Cosmic Microwave Background Temperature at Galaxy Clusters

We have deduced the cosmic microwave background (CMB) temperature in the Coma cluster (A1656, $z=0.0231$), and in A2163 ($z=0.203$) from spectral measurements of the Sunyaev-Zel’dovich (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 $T_{A2163} = 3.377^{+0.101}_{-0.102}$ K, respectively. These values confirm the expected relation $T(z)=T_0(1+z)$, where $T_0=2.725\pm 0.002$ K is the value measured by the COBE/FIRAS experiment. Alternative scaling relations that are conjectured in non-standard cosmologies can be constrained by the data; for example, if $T(z)=T_0(1+z{)}^{1-a}$ or $T(z)=T_0[1+(1+d)z]$, then $a=-0.16_{-0.32}^{+0.34}$ and $d=0.17\pm 0.36$ (at 95% confidence). We briefly discuss future prospects for more precise SZ measurements of $T(z)$ at higher redshifts.