  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.
