Gravity waves from cosmic bubble collisions
  Our local Hubble volume might be contained within a bubble that nucleated in a false vacuum with only two large spatial dimensions. We study bubble collisions in this scenario and find that they generate gravity waves, which are made possible in this context by the reduced symmetry of the global geometry. These gravity waves would produce B-mode polarization in the cosmic microwave background, which could in principle dominate over the inflationary background. 
Michael P. Salem, Prashant Saraswat, and Edgar Shaghoulian
hep-th astro-ph.CO gr-qc hep-ph