The QCD axion can solve the Strong CP Problem and be the dark matter of our universe. If the PQ symmetry breaking scale associated with the axion is below the inflationary reheating temperature, axion strings and domain walls populate the universe. Most of these strings and walls decay away into axion dark matter, but a small subset of the walls will be self-enclosed surfaces that are not attached to any strings. These enclosed walls can collapse in on themselves, compressing a large amount of energy into a small volume and potentially forming primordial black holes (PBHs). We study the number density and dynamics of these self-enclosed walls, taking into account their size distribution, Hubble expansion, asphericities, and all stages of domain wall dynamics using a combination of semi-analytic and numerical approaches. We find that axion models with a high axion decay constant $f_a$, such as those of interest in early matter-dominated cosmologies, yield a PBH abundance potentially observable by future gravitational lensing surveys. We note that the formalism developed here is also useful for predicting relic PBH abundances in other models that exhibit unstable domain walls.
We argue that the existence of the cold dark matter is explained by primordial black holes.We show that a significant number of primordial black holes can be formed in an axion-like curvaton model, in which the highly blue-tilted power spectrum of primordial curvature perturbations is achieved.It is found that the produced black holes with masses $\sim 10^{20} -10^{38} \mathrm{g}$ account for the present cold dark matter.We also argue the possibility of forming the primordial black holes with mass $\sim 10^5 M_{\odot}$ as seeds of the supermassive black holes.
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}…
We calculate the rate for thermal production of axions and saxions via scattering of quarks, gluons, squarks, and gluinos in the primordial supersymmetric plasma. Systematic field theoretical methods such as hard thermal loop resummation are applied to obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling. We calculate the thermally produced yield and the decoupling temperature for both axions and saxions. For the generic case in which saxion decays into axions are possible, the emitted axions can constitute extra radiation already pri…
Production of axionlike particles (ALPs) by primordial magnetic fields may have significant impacts on cosmology. We discuss the production of ALPs in the presence of the primordial magnetic fields. We find a region of the ALP mass and photon coupling which realizes the observed properties of the dark matter with appropriate initial conditions for the magnetic fields. This region may be interesting in light of recent indications for the 3.5 keV lines from galaxy clusters. For a small axion mass, a region of previously allowed parameter spaces is excluded by overproduction of ALPs as a hot/warm…
The serious cosmological problems created by the axion-string/axion-domain-wall system in standard axion models are alleviated by positing the existence of a new confining force. The instantons of this force can generate an axion potential that erases the axion strings long before QCD effects become important, thus preventing QCD-generated axion walls from ever appearing. Axion walls generated by the new confining force would decay so early as not to contribute significantly to the energy in axion dark matter.
We discuss a possible connection between the recent NANOGrav results and the primordial black holes (PBHs) for the LIGO-Virgo events. In particular, we focus on the axion-like curvaton model, which provides a sizable amount of PBHs and GWs induced by scalar perturbations around the NANOGrav frequency range. The inevitable non-Gaussianity of this model suppresses the induced GWs associated with PBHs for the LIGO-Virgo events to be compatible with the NANOGrav results. We show that the axion-like curvaton model can account for PBHs for the LIGO-Virgo events and the NANOGrav results simultaneousl…
We study the formation of primordial black holes (PBHs) from the collapse of closed domain walls (DWs) which naturally arise in QCD axion models near the QCD scale together with the main string-wall network. The size distribution of the closed DWs is determined by percolation theory, from which we further obtain PBH mass distribution and abundance. Various observational constraints on PBH abundance in turn also constrain axion parameters. Our model prefers axion mass around the meV scale ($f_{a}\sim 10^{9}$ GeV). The corresponding PBHs are in the sublunar-mass window $10^{20}$-$10^{22}$ g (i.e…
An absorption dip in the spectrum of the cosmic microwave background observed by the EDGES experiment suggests an unexplained reduction of the hydrogen spin temperature at cosmic redshift z ~ 17. The mass of dark-matter axions could correspond to the hyperfine splitting of 5.9 micro-eV, between the triplet (H1) and singlet (H0) state. We calculate the rate for a+ H0 <-> H1 in two ways, and find that it is orders of magnitude smaller than the CMB-mediated transition rate, so irrelevant. As a result, this process cannot be used to rule in or out dark matter axions of mass = hyperfine splitting. …
