Kaufer and M. Chadid for their kind and efficient assistance at the telescope and IUCAA for hospitality during the time this work was being done. We thank T. Padmanabhan for useful comments. We gratefully acknowledge support from the Indo-French Centre for the Promotion of Advanced Research (Centre Franco-Indien pour la Promotion de la Recherche Avancée).

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Srianand, R., Petitjean, P. & Ledoux, C. The cosmic microwave background radiation temperature at a redshift of 2.34. Nature 408, 931–935 (2000). https://doi.org/10.1038/35050020

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