Microwave Background Temperature at a Redshift of 6.34 from Water Absorption
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DrPierre Cox(IAP)
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Europe/Paris
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Description
Distortions of the observed cosmic microwave background (CMB) through the SZ effect provide direct measurements of its temperature at redshifts from 0 to 1. Additional background temperature estimates exist at higher redshifts, from 1.8 to 3.3, based on molecular and atomic line-excitation temperatures in quasar absorption-line systems. No deviations from the expected (1+z) scaling behaviour of the CMB temperature have been seen to date, but the measurements have not extended deeply into the matter-dominated area of the Universe at z>3.3.
We will describe recent observations, obtained with NOEMA, of sub-millimetre line absorption from the water molecule against the cosmic microwave background in a massive starburst galaxy at z=6.34 (Riechers et al. 2022). These new measurements, providing the most direct constraint on Tcmb currently available at z>1, yield a temperature for the cosmic background of 16.4-30.2 K (1 sigma range) at z=6.34, that is consistent with the standard Lambda-CDM cosmology prediction. Future observations and ways to improve on this result will be briefly mentioned.