Testing physical models for dipolar asymmetry with CMB polarization

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.Comment: 15 pages, 11 figures, 2 table

Joint Bayesian analysis of large angular scale CMB temperature anomalies

“…Hence, the retrieved signal profile obtained in this paper satisfies the observational requirement at both small and large angular scales. The profile of the power-law signal obtained in this analysis will be useful guide in any pursuit of a theoretical understanding of CHA signal and also to make predictions for other cosmological probes including CMB polarization [66][67][68][69][70][71]. Bayesian inference of the BipoSH coefficients using HMC discussed in this work can be generalized to CMB polarization.…”

Statistical anisotropies in temperature and polarization fluctuations from a scale-dependent trispectrum

“…where g x (k) is the CMB transfer function with x = T, E describing temperature and E-mode polarization fluctuations respectively. The role of polarization fluctuations in helping pin down whether the modulations observed in the temperature fluctuations are primordial or not has been previously studied in [17,32,42]. Here we write down the general expressions for the covariances of modulation estimators in the presence of a trispectrum.…”

“…where w, x can be either T, E and C s are the CMB angular power spectrum of the best-fit cosmology. (Note that, while we use the notation from [17] of ∆X M s, our definition does not include additional -dependent factors that exactly map the , + 1 correlations to the Cartesian components of dipole modulation parameter A as defined as in Eq.(1).) Similar estimators can be defined for higher-order modulations, by considering , + 2 correlations, for example for quadrupolar modulation.…”

Covariance of CMB anomalies

“…Much of the study of large-angle anomalies has thus been focused on disentangling these three logical possibilities: whether large-angle CMB anomalies are cosmological, are due to systematics, or are statistical flukes. Better understanding of the anomalies will be driven in the future by observations of new quantities on very large spatial scales, such as CMB polarization [14][15][16][17][18] and lensing [19], as well as large-scale structure [20]. Whether or not new insights about the early universe become readily apparent, studying large-angle anomalies has and will thus continue to provide an opportunity to build a deeper understanding of our measurements of the large-angle CMB.…”