Rethinking CMB foregrounds: systematic extension of foreground parameterizations

TL;DR

This paper introduces a moment-based approach to model and extend the spectral parameterization of CMB foregrounds, accounting for spatial averaging effects, thereby improving foreground modeling for future high-sensitivity CMB measurements.

Contribution

The paper develops a novel moment method that extends foreground parameterizations by capturing scale-dependent spectral shapes due to spatial averaging effects.

Findings

Identifies natural new spectral shapes for foreground components.

Demonstrates the method on power-laws, free-free, and blackbody spectra.

Provides a framework for propagating spectral information across scales.

Abstract

Future high-sensitivity measurements of the cosmic microwave background (CMB) anisotropies and energy spectrum will be limited by our understanding and modeling of foregrounds. Not only does more information need to be gathered and combined, but also novel approaches for the modeling of foregrounds, commensurate with the vast improvements in sensitivity, have to be explored. Here, we study the inevitable effects of spatial averaging on the spectral shapes of typical foreground components, introducing a moment approach, which naturally extends the list of foreground parameters that have to be determined through measurements or constrained by theoretical models. Foregrounds are thought of as a superposition of individual emitting volume elements along the line of sight and across the sky, which then are observed through an instrumental beam. The beam and line of sight averages are inevitable. Instead of assuming a specific model for the distributions of physical parameters, our method identifies natural new spectral shapes for each foreground component that can be used to extract parameter moments (e.g., mean, dispersion, cross-terms, etc.). The method is illustrated for the superposition of power-laws, free-free spectra, gray-body and modified blackbody spectra, but can be applied to more complicated fundamental spectral energy distributions. Here, we focus on intensity signals but the method can be extended to the case of polarized emission. The averaging process automatically produces scale-dependent spectral shapes and the moment method can be used to propagate the required information across scales in power spectrum estimates. The approach is not limited to applications to CMB foregrounds but could also be useful for the modeling of X-ray emission in clusters of galaxies.