We propose a mechanism to generate Primordial Black Holes (PBHs) which is independent of cosmological inflation and occurs slightly below the QCD phase transition. Our setup relies on the collapse of long-lived string-domain wall networks and is naturally realized in QCD axion models with domain wall number $N_{DW}>1$ and Peccei-Quinn symmetry broken after inflation. In our framework, dark matter is mostly composed of axions in the meV mass range along with a small fraction, $Ω_{\text{PBH}} \gtrsim 10^{-6} Ω_{\text{CDM}} $ of heavy $M \sim 10^4-10^7 M_\odot$ PBHs. The latter could play a role …
Lattice study of primordial black hole formation in bumpy axion inflation
Kavli IPMU (WPI), UTIAS, University of Tokyo（东京大学）；ICRR, University of Tokyo（东京大学）；Physics Division, Faculty of Science, Kanagawa University（神奈川大学）；RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS), RIKEN（理化学研究所）；Department of Physics, Tohoku University（东北大学）
We study the process of primordial black hole (PBH) formation at the beginning of radiation era for the cosmological scenario in which the inflaton is a pseudo-Nambu-Goldstone boson (axion) and there is a coupling of the inflaton with some gauge field. In this model inflation is accompanied by the gauge quanta production and a strong rise of the curvature power spectrum amplitude at small scales (along with non-Gaussianity) is predicted. We show that data on PBH searches can be used for a derivation of essential constraints on the model parameters in such an axion inflation scenario. We compar…
We study the cosmological evolution of domain walls bounded by strings which arise naturally in axion models. If we introduce a bias in the potential, walls become metastable and finally disappear. We perform two dimensional lattice simulations of domain wall networks and estimate the decay rate of domain walls. By using the numerical results, we give a constraint for the bias parameter and the Peccei-Quinn scale. We also discuss the possibility to probe axion models by direct detection of gravitational waves produced by domain walls.
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.
We show that an axionlike field coupled to a new confining gauge group can generate the primordial density perturbation in the post-inflation universe. The axion decay constant and strong coupling scale are uniquely determined by observations of the density perturbation, which further suggest a temporal deconfinement of the gauge group after inflation. The resulting temperature-dependent axion potential, together with its periodic nature, gives rise to the red-tilted density perturbation, with a positive local-type non-Gaussianity of order unity.
We revise primordial black hole (PBH) formation in the axion-like curvaton model and investigate whether PBHs formed in this model can be the origin of the gravtitational wave (GW) signals detected by the Advanced LIGO. In this model, small-scale curvature perturbations with large amplitude are generated, which is essential for PBH formation. On the other hand, large curvature perturbations also become a source of primordial GWs by their second-order effects. Severe constraints are imposed on such GWs by pulsar timing array (PTA) experiments. We also check the consistency of the model with the…
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 …
Axion Isocurvature Perturbations Survive the Scaling Evolution of Axion Domain Walls
Frontier Research Institute for Interdisciplinary Sciences, Tohoku University（东北大学前沿研究综合科学研究所）；Department of Physics, Tohoku University（东北大学物理系）；RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS), RIKEN（理化学研究所交叉理论数学科学中心(iTHEMS)）；Kavli IPMU (WPI), UTIAS, University of Tokyo（东京大学卡弗里宇宙物理学起源国际研究联盟(WPI)、UTIAS）；Department of Physics, Tokyo Metropolitan University（东京都立大学物理系）
LIGO-Virgo collaboration has found black holes as heavy as $M \sim 30M_\odot$ through the detections of the gravitational waves emitted during their mergers. Primordial black holes (PBHs) produced by inflation could be an origin of such events. While it is tempting to presume that these PBHs constitute all Dark Matter (DM), there exists a number of constraints for PBHs with $\mathcal{O} (10) M_\odot$ which contradict with the idea of PBHs as all DM. Also, it is known that weakly interacting massive particle (WIMP) that is a common DM candidate is almost impossible to coexist with PBHs. These o…
The primordial irreducible gravitational-wave background due to quantum vacuum tensor fluctuations produced during inflation spans a large range of frequencies with an almost scale-invariant spectrum but is too low to be detected by the next generation of gravitational-wave interferometers. We show how this signal is enhanced by a short temporary kination era in the cosmological history (less than 10 e-folds), that can arise at any energy scale between a GeV and the inflationary scale $10^{16}$ GeV. We argue that such kination era is naturally generated by a spinning axion before it gets trapp…
The domain wall problem of the Peccei-Quinn mechanism can be solved if the Peccei-Quinn symmetry is explicitly broken by a small amount. Domain walls decay into axions, which may account for dark matter of the universe. This scheme is however strongly constrained by overproduction of axions unless the phase of the explicit breaking term is tuned. We investigate the case where the universe is matter-dominated around the temperature of the MeV scale and domain walls decay during this matter dominated epoch. We show how the viable parameter space is expanded.
