Astrophysics Colloquium: Probing the physics of the early universe with pulsating stars

Abstract: Ancient stars can be thought of as fossils whose pulsations encode their distances, chemistry, and ages, which in turn preserve an archaeological record of the early universe. Accurately decoding these records requires stellar evolution and pulsation simulations to interpret the observations. I will present recent theoretical advancements and machine-learning speedups that have matured these simulations to the point where the resulting records can now be used to directly confront ideas of galaxy formation in the early universe. I will focus on two areas that respectively probe the local and distant universe: using RR Lyrae stars to examine the Galactic halo and the Milky Way's dwarf galaxies, and trying to uncover the true nature of JWST's Little Red Dots (LRDs). I will show how our new physics-informed method for modelling observations of pulsating stars is unlocking 1% distance measurements nearly out to the virial radius of the Milky Way, which in turn will enable a new era of galactic archaeology and near-field cosmology. I will furthermore present evidence that LRDs are pulsating quasi-stars: black holes embedded in a supermassive Cepheid-like stellar envelope. If confirmed, this interpretation would simultaneously solve two major problems: the underlying nature of LRDs, and the origin of supermassive black holes in the early universe.