CMB Anomalies after Planck

TL;DR

This paper reviews unexpected large-scale anomalies in the CMB observed by Planck and WMAP, highlighting their statistical significance and the challenge they pose to standard inflationary models.

Contribution

It provides a critical analysis of current understanding of CMB anomalies and discusses potential directions for future research to explain these features.

Findings

Multiple large-scale anomalies are statistically significant.

Some anomalies are uncorrelated, increasing their combined significance.

Current models do not fully explain these anomalies.

Abstract

Several unexpected features have been observed in the microwave sky at large angular scales, both by WMAP an by Planck. Among those features is a lack of both variance and correlation on the largest angular scales, alignment of the lowest multipole moments with one another and with the motion and geometry of the Solar System, a hemispherical power asymmetry or dipolar power modulation, a preference for odd parity modes and an unexpectedly large cold spot in the Southern hemisphere. The individual p-values of the significance of these features are in the per mille to per cent level, when compared to the expectations of the best-fit inflationary $\Lambda$CDM model. Some pairs of those features are demonstrably uncorrelated, increasing their combined statistical significance and indicating a significant detection of CMB features at angular scales larger than a few degrees on top of the standard model. Despite numerous detailed investigations, we still lack a clear understanding of these large-scale features, which seem to imply a violation of statistical isotropy and scale invariance of inflationary perturbations. In this contribution we present a critical analysis of our current understanding and discuss several ideas of how to make further progress.