Cosmology with Rayleigh Scattering of the Cosmic Microwave Background

TL;DR

Measuring Rayleigh scattering of the CMB can provide additional cosmological information, significantly improving constraints on parameters like neutrino properties without major changes to upcoming experiments.

Contribution

This paper demonstrates that detecting Rayleigh scattering signals in the CMB enhances cosmological parameter constraints, especially for $N_{\rm eff}$ and neutrino mass, with minimal experimental modifications.

Findings

Rayleigh scattering signal is detectable with sufficient frequency sensitivity.

Measurement of Rayleigh scattering improves constraints on $N_{\rm eff}$.

Measurement of Rayleigh scattering improves constraints on neutrino mass.

Abstract

The cosmic microwave background (CMB) has been a treasure trove for cosmology. Over the next decade, current and planned CMB experiments are expected to exhaust nearly all primary CMB information. To further constrain cosmological models, there is a great benefit to measuring signals beyond the primary modes. Rayleigh scattering of the CMB is one source of additional cosmological information. It is caused by the additional scattering of CMB photons by neutral species formed during recombination and exhibits a strong and unique frequency scaling ($\propto \nu^4$). We will show that with sufficient sensitivity across frequency channels, the Rayleigh scattering signal should not only be detectable but can significantly improve constraining power for cosmological parameters, with limited or no additional modifications to planned experiments. We will provide heuristic explanations for why certain cosmological parameters benefit from measurement of the Rayleigh scattering signal, and confirm these intuitions using the Fisher formalism. In particular, observation of Rayleigh scattering allows significant improvements on measurements of $N_{\rm eff}$ and $\sum m_\nu$.