Cosmic Microwave Background Polarization as a Direct Test of Inflation

We study the autocorrelation function of cosmic microwave background polarization anisotropies and their cross correlation with temperature fluctuations as a 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 subhorizon scales cannot generate correlations on scales larger than $\sim 2$°. 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.