Institute for Advanced Study Informal Astrophysics Seminar

ABSTRACT: I will demonstrate how we can gain powerful insights into the nature of dark matter and dark energy by investigating how they behave in the most energetic environment ever realized: the early Universe. First, I will show how the early Universe exposes a critical flaw in chameleon gravity, which is a scalar-tensor theory that provides dark energy while mimicking general relativity in the Solar System. The same mechanism that hides the chameleon scalar field makes that field roll when particles become nonrelativistic during the first minutes after the Big Bang. I will show that these "kicks" to the chameleon field have catastrophic consequences for chameleon gravity. Second, I will describe how dark matter microhalos provide a window into the end of inflation and the subsequent transition to a radiation-dominated Universe. Specifically, we can use their abundances to study the density fluctuations that were created in the later stages of inflation, and the transition from inflation to a hot Universe can generate microhalos by enhancing the growth of small-scale density fluctuations. As relics from the first minutes after the Big Bang, these microhalos can provide new insights into the thermal history of the early Universe and the origins of dark matter.