  The eternal inflation scenario predicts that our observable universe resides
inside a single bubble embedded in a vast inflating multiverse. We present the
first observational tests of eternal inflation, performing a search for
cosmological signatures of collisions with other bubble universes in cosmic
microwave background data from the WMAP satellite. We conclude that the WMAP
7-year data do not warrant augmenting LCDM with bubble collisions, constraining
the average number of detectable bubble collisions on the full sky to be less
than 1.6 at 68% CL. Data from the Planck satellite can be used to more
definitively test the bubble collision hypothesis.
