Astrophysical observations of spinning BHs, which span $ 5M_\odot\lesssim M_{\rm BH}\lesssim 5\times 10^8 M_\odot$, can be used to exclude the existence of certain massive bosons via the superradiance phenomenon. In this work, we explore for the first time how these measurements can be used to constrain properties of statistical distributions for the masses of multiple bosonic fields. Quite generally, our methodology excludes $N_{\rm ax}\gtrsim 30$ scalar fields with a range of mass distribution widths and central values spanning many orders of magnitude. We demonstrate this for the specific example of axions in string theory and M-theory, where the mass distributions in certain cases take universal forms. We place upper bounds on $N_{\rm ax}$ for certain scenarios of interest realised approximately as mass distributions in M-theory, including the QCD axion, grand unified theories, and fuzzy dark matter.
The superradiant instability can lead to the generation of extremely dense axion clouds around rotating black holes. We show that, despite the long lifetime of the QCD axion with respect to spontaneous decay into photon pairs, stimulated decay becomes significant above a minimum axion density and leads to extremely bright lasers. The lasing threshold can be attained for axion masses $μ\gtrsim 10^{-8}\ \mathrm{eV}$, which implies superradiant instabilities around spinning primordial black holes with mass $\lesssim 0.01M_\odot$. Although the latter are expected to be non-rotating at formation, a…
Black holes in binaries with other compact objects can provide natural venues for indirect detection of axions or other ultralight fields. The superradiant instability associated with a rapidly spinning black hole leads to the creation of an axion cloud which carries energy and angular momentum from the black hole. This cloud will then decay via gravitational wave emission. We show that the energy lost as a result of this process tends toward an outspiraling of the binary orbit. A given binary system is sensitive to a narrow range of axion masses, determined by the mass of the black hole. Puls…
A black hole image contains a bright ring of photons that have closely circled the black hole on their way from the source to the detector. Here, we study the photon ring of a rotating black hole which is pierced by a global hyper-light axion-type cosmic string. We show that the coupling $ϕF \tilde{F}$ between the axion $ϕ$ and the photon can give rise to a unique polarimetric structure of the photon ring. The structure emerges due to an Aharonov-Bohm type effect that leads to a change of the polarization directions of linear polarized photons when they circle the black hole. For several param…
The coupling between scalar and vector fields has a long and interesting history. Axions are one key possibility to solve the strong CP problem and axion-like particles could be one solution to the dark matter puzzle. Given the nature of the coupling, and the universality of free fall, nontrivial important effects are expected in regions where gravity is strong. Here, we show that i. A background EM field induces an axionic instability in flat space, for large enough electric fields. Conversely, a homogeneous harmonic axion field induces an instability in the Maxwell sector. When carried over …
Discovering the QCD Axion with Black Holes and Gravitational Waves
先进的LIGO可能是第一个探测引力波的实验。通过恒星黑洞的超辐射，它也可能是第一个发现衰变常数在大统一尺度以上的QCD轴子的实验。当轴子的康普顿波长与黑洞的大小相当时，轴子与黑洞结合，形成“引力原子”。“通过超辐射过程，占据束缚能级的轴子数量呈指数增长，从黑洞中提取能量和角动量。轴子在引力原子能级之间的跃迁以及轴子湮灭为引力子的过程中可以产生可观测到的引力波信号。这些信号是持久的、单色的，可以与普通的天体物理源区分开来。对于10^-11和10^-10 eV之间的轴子，我们估计在aLIGO上的跃迁事件高达O（1），对于10^-13和10^-11 eV之间的轴子，我们估计湮灭事件高达10^4。在零搜索的情况下，ALIGO可以限制轴子质量的范围内快速旋转的黑洞形成率。在未来的低频引力波观测站中，质量更轻的轴子湮灭也是有希望的;湮灭率有很大的不确定性，主要由超大质量黑洞的自旋分布决定。我们对aLIGO的预测对于来自黑洞环境的扰动是稳健的，并且解释了我们对恒星黑洞自旋测量所建议的6*10^-13 eV < ma < 2*10^-11 eV的QCD轴子的更新排除。
Ultra light axion fields, motivated by the string theory, form a large condensate (axion cloud) around rotating black holes through superradiant instability. Several effects due to the axion cloud, such as the spin-down of black holes and the emission of monochromatic gravitational waves, open a new window to search for axions by astrophysical observations. When the axion self-interaction is considered, the evolution of cloud is altered significantly, and an explosive phenomenon called bosenova can happen. Thus, it is necessary to understand the precise evolution of self-interacting clouds for…
We reconsider entropy arguments which have been previously argued to support the idea that the dark matter constituents are primordial black holes with many solar masses. It has recently been shown that QCD axions which solve the strong CP problem may have masses $m_a$ in the extended range $10^{-3}eV > m_a > 10^{-33} eV$. Ultralight axions provide so many degrees of freedom that their entropy can exceed that of primordial black holes. This suggests that ultralight axions are more suited than primordial black holes to be constituents of dark matter.
