First Observational Tests of Eternal Inflation
Stephen M. Feeney (UCL), Matthew C. Johnson (Perimeter Institute),, Daniel J. Mortlock (Imperial College London), Hiranya V. Peiris (UCL)

TL;DR
This paper reports the first observational search for signatures of bubble universe collisions predicted by eternal inflation, using WMAP data, and constrains their possible occurrence.
Contribution
It provides the first observational constraints on bubble collisions in eternal inflation using cosmic microwave background data.
Findings
No evidence for bubble collisions in WMAP data
Constraints limit detectable collisions to fewer than 1.6 on the full sky
Planck data can further test the bubble collision hypothesis
Abstract
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.
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