2002ApJ...580L.101B

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^+0.080_–0.065 K and T_A2163=3.377^+0.101_–0.102K, 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^+0.34_–0.32and d=0.17±0.36 (at 95% confidence). We briefly discuss future prospects for more precise SZ measurements of T(z) at higher redshifts.