arXiv subjects

Masahiro Kawasaki

Publications and source records attributed to Masahiro Kawasaki.

At least 55 records · Page 3Linked to original sources

arXiv2021

NANOGrav results and LIGO-Virgo primordial black holes in axion-like curvaton model

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 simultaneously.

astro-ph.CO↗

arXiv2021

Oscillons of Axion-Like Particle: Mass distribution and power spectrum

In string theory, the simultaneous existence of many Axion-Like Particles (ALPs) are suggested over a vast mass range, and a variety of potentials have been developed in the context of inflation. In such potentials shallower than quadratic, the prominent instability can produce localized dense objects, oscillons. Because of the approximate conservation of their adiabatic invariant, oscillons generally survive quite long, maybe up to the current age of the universe in the case of ultra-light ALPs with $m \sim 10^{-22}\ {\rm eV}$. Such oscillons can have significant effects on the evolution of the recent universe. In this paper, we investigate the oscillons of the pure-natural type potential by classical lattice simulation to explore the key quantities necessary for phenomenological application: the number density of oscillons, the oscillon mass distribution, the energy ratio of oscillons to the ALP field, and the power spectrum. Then, we evolve these values in consideration of the analytic decay rate.

astro-ph.CO↗

arXiv2020

Affleck-Dine inflation in supergravity

Affleck-Dine inflation is a recently proposed model in which a single complex scalar field, nonminimally coupled to gravity, drives inflation and simultaneously generates the baryon asymmetry of universe via Affleck-Dine mechanism. In this paper we investigate the supersymmetric implementation of Affleck-Dine inflation in the use of two chiral superfields with appropriate superpotential and Kähler potential. The scalar potential has a similar form to the potential of original Affleck-Dine inflation, and it gives successful inflation and baryogenesis. We also consider the isocurvature perturbation evolving after crossing the horizon, and find that it is ignorable and hence consistent with the observations.

hep-ph↗

arXiv2020

Probing Oscillons of Ultra-Light Axion-like Particle by 21cm Forest

Ultra-Light Axion-like Particle (ULAP) is motivated as one of the solutions to the small scale problems in astrophysics. When such a scalar particle oscillates with an $\mathcal{O}(1)$ amplitude in a potential shallower than quadratic, it can form a localized dense object, oscillon. Because of its longevity due to the approximate conservation of the adiabatic invariant, it can survive up to the recent universe as redshift $z \sim \mathcal{O}(10)$. The scale affected by these oscillons is determined by the ULAP mass $m$ and detectable by observations of 21cm line. In this paper, we examine the possibility to detect ULAP by 21cm line and find that the oscillon can enhance the signals of 21cm line observations when $m \lesssim 10^{-19}\ {\rm eV}$ and the fraction of ULAP to dark matter is much larger than $10^{-2}$ depending on the form of the potential.

astro-ph.CO↗

arXiv2020

Big Bang Nucleosynthesis constraints on sterile neutrino and lepton asymmetry of the Universe

We consider the cosmological effects of sterile neutrinos with the masses of $150- 450$ MeV. The decay of sterile neutrinos changes the thermal history of the Universe and affects the energy density of radiation at the recombination and the big bang nucleosynthesis (BBN) results. We derive severe constraints on the parameters of sterile neutrinos from the primordial abundances of helium-4 and deuterium. We also find that in a particular model the constraints can be considerably relaxed by assuming a large lepton asymmetry in the active neutrinos. In this case, the consistent parameters result in $N_{\mathrm{eff}} \simeq 3.2- 3.4$ and can alleviate the Hubble tension.

hep-ph↗

arXiv2020

Big-bang nucleosynthesis with sub-GeV massive decaying particles

We consider the effects of the injections of energetic photon and electron (or positron) on the big-bang nucleosynthesis. We study the photodissociation of light elements in the early Universe paying particular attention to the case that the injection energy is sub-GeV and derive upper bounds on the primordial abundances of the massive decaying particle as a function of its lifetime. We also discuss a solution of the $^7$Li problem in this framework.

