The LargeMisalignment Mechanism for the Formation of Compact Axion Structures: Signatures from the QCD Axion to Fuzzy Dark Matter
Abstract
Axions are some of the best motivated particles beyond the Standard Model. We show how the attractive selfinteractions 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 radiationdominated era. This effect can turn an otherwise nearly scaleinvariant spectrum of adiabatic perturbations into one that has a spike at the aforementioned scales, producing objects ranging from dense DM halos to scalarfield configurations such as solitons and oscillons. We call this class of cosmological scenarios for axion DM production "the largemisalignment mechanism." We explore observational consequences of this mechanism for axions with masses up to $10$ eV. For axions heavier than $10^{5}$ eV, the compact axion halos are numerous enough to significantly impact Earthbound direct detection experiments, yielding intermittent but coherent signals with repetition rates exceeding one per decade and crossing times less than a day. These episodic increases in the axion density and kinematic coherence suggest new approaches for axion DM searches, including for the QCD axion. Dense structures made up of axions from $10^{22}$ eV to $10^{5}$ eV are detectable through gravitational lensing searches, and their gravitational interactions can also perturb baryonic structures and alter star formation. At very high misalignment amplitudes, the axion field can undergo selfinteractioninduced implosions long before matterradiation equality, producing potentiallydetectable lowfrequency stochastic gravitational waves.
 Publication:

arXiv eprints
 Pub Date:
 September 2019
 arXiv:
 arXiv:1909.11665
 Bibcode:
 2019arXiv190911665A
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Phenomenology;
 High Energy Physics  Theory
 EPrint:
 45 pages, 28 figures