Large-misalignment 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 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. 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 10 −5 eV, the compact axion halos are numerous enough to significantly impact Earth-bound 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 self-interaction-induced implosions long before matter-radiation equality, producing potentially-detectable low-frequency stochastic gravitational waves.

On the wondrous stability of ALP dark matter

“…Indeed, while self-couplings have been found to be safe for QCD axions [1,7], they can be relevant in models that go beyond the simplest pseudo-Goldstone ALP. Among other effects, they can lead to a fragmentation of the condensate [53] 8 . Similarly, some care is needed when couplings, e.g.…”

“…The decay of the condensate into relativistic axion modes can also be described as a growth of quantum fluctuations caused by anharmonicities in the potential. This was done in [1,7], once more finding that the effect is negligible in an expanding background, unless extreme misalignment angles occur [8].…”

Imprints of the early Universe on axion dark matter substructure

“…For a/f a 1 these effects are unimportant. Nonlinear terms in the ALP equation of motion can also lead to important effects on small scales, such as soliton-like configuration known as axitons or oscillons [33,[44][45][46]. The precise effect of self-interactions depends on the interaction terms and temperature dependence of the ALP mass.…”

“…To estimate how sensitive this technique would be to the presence of ALP minicluster we follow the simplified analysis of Ref. [46], based on the proposal of Ref. [42].…”

New application of the Killing vector field formalism: modified periodic potential and two-level profiles of the axionic dark matter distribution

“…The third trend is connected with the modeling of axionic structures of different scales, associated with the galactic halos, miniclusters, halos surrounding the magnetars, dyons, etc. (see, e.g., [26][27][28][29]). We have to emphasize, that one of the most influential idea in these new models is the idea of internal self-interaction in the axionic systems.…”