Spectral Variations of the Sky: Constraints on Alternate Universes
  The fine tuning of parameters required to reproduce our present day Universe
suggests that our Universe may simply be a region within an eternally inflating
super-region. Many other regions beyond our observable Universe would exist
with each such region governed by a different set of physical parameters.
Collision between these regions, if they occur, should leave signatures of
anisotropy in the cosmic microwave background (CMB) but have not been seen. We
analyze the spectral properties of masked, foreground-cleaned maps between 100
and 545 GHz constructed from the Planck dataset. Four distinct $\sim2-4\arcdeg$
regions associated with CMB cold spots show anomalously strong 143 GHz emission
but no correspondingly strong emission at either 100 or 217 GHz. The signal to
noise of this 143 GHz residual emission is at the $\gtrsim$6$\sigma$ level
which reduces to $3.2-5.4\sigma$ after subtraction of remaining
synchrotron/free-free foregrounds. We assess different mechanisms for this
residual emission and conclude that although there is a 30\% probability that
noise fluctuations may cause foregrounds to fall within 3$\sigma$ of the
excess, there is less than a 0.5\% probability that foregrounds can explain all
the excess. A plausible explanation is that the collision of our Universe with
an alternate Universe whose baryon to photon ratio is a factor of $\sim$4500
larger than ours, could produce enhanced Hydrogen Paschen-series emission at
the epoch of recombination. Future spectral mapping and deeper observations at
100 and 217 GHz are needed to mitigate systematics arising from unknown
Galactic foregrounds and to confirm this unusual hypothesis.
