Cosmic Birefringence from Planck Data Release 4 (2201.07682v2)

Abstract: We search for the signature of parity-violating physics in the cosmic microwave background, called cosmic birefringence, using the Planck data release 4. We initially find a birefringence angle of $\beta=0.30\pm0.11$ (68% C.L.) for nearly full-sky data. The values of $\beta$ decrease as we enlarge the Galactic mask, which can be interpreted as the effect of polarized foreground emission. Two independent ways to model this effect are used to mitigate the systematic impact on $\beta$ for different sky fractions. We choose not to assign cosmological significance to the measured value of $\beta$ until we improve our knowledge of the foreground polarization.

• The paper reports a cosmic birefringence angle of 0.30° ± 0.11° using nearly full-sky Planck HFI polarization data.
• It employs a novel analysis pipeline to decompose cosmological signals from instrumental calibration errors.
• Foreground contamination from polarized dust, modeled through filament alignment, significantly impacts the observed signal.

Cosmic Birefringence from Planck Data Release 4

This paper investigates the presence of cosmic birefringence in the cosmic microwave background (CMB), as observed through Planck data release 4 (PR4). Cosmic birefringence is a parity-violating effect that can arise from pseudoscalar fields, such as axions, which couple to electromagnetic fields. This coupling can cause a rotation in the plane of polarization of photons, leading to an observable effect on the CMB polarization patterns.

The paper utilizes the polarization data from Planck’s high-frequency instrument (HFI), examining its angular power spectra to detect signals of cosmic birefringence. The authors report a measured birefringence angle of $\beta = 0.30^\circ \pm 0.11^\circ$, utilizing nearly full-sky data. However, they note that the measured $\beta$ decreases as more of the Galactic plane is masked, suggesting contamination from foregrounds, specifically polarized dust emissions which can exhibit intrinsic $EB$ correlations.

The analysis pipeline implemented for this paper deploys a technique to separate the cosmological signal from instrument calibration effects. This method involves decomposing observed power spectra into two components: one sensitive to cosmological signals and another to systematic errors, thus allowing a more accurate measurement of $\beta$ by accounting for instrumental angle miscalibration $\alpha_i$.

Several independent analysis pipelines corroborate the results, each utilizing varying approaches and tools like PolSpice, NaMaster, and Xpol for computing $C_\ell$. Cross-checking with simulation data from the Planck NPIPE reprocessing, which accurately models systematic effects, strengthens the robustness of the observed signal. Analysis of the simulations indicates negligible systematic influence on $\beta$ within statistical errors.

The authors discuss the implications of the foreground $EB$ correlation. They introduce a filament-based physical model that attributes $EB$ foreground correlation to the alignment variance between interstellar magnetic fields and dust filaments. Through this model and analysis adjustments, the paper posits that the observed variation in $\beta$ can be largely attributed to these foreground effects.

In conclusion, although a positive cosmic birefringence angle is detected, determining its origin conclusively as a cosmological signal remains impeded by uncertainties in foreground polarization modeling. Future CMB observations, especially missions with full-sky coverage such as LiteBIRD, are anticipated to refine these measurements and clarify underlying cosmological models. The findings emphasize that understanding polarized foreground emissions is crucial in advancing cosmic birefringence studies, offering potential insights into the fundamental physics beyond the standard model.