Multiple Cosmic Collisions and the Microwave Background Power Spectrum
  Collisions between cosmic bubbles of different vacua are a generic feature of
false vacuum eternal inflation scenarios. While previous studies have focused
on the consequences of a single collision event in an observer's past, we begin
here an investigation of the more general scenario allowing for many "mild"
collisions intersecting our past light cone (and one another). We discuss the
general features of multiple collision scenarios and consider their impact on
the cosmic microwave background (CMB) temperature power spectrum, treating the
collisions perturbatively. In a large class of models, one can approximate a
multiple collision scenario as a superposition of individual collision events
governed by nearly isotropic and scale-invariant distributions, most appearing
to take up less than half of the sky. In this case, the shape of the expected
CMB temperature spectrum maintains statistical isotropy and typically features
a dramatic increase in power in the low multipoles relative to that of the
best-fit $\Lambda$CDM model, which is in tension particularly with the observed
quadrupole. We argue that this predicted spectrum is largely model-independent
and can be used to outline features of the underlying statistical distributions
of colliding bubbles consistent with CMB temperature measurements.
