An anisotropic universe due to dimension-changing vacuum decay

TL;DR

This paper investigates how a dimension-changing false vacuum decay in the early universe could produce observable anisotropies in the CMB, especially affecting tensor modes at early times.

Contribution

It extends previous work by analyzing the full cosmological perturbation theory, revealing that early-time tensor anisotropies can dominate late-time effects, which is a novel insight.

Findings

Tensor modes show early-time anisotropic effects that grow with multipole

Scalar mode effects are consistent with expectations and sub-dominant

Early-time tensor anisotropies can surpass late-time effects, impacting observational searches

Abstract

In this paper we consider the question of observational signatures of a false vacuum decay event in the early universe followed by a period of inflation; in particular, motivated by the string landscape, we consider decays in which the parent vacuum has a smaller number of large dimensions than the current vacuum, which leads to an anisotropic universe. We go beyond previous studies, and examine the effects on the CMB temperature and polarisation power spectra, due to both scalar and tensor modes, and consider not only late-time effects but also the full cosmological perturbation theory at early times. We find that whilst the scalar mode behaves as one would expect, and the effects of anisotropy at early times are sub-dominant to the late-time effects already studied, for the tensor modes in fact the the early-time effects grow with multipole and can become much larger than one would expect, even dominating over the late-time effects. Thus these effects should be included if one is looking for such a signal in the tensor modes.