Can Relic Superhorizon Inhomogeneities be Responsible for Large-Scale CMB Anomalies?

TL;DR

This paper explores how relic superhorizon inhomogeneities during inflation can cause large-scale CMB anomalies, including anisotropic alignments and power suppression, through a scale-dependent quadrupole modification.

Contribution

It introduces a model where relic superhorizon inhomogeneities induce scale-dependent statistical anisotropy and power suppression in the CMB spectrum, explaining observed anomalies.

Findings

Large-scale anisotropy is caused by superhorizon inhomogeneities.

Power on large scales is suppressed by a -1/k^2 factor.

Model explains low-$ll$ multipole alignments and power deficits.

Abstract

We investigate the effects of the presence of relic classical superhorizon inhomogeneities during inflation. This superhorizon inhomogeneity appears as a gradient locally and picks out a preferred direction. Quantum fluctuations on this slightly inhomogeneous background are generally statistical anisotropic. We find a quadrupole modification to the ordinary isotropic spectrum. Moreover, this deviation from statistical isotropy is scale-dependent, with a $\sim -1/k^2$ factor. This result implies that the statistical anisotropy mainly appears on large scales, while the spectrum on small scales remains highly isotropic. Moreover, due to this $-1/k^2$ factor, the power on large scales is suppressed. Thus, our model can simultaneously explain the observed anisotropic alignments of the low-$\ell$ multipoles and their low power.