We analyze the spectrum of axions radiated from collapse of domain walls, which have received less attention in the literature. The evolution of topological defects related to the axion models is investigated by performing field-theoretic lattice simulations. We simulate the whole process of evolution of the defects, including the formation of global strings, the formation of domain walls and the annihilation of the defects due to the tension of walls. The spectrum of radiated axions has a peak at the low frequency, which implies that axions produced by the collapse of domain walls are not hig…
Axion models have two serious cosmological problems, domain wall and isocurvature perturbation problems. In order to solve these problems we investigate the Linde's model in which the field value of the Peccei-Quinn (PQ) scalar is large during inflation. In this model the fluctuations of the PQ field grow after inflation through the parametric resonance and stable axionic strings may be produced, which results in the domain wall problem. We study formation of axionic strings using lattice simulations. It is found that in chaotic inflation the axion model is free from both the domain wall and t…
We calculate the rate for thermal production of axions via scattering of quarks and gluons in the primordial quark-gluon plasma. To obtain a finite result in a gauge-invariant way that is consistent to leading order in the strong gauge coupling, we use systematic field theoretical methods such as hard thermal loop resummation and the Braaten-Yuan prescription. The thermally produced yield, the decoupling temperature, and the density parameter are computed for axions with a mass below 10 meV. In this regime, with a Peccei-Quinn scale above 6x10^8 GeV, the associated axion population can still b…
We investigate the cosmological constraints on axion models where the domain wall number is greater than one. In these models, multiple domain walls attached to strings are formed, and they survive for a long time. Their annihilation occurs due to the effects of explicit symmetry breaking term which might be raised by Planck-scale physics. We perform three-dimensional lattice simulations and compute the spectra of axions and gravitational waves produced by long-lived domain walls. Using the numerical results, we estimated relic density of axions and gravitational waves. We find that the existe…
The axion causes two serious cosmological problems, domain wall and isocurvature perturbation problems. Linde pointed that the isocurvature perturbations are suppressed when the Peccei-Quinn (PQ) scalar field takes a large value $\sim M_{\text{pl}}$ (Planck scale) during inflation. In this case, however, the PQ field with large amplitude starts to oscillate after inflation and large fluctuations of the PQ field are produced through parametric resonance, which leads to the formation of domain walls. We consider a supersymmetric axion model and examine whether domain walls are formed by using la…
We suggest the new feature of primordial gravitational waves sourced by the axion-gauge couplings, whose forms are motivated by the dimensional reduction of the form field in the string theory. In our inflationary model, as an inflaton we adopt two types of axion, dubbed the model-independent axion and the model-dependent axion, which couple with two gauge groups with different sign combination each other. Due to these forms both polarization modes of gauge fields are amplified and enhance both helicies of tensor modes during inflation. We point out the possibility that a primordial blue-tilte…
In the presence of primordial magnetic fields the oscillating cosmic axion field drives an oscillating electric field. The ensuing dissipation of axions is found to be inversely proportional to the conductivity of the primordial plasma. This counterintuitive result is essentially equivalent to ``Zeno's paradox'' or the ``watched-pot effect'' of quantum mechanics. It implies that the standard predictions of the cosmic axion density remain unaltered even if primordial magnetic fields are strong.
The recent BICEP2 result shows that the Universe once has gone through the vacuum with the GUT scale energy density. The implied high scale inflation nullifies the dilution idea of topological defects, strings and domain walls, of the axionic system. In particular, domain walls are disastrous if the domain wall problem with domain-wall number greater than 1 is present. We argue that the model-independent axion in string compactification with the anomalous U(1) gauge symmetry resolves the domain wall problem naturally with a symmetry principle.
