High Energy Physics Seminar

Axions are some of the best motivated particles beyond the Standard Model. I will show how the attractive self-interactions of dark matter (DM) axions over a broad range of masses, from 10^−22 eV to 10^7 GeV, can lead to nongravitational growth of density fluctuations and the formation of bound objects. This structure formation enhancement is driven by parametric resonance when the initial field misalignment is large, and it affects axion density perturbations on length scales of order the Hubble horizon when the axion field starts oscillating, deep inside the radiation-dominated era. This effect can turn an otherwise nearly scale-invariant spectrum of adiabatic perturbations into one that has a spike at the aforementioned scales, producing objects ranging from dense DM halos to scalar-field configurations such as solitons and oscillons. This "large-misalignment mechanism" leads to various observational consequences in gravitational lensing and interactions, baryonic structures and star formation, direct detection (including for the QCD axion), and stochastic gravitational waves.