Impact of Dust on Spectral Distortion Measurements of the Cosmic Microwave Background

Abstract

Spectral distortions of the cosmic microwave background (CMB) are sensitive to energy injection by exotic physics in the early universe. The proposed Primordial Inflation Explorer (PIXIE) mission has the raw sensitivity to provide meaningful limits on new physics, but only if foreground emission can be adequately modeled. We quantify the impact of interstellar dust on Compton $y$ and $\mu$ measurements by considering a range of grain size distributions and compositions constrained by theoretical and observational priors (Zelko & Finkbeiner 2020). We find that PIXIE can marginalize over a modest number of dust parameters and still recover $y$ and $\mu$ estimates, though with increased uncertainty. As more foreground components are included (synchrotron, free-free), the estimates of $y$ degrade, and measurement of $\mu$ in the range sometimes considered for the standard $\Lambda$CDM of $2\times 10^{-8}$ becomes infeasible without ancillary low-frequency foreground information. An additional concern is dust absorption of the CMB monopole, a subtle effect that must be included. We quantify one form of model discrepancy error, finding that the error introduced by fitting our interstellar medium dust model with a modified blackbody is too large for CMB spectral distortions to be detectable. The greatest challenge may be the cosmic infrared background (CIB). We find that $\mu$ and $y$ are extremely sensitive to modeling choices for the CIB, and quantify biases expected for a range of assumptions.

Published in the Astrophysical Journal. 25 pages, 12 figures, 10 tables. Updated to reflect the journal version. Corrected a typo in the Appendix Eq A6 and A9; updated the tables 5,6,7 (first column). There were no changes to the qualitative nature of the results

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• Dust in Clusters of Galaxies
• Separation of CMB $\mu$ spectral distortions from foregrounds with poorly defined spectral shapes
• Diffuse interstellar bands as dust indicators: the contribution from 3D maps