CMB polarization as a direct test of Inflation

We study the auto-correlation function of CMB polarization anisotropies and their cross correlation with temperature fluctuations as probe of the causal structure of the universe. Because polarization is generated at the last scattering surface, models in which fluctuations are causally produced on sub-horizon scales cannot generate correlations on scales larger then $\sim 2^o$. Inflationary models, on the other hand, predict a peak in the correlation functions at these scales: its detection would be definitive evidence in favor of a period of inflation. This signal could be detected with the next generation of satellites.