hep-ph↗

arXiv2020

Gravitational Wave Production right after a Primordial Black Hole Evaporation

We discuss the footprint of evaporation of primordial black holes (PBHs) on stochastic gravitational waves (GWs) induced by scalar perturbations. We consider the case where PBHs once dominated the Universe but eventually evaporated before the big bang nucleosynthesis. The reheating through the PBH evaporation could end with a sudden change in the equation of state of the Universe compared to the conventional reheating caused by particle decay. We show that this "sudden reheating" by the PBH evaporation enhances the induced GWs, whose amount depends on the length of the PBH-dominated era and the width of the PBH mass function. We explore the possibility to constrain the primordial abundance of the evaporating PBHs by observing the induced GWs. We find that the abundance parameter $β\gtrsim 10^{-5} \text{ - }10^{-8}$ for $\mathcal{O}(10^3 \text{ - } 10^5) \, \text{g}$ PBHs can be constrained by future GW observations if the width of the mass function is smaller than about a hundredth of the mass.

astro-ph.CO↗

arXiv2020

Q-ball decay through A-term in the gauge-mediated SUSY breaking scenario

Q-balls are non-topological solitons whose classical stability is ensured by a global $U(1)$ charge. In particular, Q-balls are produced in the framework of the Affleck-Dine baryogenesis where $U(1)$ charge is the baryon number. Since some type of Q-balls is stable against quantum decay into nucleons, it can work as the dark matter. When the dark matter Q-balls are captured into the neutron star, they absorb the surrounding neutrons and grow to consume all the neutrons, which leads to a stringent constraint on the Q-ball dark matter. However, the Q-ball growth stops due to the $U(1)$ breaking A-term. We revisit the constraint by the neutron star for the gauge-mediation type and the new type Q-ball, and find the allowed parameter space, in which Q-balls work as the dark matter.

hep-ph↗

arXiv2020

Revisiting oscillon formation in KKLT scenario

KKLT scenario has succeeded in stabilizing the volume modulus and constructing metastable de Sitter (dS) vacua in type IIB string theory. We revisit to investigate the possibility of the oscillon (or I-ball) formation in the KKLT scenario when the volume modulus is initially displaced from the dS minimum. Special attention is paid to physically realistic initial conditions of the volume modulus, which was not taken in the literature. Using lattice simulations, we find that oscillons do not form unless the volume modulus is initially placed at very near the local maximum, which requires severe fine-tuning.

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arXiv2019

Generation of Primordial Black Holes and Gravitational Waves from Dilaton-Gauge Field Dynamics

We study the observational signatures from particle production of a $U(1)$ gauge field kinetically coupled to an inflaton. Regarding the form of gauge kinetic function, we consider the possibility that it becomes stabilized at a certain time, which makes the growing power of the gauge field evolve non-monotonically with a sharp transition. Remarkably, the copious production of the gauge field occurs on super-horizon scales at the late stage of inflation and perturbations are enhanced on the intermediate scales during inflation. We find that it can predict a bumpy shape of the curvature power spectrum which leads to the generation of primordial black holes as a dark matter after inflation. We also estimate two types of tensor modes sourced by the gauge field: the primordial gravitational waves generated during inflation and the induced gravitational waves provided by the enhanced curvature perturbation after inflation. We show that both of them are potentially testable with the future space-based gravitational wave interferometers.

astro-ph.CO↗

arXiv2019

Oscillon of Ultra-Light Axion-like Particle

Ultra-light axion-like particle (ULAP) is one of attractive candidates for cold dark matter. Because the de Broglie wavelength of ULAP with mass $\sim 10^{-22} {\rm eV}$ is $\mathcal{O}({\rm kpc})$, the suppression of the small scale structure by the uncertainty principle can solve the core-cusp problem. Frequently, ULAP is assumed to be uniformly distributed in the present universe. In typical ULAP potentials, however, strong self-resonance at the beginning of oscillation invokes the large fluctuations, which may cause the formation of the dense localized object, oscillon. % Such a dense object lives for a long time, it may affect the cosmological evolution. In this paper, we confirm the oscillon formation in a ULAP potential by numerical simulation and analytically derive its lifetime.

astro-ph.CO↗

arXiv2019

Cosmologically allowed regions for the axion decay constant $F_a$

If the Peccei-Quinn symmetry is already broken during inflation, the decay constant $F_a$ of the axion can be in a wide region from $10^{11}$ GeV to $10^{18}$ GeV for the axion being the dominant dark matter. In this case, however, the axion causes the serious cosmological problem, isocurvature perturbation problem, which severely constrains the Hubble parameter during inflation. The constraint is relaxed when Peccei-Quinn scalar field takes a large value $\sim M_{p}$ (Planck scale) during inflation. In this letter, we point out that the allowed region of the decay constant $F_a$ is reduced to a rather narrow region for a given tensor-to-scalar ratio $r$ when Peccei-Quinn scalar field takes $\sim M_{p}$ during inflation. For example, if the ratio $r$ is determined as $r \gtrsim 10^{-3}$ in future measurements, we can predict $F_a \simeq (0.1-1.4)\times 10^{12}$ GeV for domain wall number $N_\text{DW}=6$.

