Abstract

The adiabatic evolution of the temperature of the cosmic microwave background (CMB) is a key prediction of standard cosmology. We study deviations from the expected adiabatic evolution of the CMB temperature of the form T(z) = T-0(1 + z)(1 - alpha) using measurements of the spectrum of the Sunyaev-Zel'dovich effect with the South Pole Telescope (SPT). We present a method for using the ratio of the Sunyaev-Zel'dovich signal measured at 95 and 150 GHz in the SPT data to constrain the temperature of the CMB. We demonstrate that this approach provides unbiased results using mock observations of clusters from a new set of hydrodynamical simulations. We apply this method to a sample of 158 SPT-selected clusters, spanning the redshift range 0.05 < z < 1.35, and measure alpha = 0.017(-0.028)(+0.030), consistent with the standard model prediction of alpha = 0. In combination with other published results, we find alpha = 0.005 +/- 0.012, an improvement of similar to 10 per cent over published constraints. This measurement also provides a strong constraint on the effective equation of state in models of decaying dark energy w(eff) =-0.994 +/- 0.010.