Cosmic microwave background temperature at Galaxy clusters

We have deduced the cosmic microwave background temperature in the Coma Cluster (A1656, z = 0.0231) and in A2163 (z = 0.203) from spectral measurements of the Sunyaev-Zeldovich (SZ) effect over four passbands at radio and microwave frequencies. The resulting temperatures at these redshifts are TComa = 2.789(-0.065)(+0.080) K and T-A2163 = 3.377(-0.102)(+0.101) K, respectively. These values confirm the expected relation T(z) = T-0(1 + z), where T-0 = 2.725 +/- 0.002 K is the value measured by the COBE Far Infrared Absolute Spectrometer experiment. Alternative scaling relations that are conjectured in nonstandard cosmologies can be constrained by the data; for example, if Y(z) = T-0(1 + z)(1-alpha) or T(z) = T-0[1 + (1 +d)z], then a = -016(-0.32)(+0.34) and d = 0.17 +/- 0.36 (at 95% confidence). We briefly discuss future prospects for more precise SZ measurements of at higher redshifts.