Creating Statistically Anisotropic and Inhomogeneous Perturbations

TL;DR

This paper proposes a model where primordial perturbations can be statistically anisotropic and inhomogeneous, challenging the standard assumption of isotropy and homogeneity in the early universe, by introducing scalar fields with specific spatial gradients.

Contribution

It introduces a novel mechanism using scalar fields with constant spatial gradients to generate anisotropic and inhomogeneous primordial perturbations without breaking the universe's overall symmetries.

Findings

Potential explanation for CMB anomalies

Perturbations can be anisotropic despite symmetric energy-momentum tensor

Model aligns with observed large-scale anomalies

Abstract

In almost all structure formation models, primordial perturbations are created within a homogeneous and isotropic universe, like the one we observe. Because their ensemble averages inherit the symmetries of the spacetime in which they are seeded, cosmological perturbations then happen to be statistically isotropic and homogeneous. Certain anomalies in the cosmic microwave background on the other hand suggest that perturbations do not satisfy these statistical properties, thereby challenging perhaps our understanding of structure formation. In this article we relax this tension. We show that if the universe contains an appropriate triad of scalar fields with spatially constant but non-zero gradients, it is possible to generate statistically anisotropic and inhomogeneous primordial perturbations, even though the energy momentum tensor of the triad itself is invariant under translations and rotations.