In the QCD axion scenario, a network of domain walls bounded by cosmic strings fragments into pieces. As these fragments collapse, some of them will form black holes. With standard QCD axion parameters, the black holes will have lunar masses ($M_{\rm bh} \sim 10^{-8}\, {\rm M}_\odot$). Even though their number density is difficult to estimate, arguments suggest that they can constitute a reasonable fraction of the critical cosmological density.
Exploring the String Axiverse with Precision Black Hole Physics
最近有人提出，存在大量的轻轴子，即一个轴宇宙，是弦论存在额外维度的证据。我们通过彭罗斯超辐射过程讨论了这些轴子对天体物理黑洞的观测结果。当一个轴子康普顿波长与黑洞的大小相当时，轴子与黑洞“核”结合，在天空中形成一个引力原子。由黑洞的能量和角动量提供的超辐射原子能级的占有数呈指数增长。黑洞向下旋转，在其周围形成一个轴子玻色-爱因斯坦凝聚体云。当吸引轴子的自相互作用变得比引力结合能更强时，轴子云坍塌，这种现象在凝聚态物理中被称为“Bosenova”。轴子的存在首先是由天体物理学上的黑洞的质量与自旋图中的空隙来诊断的。对于年轻的黑洞，自旋的允许值是量子化的，从而在能隙区域内产生“Regge轨迹”。轴子云也可以通过精确的近视界几何映射或通过来自Bosenova爆炸的引力波以及引力原子中的轴子跃迁和湮灭直接观察到。我们的估计表明，这些信号在即将到来的实验中是可以检测到的，例如先进的LIGO，AGIS和LISA。目前的黑洞自旋测量表明，QCD轴子衰变常数的上限为2 x 10^17 GeV，而高级LIGO可以检测到来自QCD轴子云的信号，其衰变常数与大统一尺度一样低。最后，我们讨论了观测与Bosenova爆炸有关的γ射线的可能性，也许，从QCD轴子到光子转换的无线电波。
We pioneer the black hole mass gap as a powerful new tool for constraining new particles. A new particle that couples to the Standard Model---such as an axion---acts as an additional source of loss in the cores of population-III stars, suppressing mass lost due to winds and quenching the pair-instability. This results in heavier astrophysical black holes. As an example, using stellar simulations we show that the solar axion explanation of the recent XENON1T excess implies astrophysical black holes of ~ 56 MS, squarely within the black hole mass gap predicted by the Standard Model.
Primordial Black Holes as a Factory of Axions: Extragalactic Photons from Axions
Yonsei University（延世大学）；Weizmann Institute of Science（魏茨曼科学研究所）；Chungnam National University（忠南国立大学）；Institute for Basic Science (IBS)（基础科学研究院）；Korea Institute for Advanced Study（韩国高等研究院）
In the next few years Advanced LIGO (aLIGO) may see gravitational waves (GWs) from thousands of black hole (BH) mergers. This marks the beginning of a new precision tool for physics. Here we show how to search for new physics beyond the standard model using this tool, in particular the QCD axion in the mass range ma ~ 10^-14 to 10^-10 eV. Axions (or any bosons) in this mass range cause rapidly rotating BHs to shed their spin into a large cloud of axions in atomic Bohr orbits around the BH, through the effect of superradiance (SR). This results in a gap in the mass vs. spin distribution of BHs …
Dimming of Photon Ring due to Photon-Axion Conversion around Kerr Black Holes
Shiv Nadar Institution of Eminence (Deemed to be University)（希夫纳达尔卓越机构（认可大学））；National Institute of Technology, Rourkela（鲁尔克拉国家理工学院）；Universität Hamburg（汉堡大学）
We consider a general class of axion models, including the QCD and string axion, in which the PQ symmetry is broken before or during inflation. Assuming the axion is the dominant component of the dark matter, we discuss axion star formation in virialized dark minihalos around primordial black holes through gravitational Bose-Einstein condensation. We determine the conditions for minihalos to kinetically produce axion stars before galaxy formation. Today, we expect up to $\sim 10^{17}$ ($\sim 10^9$) axion stars in a radius of 100 parsecs around the Sun for the case of the QCD (string) axion.
