Presentation Archive

The Large Misalignment Mechanism for Compact Halo Formation

Asimina Arvanitaki (Perimeter Institute)

September 23, 2019

Abstract: Axions are some of the best motivated particles beyond the Standard Model. We show how the attractive self-interactions of dark matter (DM) axions over a broad range of masses, from 10E-22 eV to 10E7 GeV, can lead to non-gravitational growth of density fluctuations and the formation of bound objects. This enhancement of structure formation is driven by parametric resonance when the initial field misalignment is large; 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 nontrivial scalar field configurations such as solitons and oscillons. We call this class of cosmological scenarios for axion DM production “the large-misalignment mechanism”. We explore observational consequences of this mechanism for axions with masses up to 10 eV. For axions heavier than 10E-5 eV, the number density of compact axion halos is high enough to significantly impact Earth-bound direct detection experiments, yielding intermittent but coherent signals with repetition rates exceeding one per decade, and with each encounter lasting for a crossing time less than a day. These episodic large increases in the axion density and kinematic coherence suggest new approaches for axion DM searches, including those for the QCD axion. Dense structures made up of ultralight axions with masses in the range from 10E-22 eV to 10E-5 eV are detectable through gravitational lensing searches, and their gravitational interactions can also perturb baryonic structures and alter the star formation history of the Universe. At sufficiently high misalignment amplitudes, the axion field can undergo self-interaction-induced implosions long before matter-radiation equality, producing potentially detectable low-frequency stochastic gravitational waves.