Microwave Background Correlations from Dipole Anisotropy Modulation

TL;DR

This paper investigates a 7% asymmetry in the cosmic microwave background, modeling it as a dipole modulation, and finds evidence supporting this anisotropy with potential implications for new physics beyond the standard cosmological model.

Contribution

It develops estimators for dipole modulation in CMB data and applies them to Planck and WMAP maps, providing evidence for anisotropy consistent with previous findings.

Findings

Dipole modulation causes specific off-diagonal correlations in CMB multipoles.

The asymmetry is statistically significant at scales larger than 2 degrees.

Future polarization measurements can enhance the detection significance.

Abstract

Full-sky maps of the cosmic microwave background temperature reveal a 7% asymmetry of fluctuation power between two halves of the sky. A common phenomenological model for this asymmetry is an overall dipole modulation of statistically isotropic fluctuations, which produces particular off-diagonal correlations between multipole coefficients. We compute these correlations and construct corresponding estimators for the amplitude and direction of the dipole modulation. Applying these estimators to various cut-sky temperature maps from Planck and WMAP data shows consistency with a dipole modulation, differing from a null signal at 2.5$\sigma$, with an amplitude and direction consistent with previous fits based on the temperature fluctuation power. The signal is scale dependent, with a statistically significant amplitude at angular scales larger than 2 degrees. Future measurements of microwave background polarization and gravitational lensing can increase the significance of the signal. If the signal is not a statistical fluke in an isotropic Universe, it requires new physics beyond the standard model of cosmology.