Dust moments: towards a new modelling of the galactic dust emission for CMB B-modes analysis (1912.09567v2)

Abstract: The characterization of the spectral energy distribution (SED) of dust emission has become a critical issue in the quest for primordial B-modes. The dust SED is often approximated by a modified black body (MBB) emission law but the extent to which this is accurate is unclear. This paper addresses this question, expanding the dust SED at the power spectrum level. The expansion is performed by means of moments around the MBB law, related to derivatives with respect to the dust spectral index. We present the mathematical formalism and apply it to simulations and Planck total intensity data, from 143 to 857 GHz, because no polarized data are yet available that provide the required sensitivity to perform this analysis. With simulations, we demonstrate the ability of high-order moments to account for spatial variations in MBB parameters. Neglecting these moments leads to poor fits and a bias in the recovered dust spectral index. We identify the main moments that are required to fit the Planck data. The comparison with simulations helps us to disentangle the respective contributions from dust and the cosmic infrared background to the high-order moments, but the simulations give an insufficient description of the actual Planck data. Extending our model to cosmic microwave background B-mode analyses within a simplified framework, we find that ignoring the dust SED distortions, or trying to model them with a single decorrelation parameter, could lead to biases that are larger than the targeted sensitivity for the next generation of CMB B-mode experiments.

• The paper proposes a moment expansion technique to model dust emission SED at the power spectrum level, improving upon the traditional Modified Black Body method.
• The study shows that neglecting higher-order moments in dust SED modeling leads to biased estimates, which can significantly affect CMB B-mode studies compared to target sensitivities.
• The moment expansion method offers a more accurate foreground model for extracting primordial B-mode signals and highlights the necessity of accounting for complex foreground properties in cosmology.

Dust Moments: Towards a New Modeling of Galactic Dust Emission for CMB B-modes Analysis

The paper "Dust moments: towards a new modeling of the galactic dust emission for CMB B-modes analysis" by Mangilli et al. addresses a crucial challenge in cosmology related to the precise characterization of the spectral energy distribution (SED) of galactic dust emission. This problem is pertinent to the search for primordial B-modes in the cosmic microwave background (CMB), a potential probe of the inflationary era in the early universe.

Methodology and Framework

The paper critiques the traditional approach of modeling dust emission using a Modified Black Body (MBB) law, highlighting the inadequacies of this method in accounting for the observed spatial variations in dust spectral parameters. The authors propose an enhanced framework by expanding the dust SED at the power spectrum level through a moment expansion technique. This technique involves the expansion of the SED in terms of moments around the MBB law, specifically through derivatives with respect to the dust spectral index β. By adopting this approach, the paper aims to account for variations that arise from the averaging effects along the line of sight and across the sky, which are significant when working with spherical harmonic decomposition of map data.

The paper provides a rigorous mathematical formulation of this moment expansion in harmonic space and applies it to both simulated and actual data at frequencies ranging from 143 to 857 GHz. Notably, the analysis primarily targets total intensity data due to the limited availability of polarized data with adequate sensitivity.

Numerical Results and Findings

The moment expansion framework demonstrated the ability to capture the complexities in dust emission that exceed the MBB assumptions. The results show that neglecting higher-order moments can result in biased estimates of the dust spectral index, which consequently affects the dust SED modeling. Through simulations, the paper identifies the primary moments necessary to fit the data accurately.

A critical finding of the paper is the potential bias that arises in cosmic microwave background B-mode studies if dust emission is simplified to a singular decorrelation parameter or omitted entirely. Such biases could be significant relative to the sensitivity targets set for upcoming CMB B-mode experiments.

Practical and Theoretical Implications

The implications of this research are twofold. Practically, the moment expansion method offers an improved model for dusty foregrounds, crucial for the accurate extraction of primordial B-mode signals in CMB data. This is particularly relevant for future experiments aimed at achieving high precision in measuring the tensor-to-scalar ratio, r, which characterizes the amplitude of inflationary gravitational waves. Theoretically, the paper sends a strong signal to the cosmology community about the necessity of accounting for complex foreground emission properties to prevent measurement biases.

Speculations on Future Developments

The framework laid out in this paper could pave the way for further advancements in the modeling of polarized dust emission, necessitating an expansion from intensity data analysis to polarization data. Given the current trajectory of CMB research, it is plausible that moment expansion techniques will become integral in the design and analysis phases of future B-mode detection missions. Additionally, extending similar methodologies to other astrophysical foregrounds could enhance the overall fidelity of cosmological measurements.

In summary, Mangilli et al.'s work provides a sophisticated tool for confronting the challenges posed by galactic dust in CMB B-mode analyses. The proposed moment expansion method offers a comprehensive approach to modeling dust SED, ensuring that future cosmological studies can improve upon the current limitations of MBB approximations.