Astrophysics and cosmology from CMB scattering

The cosmic microwave background is a bath of photons permeating the Universe that was produced when the Universe was only 380,000 years old. Since their production, the CMB photons have propagated through the Universe. Along their journey through the Universe, a small fraction of these photons interact with the intervening matter: some of the photons are deflected by massive objects, others are scattered by hot electrons and more. These interactions create distortions in the observed properties of the CMB and are known as CMB secondary anisotropies. These CMB secondary anisotropies are a rich source of astrophysical and cosmological information. For example, hot cosmic electrons induce anisotropies, known as the thermal Sunyaev-Zel’dovich (tSZ) effect, and these inform us about the growth rate of cosmological structures and the pressure in galaxy groups and clusters. Similarly, electrons moving with large velocities imprint anisotropies, known as the kinetic Sunyaev-Zel’dovich (kSZ) effect, and provide a measure of the density of free electrons as well as a view of the cosmic velocity field. My research proposal focuses on detecting CMB secondaries and using these measurements to study cosmological and astrophysical processes. The Simons Observatory (SO) is a CMB telescope that started taking data in 2024. My first goal is to exploit the unparalleled sensitivity and resolution of SO to make maps of a diverse range of CMB secondary anisotropies. I will also search for a range of dark matter models that produce unique spectral signatures in CMB datasets. Isolated maps of the different sky signals are an essential starting point for many analyses, which can otherwise be biased by the other sky signals. Thus, the public maps that I will make are a key scientific product of my proposal. Using isolated maps of the tSZ, I will test our cosmological model and search for novel physics. In detail, I will use beyond two-point statistics, such as the skewness, to extract information on the growth of structure from the tSZ maps. Beyond two-point statistics are very sensitive cosmological probe and are highly complementary to the standard methods. These measurements will weigh-in on the “sigma-8” cosmological tension and improve upcoming constraints on the sum of the masses of the neutrinos. In parallel, I will explore the rotational kSZ effect, which arises from gas rotations. Through measurements of this, I will study how clusters evolve and test fundamental symmetries of our Universe. Current CMB experiments are on the cusp of detecting two novel CMB secondary anisotropies: patchy-screening effect and the relativistic Sunyaev-Zel’dovich effect. These effects provide new views into galaxy groups and clusters by providing direct access to the electron density and electron temperature. With Simons Observatory data, I will be able to transform the nascent studies of these effects into high precision measurements. The physics governing the environments of groups and clusters is key in regulating galaxy formation and evolution, however it is currently poorly constrained. My measurements will change this picture and thus will inform future galaxy evolution models. In summary, the next generation of CMB experiments will enable high precision measurements of CMB secondaries. These CMB secondaries contain a wealth of cosmological and astrophysical information. Through precision measurements of these effects with the Simons Observatory, I will advance our understanding of the physics of galaxy groups and clusters and produce novel tests of our cosmological model.