Sub-percent constraints on cosmological temperature evolution

The redshift dependence of the cosmic microwave background temperature is one of the key cosmological observables. In the standard cosmological model one has $T(z)=T_0(1+z)$, where $T_0$ is the present-day temperature. Deviations from this behavior would imply the presence of new physics. Here we discuss how the combination of all currently available direct and indirect measurements of $T(z)$ constrains the common phenomenological parametrization $T(z)=T_0(1+z)^{1-β}$, and obtain the first sub-percent constraint on the $β$ parameter, specifically $β=(7.6\pm8.0)\times10^{-3}$ at the $68.3\%$ confidence level.

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