The scattering of microwave background photons by hot gas in galaxy clusters induces a small degree of polarization, providing otherwise inaccessible information on large-scale CMB anisotropies. This signal is potentially a probe of cosmology inside our past light-cone, and is a fairly sensitive tracer of late-time structure formation and dark energy. However, it remains undetected. I will briefly discuss the physics of this effect, and outline how it may be detected with present and upcoming Sunyaev-Zeldovich surveys such as those of the Planck satellite and high-resolution ground-based telescopes.
