Looking for Axion Particles in the Universe

Speaker: Liang Dai (UC Berkeley) In Person and zoom

Zoom info: https://stanford.zoom.us/my/sihanyuan?pwd=QnpsUHZWWGJ2ekVYWmZVL3BmM0gzZ…

Axion particles are a generic class of hypothetical spin-zero particles that are of great interest to a range of fundamental questions in particle physics and cosmology. I will discuss several ideas to find their signatures with cosmological observations. An ultralight axion particle may cause the formation of a cosmic string network following spontaneous symmetry breaking in the early Universe and may rotate the polarization direction of photons through a Chern-Simons interaction with electromagnetism. I will discuss how traditional and novel statistical estimators can be employed to extract polarization rotation signatures from Cosmic Microwave Background anisotropies. Complementary to CMB observables, I will discuss a novel method to measure polarization rotation using forthcoming large radio galaxy surveys. The method will exploit an alignment between the polarization direction of integrated radio continuum and the apparent galaxy shape, and is specifically sensitive to cosmic birefringence sources at low redshifts z < 1.5. Motivated by empirical findings reported for a small sample of local spiral galaxies, I will discuss new results based on cosmological galaxy formation simulations, which corroborate this effect for the cosmological population of disk galaxies and allow us to make more reliable forecasts. Furthermore, the QCD axion as a promising solution to the Strong CP Problem may constitute dark matter and may cluster in bound axion mini-halos of asteroid to planet masses. I will explain how the most extremely magnified stars uncovered in galaxy cluster lensing fields can be exploited to reveal or constrain such minuscule structures.