We study the evolution of axions interacting with primordial magnetic fields (PMFs) starting just from the QCD phase transition in the expanding universe. This interaction is owing to the Primakoff effect. Adopting the zero mode approximation for axions, we derive the system of equations for axions and magnetic fields, where the expansion of the universe and the spectra of magnetic fields are accounted for exactly. We find that the contribution of the Primakoff effect to the dynamics of axions and magnetic fields is rather weak. It confirms some previous estimates leading to analogous conclusi…
We consider the scenario in which the Peccei-Quinn symmetry breaking is followed by a period of inflation. A particularly interesting case is that the string-domain wall network produced by the symmetry breaking enters the horizon after the QCD phase transition. We show that the abundance of axions produced by such a string-domain wall network is counterintuitively much larger than the conventional post-inflationary Peccei-Quinn symmetry breaking scenario. As a result, a scenario with the axion decay constant even as low as the astrophysical bound of about $10^8$ GeV can explain the observed a…
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.
Axion-like particles (ALPs) are a compelling candidate for dark matter (DM), whose production is associated with the formation of a string-wall network. If walls bounded by strings persist, which requires the potential to have multiple local minima ($N>1$), they must annihilate before they become dominant. They annihilate mostly into gravitational waves and non-relativistic ALPs. We show that for ALPs other than the QCD axion these gravitational waves, if produced at temperatures below 100 eV, could be detected by future cosmological probes for ALPs with mass from $10^{-16}$ to $10^{6}$ eV tha…
We propose a scenario where the spontaneous breakdown of the Peccei-Quinn symmetry leads to monopole production. Both the axion and the monopole contribute to dark matter, and the Witten effect on the axion mass is a built-in feature. In the KSVZ-type axion model, seemingly different vacua are actually connected by the hidden gauge symmetry, which makes the axionic string unstable and separate into two Alice strings. The Alice string is attached to a single domain wall due to the QCD instanton effect, solving the domain-wall problem. This is in the same spirit of the Lazarides-Shafi mechanism,…
We consider a cosmological scenario in which the very early Universe experienced a transient epoch of matter domination due to the formation of a large population of primordial black holes (PBHs) with masses $M \lesssim 10^{9}\,\textrm{g}$, that evaporate before Big Bang nucleosynthesis. In this context, Hawking radiation would be a non-thermal mechanism to produce a cosmic background of axion-like particles (ALPs). We assume the minimal scenario in which these ALPs couple only with photons. In the case of ultralight ALPs ($m_a \lesssim 10^{-9}\,\textrm{eV}$) the cosmic magnetic fields might t…
We show that axion domain walls gain spontaneous magnetization in early universe by trapping either electrons or positrons with their spins polarized. The reason is that the walls produces an attractive potential for these particles. We argue that the wall bounded by an axionic superconducting string leaves a magnetic field after its decay. We obtain a field $\sim 10^{-23}$ Gauss on the scale of horizon at the recombination.
Spontaneous Baryogenesis from Axions on Induced Electroweak Walls
Departament de Física Quàntica i Astrofísica and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona（巴塞罗那大学量子物理与天体物理系及宇宙科学研究所）；Department of Physics, Tokyo Metropolitan University（东京都立大学物理系）
Axions seem ubiquitous in string theories and some of them may be produced non-thermally by heavy scalar decays, contributing to dark radiation. We study various cosmological effects of photons produced from the axionic dark radiation through axion-photon conversion in the presence of primordial magnetic fields, and derive tight constraints on the combination of the axion-photon coupling and the primordial magnetic field.
The primordial black holes (PBHs) play as a novel source to radiate light elementary particles of energies in the region of a few hundred MeV. We explore the possibility that the axion-like particles (ALPs) with mass less than 1 MeV are produced from PBH evaporation. The absorption of light ALPs in the underground detector targets then induces energetic photoelectron signatures in current and future neutrino experiments. Utilizing the PBH ALP event rate, we place general exclusion limits on the axion couplings at Super-K and Hyper-K. We also translate these limits into the upper bound on the f…
The damping of the axionic string motion due to the surrounding plasma is estimated by a detailed calculation of the fermion scatter from a global string and it is shown that the friction force on the string can be neglected before the axionic domain wall formation in most cases, which is one of the important necessary conditions for the scaling distribution of the strings. The energy density of the axion produced by the collapse of the axionic domain walls is also evaluated using the axionic string distribution, and it is found to be larger or at least comparable to the axion density which or…
