Planck PR4 (NPIPE) map-space cosmic birefringence

TL;DR

This paper analyzes Planck PR4 CMB data to measure cosmic birefringence, finding results consistent with no parity violation but highlighting potential systematic uncertainties and the advantages of map-space analysis.

Contribution

It introduces a map-space analysis method for cosmic birefringence using Planck PR4 data, emphasizing systematic uncertainty assessment and spatial variation investigation.

Findings

Measured birefringence angle around 0.47 degrees with uncertainties dominated by calibration systematics.

Results are consistent with no parity violation in the universe.

No evidence found for anisotropic birefringence or a cosmic birefringence dipole.

Abstract

Cosmic birefringence is an effect where the plane of polarisation of the cosmic microwave background (CMB) is rotated by an angle $\beta$ through coupling to a hypothetical parity-violating field. We analyse the Planck Public Release 4 (PR4 or NPIPE) data using a map-space analysis method and find $\beta=0.46^\circ\pm 0.04^\circ(\mathrm{stat.})\pm0.28^\circ(\mathrm{syst.})$ for SEVEM CMB maps and $\beta=0.48^\circ\pm 0.04^\circ(\mathrm{stat.})\pm 0.28^\circ(\mathrm{syst.})$ for Commander CMB maps. These values are slightly higher than previously published results, which may be explained by the fact that we have not attempted to remove any potential bias from miscalibration of the Planck polarimeters. The uncertainty in this miscalibration dominates the systematic uncertainty, which also means that our results are consistent with no parity violation. An advantage of the map-space analysis is that it is easy to investigate any variations on the sky, for example caused by foreground contamination. Our results for isotropic birefringence are fairly robust against different spatial data cuts, but there may be hints of a foreground systematic (north versus south hemispheres) or uncontrolled miscalibration effect (T peaks versus E peaks) that should be followed up in future studies. We additionally find no evidence of a cosmic birefringence dipole (anisotropic birefringence).