Grain Alignment and CMB Polarization Studies

TL;DR

This paper reviews the current understanding of dust grain alignment mechanisms, especially radiative torques, and their implications for polarized microwave emission contamination in CMB studies, emphasizing the predictive power of recent models.

Contribution

It highlights recent advances in quantitative grain alignment theory, particularly the analytical model of radiative torques, and discusses its implications for CMB polarization measurements.

Findings

Radiative torques are the dominant grain alignment mechanism.

Recent models match observational data well.

Alignment theory will improve CMB foreground modeling.

Abstract

Polarized microwave emission from dust is an important foreground that may contaminate polarized CMB studies unless carefully accounted for. Modeling of polarization from dust requires a quantitative understanding of grain alignment. I review the current status of grain alignment theory outlining recent advances in quantitative description of the alignment. In particular, I show that the grain-alignment theory is a predictive one, and its results nicely match observations. Those indicate that the most important process of alignment is related to radiative torques acting on irregular grains. The recently developed analytical model of radiative torque alignment has proven to be a very efficient tool for predicting the degree of grain alignment. We expect the alignment theory to further mature before CMBPol flight, which would ensure a better accounting for the dust-related polarization. At the same time, CMBPol should provide the additional testing of grain alignment, clarifying the reliability of polarimetry for tracing of magnetic field.