Microwave Background Temperature at a Redshift of 6.34 from H2O Absorption

Understand

Distortions of the observed cosmic microwave background imprinted by the Sunyaev-Zel'dovich effect toward massive galaxy clusters due to inverse Compton scattering of microwave photons by high-energy electrons provide a direct measurement of the microwave background temperature at redshifts from 0 to 1.

• Some additional background temperature estimates exist at redshifts from 1.8 to 3.3 based on molecular and atomic line excitation temperatures in quasar absorption line systems, but are model dependent.
• To date, no deviations from the expected (1+z) scaling behavior of the microwave background temperature have been seen, but the measurements have not extended deeply into the matter-dominated era of the universe at redshifts z>3.3.
• Here we report the detection of sub-millimeter line absorption from the water molecule against the cosmic microwave background at z=6.34 in a massive starburst galaxy, corresponding to a lookback time of 12.8 Gyr.