  Perhaps the most controversial idea in modern cosmology is that our
observable universe is contained within one bubble among many, all inhabiting
the eternally inflating multiverse. One of the few way to test this idea is to
look for evidence of the relic inhomogeneities left by the collisions between
other bubbles and our own. Such relic inhomogeneities induces a coherent bulk
flow over gigaparsec scales. Therefore, bubble collisions leave unique imprints
in the cosmic microwave background (CMB) through the kinetic Sunyaev Zel'dovich
(kSZ) effect, temperature anisotropies induced by the scattering of photons
from coherently moving free electrons in the diffuse intergalactic medium. The
kSZ signature produced by bubble collisions has a unique directional dependence
and is tightly correlated with the galaxy distribution; it can therefore be
distinguished from other contributions to the CMB anisotropies. An important
advantage of the kSZ signature is that it peaks on arcminute angular scales,
where the limiting factors in making a detection are instrumental noise and
foreground subtraction. This is in contrast to the collision signature in the
primary CMB, which peaks on angular scales much larger than one degree, and
whose detection is therefore limited by cosmic variance. In this paper, we
examine the prospects for probing the inhomogeneities left by bubble collisions
using the kSZ effect. We provide a forecast for detection using
cross-correlations between CMB and galaxy surveys, finding that the
detectability using the kSZ effect can be competitive with constraints from CMB
temperature and polarization data.
