Microwave Background Polarization as a Probe of Large-Angle Correlations

TL;DR

This paper explores how polarization correlation functions in the cosmic microwave background can independently test the observed large-scale suppression of temperature correlations, potentially revealing new physics or confirming statistical flukes.

Contribution

It analyzes polarization correlation functions from Q/U and E/B components within the standard ΛCDM model to assess their suppression at large scales.

Findings

Correlation functions are dominated by local effects at last scattering or reionization.

Suppressed temperature correlations imply similar suppression in polarization functions if due to fundamental reasons.

Future polarization maps can distinguish between statistical fluke and fundamental model issues.

Abstract

Two-point correlation functions of cosmic microwave background polarization provide a physically independent probe of the surprising suppression of correlations in the cosmic microwave background temperature anisotropies at large angular scales. We investigate correlation functions constructed from both the Q and U Stokes parameters and from the E and B polarization components. The dominant contribution to these correlation functions comes from local physical effects at the last scattering surface or from the epoch of reionization at high redshift, so all should be suppressed if the temperature suppression is due to an underlying lack of correlations in the cosmological metric perturbations larger than a given scale. We evaluate the correlation functions for the standard $\Lambda$CDM cosmology constrained by the observed temperature correlation function, and compute statistics characterizing their suppression on large angular scales. Future full-sky polarization maps with minimal systematic errors on large angular scales will provide strong tests of whether the observed temperature correlation function is a statistical fluke or reflects a fundamental shortcoming of the standard cosmological model.