Constraints on the CMB Temperature Evolution using Multi-Band
  Measurements of the Sunyaev Zel'dovich Effect with the South Pole Telescope
  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 $\alpha = 0.017^{+0.030}_{-0.028}$, consistent with the standard model
prediction of $\alpha=0$. In combination with other published results, we
constrain $\alpha = 0.011 \pm 0.016$, an improvement of $\sim 20\%$ over
published constraints. This measurement also provides a strong constraint on
the effective equation of state in models of decaying dark energy
$w_\mathrm{eff} = -0.987^{+0.016}_{-0.017}$.
