  Scattering of the temperature anisotropy quadrupole by free electrons in
galaxy clusters leads to a secondary polarization signal in the cosmic
microwave background (CMB) fluctuations. At low redshifts, the temperature
quadrupole contains a significant contribution from the integrated Sachs-Wolfe
(ISW) effect 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 its
redshift evolution. Given the strong dependence of the ISW effect on the
background cosmology, cluster polarization can eventually be used as a powerful
probe of dark energy. As a further application, we also discuss how it might be
used to understand the potential lack of power on large scales.
