Large-misalignment mechanism for the formation of compact axion structures: Signatures from the QCD axion to fuzzy dark matter
Large-misalignment mechanism for the formation of compact axion structures: Signatures from the QCD axion to fuzzy dark matter
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DOI：
10.1103/physrevd.101.083014
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发表时间：
2019-09
影响因子：
5
通讯作者：
A. Arvanitaki;S. Dimopoulos;Marios Galanis;L. Lehner;J. Thompson;Ken Van Tilburg
中科院分区：
文献类型：
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作者：
A. Arvanitaki;S. Dimopoulos;Marios Galanis;L. Lehner;J. Thompson;Ken Van Tilburg
轴子是标准模型之外最好的激发粒子之一。我们展示了暗物质（DM）轴子在从$10^{-22}$ eV到$10^7$ GeV的广泛质量范围内的吸引自相互作用如何导致密度波动的非引力增长和束缚物体的形成。当初始场偏差较大时，这种结构形成的增强是由参数共振驱动的，并且当轴子场开始振荡时，在辐射主导时代的深处，它会影响哈勃视界有序长度尺度上的轴子密度扰动。这种效应可以将绝热扰动的几乎尺度不变的频谱转变为在上述尺度上具有尖峰的频谱，产生从密集的DM晕到标量场结构（如孤子和振荡）的物体。我们把这类轴子DM产生的宇宙学场景称为“大错位机制”。我们探索了这种机制对质量高达$10$ eV的轴子的观测结果。对于比$10^{-5}$ eV重的轴子，紧凑的轴子晕的数量足以显著影响地球上的直接探测实验，产生间歇性但连贯的信号，重复率超过每十年一次，交叉次数不到一天。这些轴子密度和运动相干性的偶发性增加为轴子DM搜索提供了新的方法，包括QCD轴子的搜索。由$10^{-22}$ eV到$10^{-5}$ eV的轴子组成的致密结构可以通过引力透镜搜索检测到，它们的引力相互作用也可以扰乱重子结构并改变恒星形成。在非常高的失调幅度下，轴子场可以在物质辐射相等之前经历自相互作用诱导的内爆，产生潜在可探测的低频随机引力波。
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.