Observable effects of anisotropic bubble nucleation

TL;DR

This paper explores how anisotropic bubble nucleation in an inflating universe could leave detectable signatures in the cosmic microwave background, specifically anomalous multipole correlations, if inflation lasted not too long.

Contribution

It provides the first calculation of the inflationary spectrum in a universe with anisotropic initial conditions caused by bubble nucleation with a compact dimension.

Findings

Spectrum shows anomalous multipole correlations

Correlations extend to large multipole moments

Amplitude linked to present curvature parameter

Abstract

Our universe may have formed via bubble nucleation in an eternally-inflating background. Furthermore, the background may have a compact dimension--the modulus of which tunnels out of a metastable minimum during bubble nucleation--which subsequently grows to become one of our three large spatial dimensions. Then the reduced symmetry of the background is equivalent to anisotropic initial conditions in our bubble universe. We compute the inflationary spectrum in such a scenario and, as a first step toward understanding the effects of anisotropy, project it onto spherical harmonics. The resulting spectrum exhibits anomalous multipole correlations, their relative amplitude set by the present curvature parameter, which extend to arbitrarily large multipole moments. This raises the possibility of future detection, if slow-roll inflation does not last too long within our bubble. A full understanding of the observational signal must account for the effects of background anisotropy on photon free streaming, and is left to future work.