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 T_Coma = 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) - T₀(1 + z), where T₀ = 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 T(z) = T₀(1 + z)^1-a or T(z) = T₀[1 + (1 + d)z], then a = -0.16_-0.32^+0.34 and d = 0.17 ± 0.36 (at 95% confidence). We briefly discuss future prospects for more precise SZ measurements of T(z) at higher redshifts.