  The scattering of temperature anisotropy quadrupole by free electrons in
galaxy clusters leads to a now well-known polarization signal in the cosmic
microwave background (CMB) fluctuations. Using multi-frequency polarization
data, one can extract the temperature quadrupole and separate it from the
contaminant polarization associated with the kinematic quadrupole due to
transverse motion of clusters. At low redshifts, the temperature quadrupole
contains a significant contribution from the integrated Sachs-Wolfe effect
(ISW) associated with the growth of density fluctuations. Using polarization
data from a sample of clusters over a wide range in redshift, one can
statistically establish the presence of the ISW effect and determine the
redshift dependence of the ISW contribution to the rms quadrupole. Given the
strong dependence of the ISW effect on the background cosmology, the cluster
polarization can eventually be used as a probe of the dark energy.
