# Testing physical models for dipolar asymmetry with CMB polarization

## TL;DR

This paper forecasts how well CMB polarization data from Planck and future experiments can test physical models explaining the large-scale dipolar asymmetry observed in CMB temperature anisotropies, emphasizing the potential for polarization to improve constraints.

## Contribution

It provides predictions for polarization asymmetry detection and constraints on physical models of dipolar modulation using current and future CMB polarization data.

## Key findings

- Planck polarization can modestly improve error bars on modulation amplitude.
- Cosmic-variance-limited polarization can significantly reduce errors beyond simple expectations.
- Planck has a 20-75 ext{%} chance to detect the modulation at 2σ if primordial fluctuations are truly modulated.

## Abstract

The cosmic microwave background (CMB) temperature anisotropies exhibit a large-scale dipolar power asymmetry. To determine whether this is due to a real, physical modulation or is simply a large statistical fluctuation requires the measurement of new modes. Here we forecast how well CMB polarization data from \Planck\ and future experiments will be able to confirm or constrain physical models for modulation. Fitting several such models to the \Planck\ temperature data allows us to provide predictions for polarization asymmetry. While for some models and parameters \Planck\ polarization will decrease error bars on the modulation amplitude by only a small percentage, we show, importantly, that cosmic-variance-limited (and in some cases even \Planck) polarization data can decrease the errors by considerably better than the expectation of $\sqrt 2$ based on simple $\ell$-space arguments. We project that if the primordial fluctuations are truly modulated (with parameters as indicated by \Planck\ temperature data) then \Planck\ will be able to make a 2$\sigma$ detection of the modulation model with 20--75\% probability, increasing to 45--99\% when cosmic-variance-limited polarization is considered. We stress that these results are quite model dependent. Cosmic variance in temperature is important: combining statistically isotropic polarization with temperature data will spuriously increase the significance of the temperature signal with 30\% probability for \Planck.