CMB Polarization towards Clusters as a Probe of the Integrated   Sachs-Wolfe Effect
  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. 
Asantha Cooray (Caltech), Daniel Baumann (Cambridge)