Topological Defects from the Multiverse
  Many theories of the early universe predict the existence of a multiverse
where bubbles continuously nucleate giving rise to observers in their interior.
In this paper, we point out that topological defects of several
dimensionalities will also be produced in de Sitter like regions of the
multiverse. In particular, defects could be spontaneously nucleated in our
parent vacuum. We study the evolution of these defects as they collide with and
propagate inside of our bubble. We estimate the present distribution of defects
in the observable part of the universe. The expected number of such nearby
defects turns out to be quite small, even for the highest nucleation rate. We
also study collisions of strings and domain walls with our bubble in our past
light cone. We obtain simulated full-sky maps of the loci of such collisions,
and find their angular size distribution. Similarly to what happens in the case
of bubble collisions, the prospect of detecting any collisions of our bubble
with ambient defects is greatly enhanced in the case where the cosmological
constant of our parent vacuum is much higher than the vacuum energy density
during inflation in our bubble.
