Spectral Variations of the Sky: Constraints on Alternate Universes

Abstract

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$6$σ$ level which reduces to $3.2-5.4σ$ 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$σ$ 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.