Subpercent constraints on the 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 sub(0)(1+z), where T sub(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 sub(0)(1+z) super(1- beta ) and obtain the first subpercent constraint on the temperature growth index 1? beta . Specifically, we find beta =(7.6+ or -8.0)x10 super(-3) at the 68.3% confidence level.