hep-ph↗

arXiv2019

Domain wall and isocurvature perturbation problems in a supersymmetric axion model

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 lattice simulation. It is found that the domain wall problem does not appear in the SUSY axion model when the initial value of the PQ field is less than $10^{3}\times v$ where $v$ is the PQ symmetry breaking scale.

hep-ph↗

arXiv2019

Fragileness of Exact I-ball/Oscillon

I-ball/oscillon is a soliton-like oscillating configuration of a real scalar field which lasts for a long time. I-ball/oscillon is a minimum energy state for a given adiabatic invariant, and its approximate conservation guarantees the longevity. In this paper, we examine the stability of a special type of I-ball/oscillon, the "exact" I-ball/oscillon, whose adiabatic invariant is exactly conserved. We show that the exact I-ball/oscillon is stable in classical field theory, but not stable against small perturbations depending on the value of its adiabatic invariant. Accordingly, the exact I-ball/oscillon breaks up in the presence of the fluctuations with corresponding instability modes. We also confirm the fragileness of the exact I-ball/oscillon by the classical lattice simulation.

hep-ph↗

arXiv2019

Formation of supermassive primordial black holes by Affleck-Dine mechanism

We study the supermassive black holes (SMBHs) observed in the galactic centers. Although the origin of SMBHs has not been well understood yet, previous studies suggest that seed black holes (BHs) with masses $10^{4-5}M_\odot$ exist at a high redshift ($z \sim 10$). We examine whether primordial black holes (PBHs) produced by inhomogeneous baryogenesis can explain those seed black holes. The inhomogeneous baryogenesis is realized in the modified Affleck-Dine mechanism. In this scenario, there is no stringent constraint from CMB $μ$-distortion in contrast to the scenario where Gaussian fluctuations collapse into PBHs. It is found that the model can account for the origin of the seed BHs of the SMBHs.

astro-ph.CO↗

arXiv2019

Decay of I-ball/Oscillon in Classical Field Theory

I-balls/oscillons are long-lived and spatially localized solutions of real scalar fields. They are produced in various contexts of the early universe in, such as, the inflaton evolution and the axion evolution. However, their decay process has long been unclear. In this paper, we derive an analytic formula of the decay rate of the I-balls/oscillons within the classical field theory. In our approach, we calculate the Poynting vector of the perturbation around the I-ball/oscillon profile by solving a relativistic field equation, with which the decay rate of the I-ball/oscillon is obtained. We also perform a classical lattice simulation and confirm the validity of our analytical formula of the decay rate numerically.

hep-ph↗

arXiv2019

Initial Li Abundances in the Protogalaxy and Globular Clusters Based upon the Chemical Separation and Hierarchical Structure Formation

The chemical separation of Li+ ions induced by a magnetic field during the hierarchical structure formation can reduce initial Li abundances in cosmic structures. It is shown that cosmological reionization of neutral Li atoms quickly completes as soon as the first star is formed. Since almost all Li is singly ionized during the main course of structure formation, it can efficiently separate from gravitationally collapsing neutral gas. The separation is more efficient in smaller structures which formed earlier. In the framework of the hierarchical structure formation, extremely metal-poor stars can have smaller Li abundances because of their earlier formations. It is found that the chemical separation by a magnetic field thus provides a reason that Li abundances in extremely metal-poor stars are lower than the Spite plateau and have a large dispersion as well as an explanation of the Spite plateau itself. In addition, the chemical separation scenario can explain Li abundances in NGC 6397 which are higher than the Spite plateau. Thus, Li abundances in metal-poor stars possibly keep information on the primordial magnetic field and the structure formation history.

astro-ph.SR↗

arXiv2019

Effect of nonlinearity between density and curvature perturbations on the primordial black hole formation

We study the effect of the nonlinear relation between density and curvature perturbations on the formation of PBHs. By calculating the variance and skewness of the density perturbation we derive the non-Gaussian property. As a criterion for PBH formation, the compaction function is used and it is found that larger curvature perturbations are required due to the nonlinear effect. We estimate the PBH abundance based on the Press-Schechter formalism with non-Gaussian probability density function during Radiation dominated era. It is found that the nonlinear effect slightly suppresses the PBH formation and the suppression is comparable to that expected if the primordial curvature perturbation would have the local form of non-Gaussianity with nonlinear parameter $f_\text{NL} \sim -1$.

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