Speaker
Description
Measurements of the Cosmic Microwave Background (CMB) temperature anisotropies over the past three decades have transformed our understanding of the Universe, while polarisation measurements have begun to provide additional powerful cosmological insights. Upcoming CMB experiments such as LiteBIRD, the Simons Observatory, and future missions including PICO will measure CMB polarisation with unprecedented sensitivity across a wide range of frequencies. In the standard cosmological framework, recombination physics is typically modelled considering only Thomson scattering. However, an additional and unavoidable process — Rayleigh scattering of CMB photons by neutral hydrogen and helium formed during recombination — becomes significant at high observational frequencies (≳200GHz). This effect introduces a characteristic frequency dependence in the CMB anisotropies and provides a new avenue for improving cosmological parameter estimation by breaking existing parameter degeneracies. In this talk, I will discuss the prospects for detecting the Rayleigh scattering signal with future CMB missions such as LiteBIRD and PICO, and quantify its impact on cosmological constraints. I will further highlight how Rayleigh scattering can serve as a sensitive probe of recombination physics and explore its role in testing alternative recombination scenarios aimed at alleviating the Hubble tension.