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

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.