In the axion monodromy inflation, the inflation is driven by the axion with super-Planckian field values in a monomial potential with superimposed sinusoidal modulations. The coupling of the axion to massless gauge fields can induce copious particle production during inflation, resulting in large non-Gaussian curvature perturbation that leads to the formation of primordial black holes. In this paper, we explore the parameter space in the axion monodromy inflation model that favors the formation of primordial black holes with masses ranging from $10^8$ grams to $20$ solar masses. We also study …
An inhomogeneous pseudo-scalar field configuration behaves like an optically active medium. Consequently, if a light ray passes through an axion cloud surrounding a Kerr black hole, it may experience a polarization-dependent bending. We explore the size and relevance of such effect considering both the QCD axion and a generic axion-like particle.
We study the axionic inflation with a modulated potential and examine if the primordial tensor power spectrum exhibits oscillatory feature, which is testable with future space-based gravitational wave experiments such as DECIGO and BBO. In the case of the single-field axion monodromy inflation, it turns out that it is difficult to detect the oscillation in the spectrum due to suppression of the sub-Planckian decay constant of axion. On the other hand, in the case of aligned chromo-natural inflation where the axion is coupled to a SU(2) gauge field, it turns out that the sizable oscillation in …
An axion cloud surrounding a supermassive black hole can be naturally produced through the superradiance process. Its existence can be examined by the axion induced birefringence effect. It predicts an oscillation of the electric vector position angle of linearly polarized radiations. Stringent constraints of the existence of the axion in a particular mass window has been obtained based on the recent Event Horizon Telescope measurement on M87$^\star$. The future Very-Long-Baseline Interferometry (VLBI) observations will be able to measure the vicinity of many supermassive black holes, thus it …
We propose a scenario in which a strong Peccei-Quinn (PQ) symmetry breaking in the early universe results in large inhomogeneities of the initial QCD axion field value, leading to the formation of very dense axion bubbles. Some of the axion bubbles subsequently collapse into primordial black holes (PBHs). The spatially homogeneous part of the QCD axion explains dark matter of the universe, while the PBHs arising from the axion bubbles can explain the LIGO events or the seed of supermassive black holes. Interestingly, the mass of PBH is determined by the axion decay constant; for $f_a = 10^{17}…
There are strong interests in considering ultra-light scalar fields (especially axion) around a rapidly rotating black hole because of the possibility of observing gravitational waves from axion condensate (axion cloud) around black holes. Motivated by this consideration, we propose a new method to study the dynamics of an ultra-light scalar field with self-interaction around a rapidly rotating black hole, which uses the dynamical renormalization group method. We find that for relativistic clouds, saturation of the superradiant instability by the scattering of the axion due to the self-interac…
If the dark matter is made of ultra-light axions, stable solitonic cores form at the centers of virialized halos. In some range for the mass $m$ of the axion particle, these cores are sufficiently compact and can mimic supermassive black holes (SMBH) residing at galactic nuclei. We use the solitonic core--halo mass relation, validated in numerical simulations, to constrain a new range of allowed axion mass from measurements of the SMBH mass in (pseudo)bulge and bulgeless galaxies. These limits are based on observations of galactic nuclei on scales smaller than 10 pc. Our analysis suggests that…
Probing the QCD axion dark matter (DM) hypothesis is extremely challenging as the axion interacts very weakly with Standard Model particles. We propose a new avenue to test the QCD axion DM via transient radio signatures coming from encounters between neutron stars (NSs) and axion minihalos around primordial black holes (PBHs). We consider a general QCD axion scenario in which the PQ symmetry breaking occurs before (or during) inflation coexisting with a small fraction of DM in the form of PBHs. The PBHs will unavoidably acquire around them axion minihalos with the typical length scale of pars…
