Implications of the CMB power asymmetry for the early universe

TL;DR

This paper investigates the CMB power asymmetry anomaly, analyzing non-Gaussian inflationary models, and finds that constructing models with sufficient asymmetry is challenging due to fine-tuning requirements, providing an explicit illustrative example.

Contribution

It extends previous analyses by identifying the non-Gaussian response function and demonstrating the difficulty of achieving the observed asymmetry without fine-tuning.

Findings

Constructed an explicit inflationary model consistent with observations.

Showed that many models require fine-tuning to produce the asymmetry.

Predicted the bispectrum signature for future Planck-like experiments.

Abstract

Observations of the microwave background fluctuations suggest a scale-dependent amplitude asymmetry of roughly 2.5 sigma significance. Inflationary explanations for this 'anomaly' require non-Gaussian fluctuations which couple observable modes to those on much larger scales. In this Letter we describe an analysis of such scenarios which significantly extends previous treatments. We identify the non-Gaussian 'response function' which characterizes the asymmetry, and show that it is non-trivial to construct a model which yields a sufficient amplitude: many independent fine tunings are required, often making such models appear less likely than the anomaly they seek to explain. We present an explicit model satisfying observational constraints and determine for the first time how large its bispectrum would appear to a Planck-like experiment. Although this model is merely illustrative, we expect it is a good proxy for the bispectrum in a sizeable class of models which generate a scale-dependent response using a large eta parameter.