Collisions with other Universes: the Optimal Analysis of the WMAP data
  An appealing theory is that our current patch of universe was born as a
nucleation bubble from a phase of false vacuum eternal inflation. We search for
evidence for this theory by looking for the signal imprinted on the CMB that is
generated when another bubble "universe" collides with our own. We create an
efficient and optimal estimator for the signal in the WMAP 7-year data. We find
no detectable signal, and constrain the amplitude, a, of the initial curvature
perturbation that would be generated by a collision: -4.66 \times 10^{-8} < a
(\sin{\thetabubble})^{4/3} < 4.73 \times 10^{-8} [Mpc^{-1}] at 95% confidence
where \thetabubble is the angular radius of the bubble signal.