If primordial black holes (PBHs) had come to dominate the energy density of the early Universe when oscillations in the axion field began, we show that the relic abundance and expected mass range of the QCD axion would be greatly modified. Since the QCD axion is a potential candidate for dark matter (DM), we refer to it as the DM axion. We predominantly explore PBHs in the mass range $(10^6 - 5\times 10^8)\,$g. We investigate the relation between the relic abundance of DM axions and the parameter space of PBHs. We numerically solve the set of Boltzmann equations, that governs the cosmological …
Ultralight bosons and axion-like particles appear naturally in different scenarios and could solve some long-standing puzzles. Their detection is challenging, and all direct methods hinge on unknown couplings to the Standard Model of particle physics. However, the universal coupling to gravity provides model-independent signatures for these fields. We explore here the superradiant instability of spinning black holes triggered in the presence of such fields. The instability taps angular momentum from and limits the maximum spin of astrophysical black holes. We compute, for the first time, the s…
Crowdsourcing Gravitational Waves from Superradiant Axions
King’s College London（伦敦国王学院）；University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；RIKEN iTHEMS（理化学研究所iTHEMS中心）
We study the prospects of detection at terrestrial and space interferometers, as well as at pulsar timing array experiments, of a stochastic gravitational wave background which can be produced in models of axion inflation. This potential signal, and the development of these experiments, open a new window on inflation on scales much smaller than those currently probed with Cosmic Microwave Background and Large Scale Structure measurements. The sourced signal generated in axion inflation is an ideal candidate for such searches, since it naturally grows at small scales, and it has specific proper…
Axions are a potential dark matter candidate, which may condense and form self gravitating compact objects, called axion stars (ASs). In this work, we study for the first time head-on collisions of relativistic ASs with black holes (BHs) and neutron stars (NSs). In the case of BH-AS mergers we find that, in general, the largest scalar clouds are produced by mergers of low compactness ASs and spinning BHs. Although in most of the cases which we study the majority of the mass is absorbed by the BH within a short time after the merger, in favourable cases the remaining cloud surrounding the final…
We consider an axion cloud around a black hole with background magnetic fields. We calculate the decay rate of the axion cloud due to the axion-photon conversion associated with the axion-photon coupling. For simplicity, we consider the situation where the axion configuration is dominated by a solution for the eigenvalue equation equivalent to that for the Hydrogen atom, and the coupling term can be evaluated by a successive perturbation method. For the monopole background, we find the decay rate of the axion cloud is given by $\sim q^2κ^2(GM)^5μ^8$, where $μ$, $M$, $G$, $κ$ and $q$ are the ax…
Gravitational wave observation has the potential of probing ultralight bosonic fields such as axion. Axion forms a cloud around a rotating black hole (BH) by superradiant instability and should affect the gravitational waveform from binary BHs. On the other hand, considering the cloud associated with a BH in a binary system, tidal interaction depletes the cloud in some cases during the inspiral phase. We made the exhaustive study of cloud depletion numerically in a wide parameter range for equal mass binaries, assuming only the quadrupolar tidal perturbation is at work. We found that clouds ca…
Lasing of ultralight axion condensate into photons can be sensitive to the presence of a background plasma owing to its coupling to electromagnetism. Such a scenario is particularly relevant for superradiant axion condensate around stellar mass black holes since the axion mass can be within a few orders of magnitude of the plasma frequency of the surrounding medium. In this paper I discuss the properties of the plasma around a black hole and analyze its effects on the lasing of a uniform axion condensate of mass of the order of the plasma frequency.
Blue axion isocurvature perturbations are both theoretically well-motivated and interesting from a detectability perspective. These power spectra generically have a break from the blue region to a flat region. Previous investigations of the power spectra were analytic, which left a gap in the predicted spectrum in the break region due to the non-applicability of the used analytic techniques. We therefore compute the isocurvature spectrum numerically for an explicit supersymmetric axion model. We find a bump that enhances the isocurvature signal for this class of scenarios. A fitting function o…
