arXiv subjects

Naoya Kitajima

Publications and source records attributed to Naoya Kitajima.

At least 19 recordsLinked to original sources

arXiv2026

Axion Isocurvature Perturbations Survive the Scaling Evolution of Axion Domain Walls

We revisit the evolution of axion domain walls seeded by inflationary fluctuations. In our previous work, we showed that such domain-wall networks retain superhorizon correlations even after entering the scaling regime. We extend our previous analysis to the case with large initial fluctuations, where many minima of the axion potential are already populated when the axion starts to oscillate. Although the conventional misalignment contribution can have suppressed long-wavelength isocurvature perturbations when many vacua are averaged over, axions produced by domain-wall collapse provide an additional contribution that can dominate when the walls enter the scaling regime before annihilation. In particular, the biased vacuum energy released during wall annihilation inherits the superhorizon correlations of the inflationary fluctuations and transfers them to the axion energy density. We find that sizable isocurvature perturbations can therefore survive even after the walls annihilate. We also discuss generic isocurvature constraints on dark matter produced by domain-wall collapse.

hep-ph↗

arXiv2026

Nambu-Goldstone emissions from the cosmological evolution of global monopoles

We show the emission of the Nambu-Goldstone (NG) bosons from the cosmological evolution of global monopoles. The NG bosons are non-thermally produced from the dynamics of global monopoles such as the pair annihilation of monopole and anti-monopole. Our numerical lattice simulations demonstrate that the spectrum of NG bosons emitted from the scaling evolution of global monopoles has a peak around the horizon scale, as in the case with the global string and the semilocal string. We also estimate the abundance of the pseudo-Nambu-Goldstone (pNG) dark matter when one (or both) of the NG modes has a soft mass.

hep-ph↗

arXiv2025

Primordial black hole formation from the merger of oscillons

We show that the merger of oscillons results in a broad spectrum of the oscillon mass. A huge number of oscillon samples obtained from numerical lattice simulations reveal that the oscillon mass distribution has an exponential tail in a heavy-mass region. This enables us to infer the fractional abundance of heavy oscillons. Using the criterion for the primordial black hole (PBH) formation from the oscillon collapse obtained in previous studies, we estimate the abundance of PBHs and conclude that a sizable number of PBHs can be produced from oscillons. It can be an alternative PBH formation mechanism without employing the tuning of the inflaton potential to enhance the small-scale density fluctuations in the conventional PBH formation scenario.

astro-ph.CO↗

arXiv2025

Primordial black hole formation from collapsing domain walls with full general relativity

We study the dynamics of isolated closed domain walls with 3+1 numerical relativity. A closed wall shrinks due to its own surface tension, and its surface energy is converted to the kinetic energy, leading to implosion. Then, it can result in the formation of a black hole. First, we focus on spherically symmetric closed domain walls and clarify whether they finally evolve into black holes. Naively, the wall can collapse if its thickness is smaller than the Schwarzschild radius which is determined by the initial surface energy. Our numerical results support this naive criterion for the black hole formation, and indicate that more than 80% of the initial wall energy falls into the black hole. We also investigate the nonspherical collapse by considering the ellipsoidal configurations for the closed domain walls, and it turns out that black holes can be formed even when the ratio of semi-major to semi-minor axes is 1.5.

gr-qc↗

arXiv2025

Multimodal axion emissions from Abelian-Higgs cosmic strings

We show that axions can be produced from Abelian-Higgs cosmic strings due to the axion-gauge coupling. The strong magnetic field is confined in the string, and the electric field is induced around the moving string, allowing axion productions from the dynamics of cosmic strings. Our numerical analysis on the string collision shows that a sizable number of axions can be produced at the reconnection, and further emissions occur from moving kinks afterward. Large-scale lattice simulations of the string network further reveal multimodal axion emissions in the sense that axions are produced in both the low-energy and high-energy regimes. The former can contribute to the cold dark matter and the latter can be regarded as dark radiation. We found that the axion with GeV or heavier mass can explain the current relic dark matter abundance and simultaneously predicts a sizable amount of dark radiation which can be probed by future observations.

hep-ph↗

arXiv2025

Emission of Nambu-Goldstone bosons from the semilocal string network

Semilocal cosmic string is a line-like non-topological soliton associated with the breakdown of the $SU(2)_{\rm global} \times U(1)_{\rm gauge}$ symmetry to the $U(1)_{\rm global}$ symmetry. The broken phase has two massless Nambu-Goldstone (NG) modes as dynamical fields, and they can be emitted by semilocal strings. In this paper, we numerically show that such NG bosons are copiously produced with the evolution of the semilocal string network in the early universe. Our numerical analysis shows that the spectrum of produced particles has a peak at low momenta corresponding to the horizon scale. If the emitted NG bosons acquire mass due to soft-breaking terms, they can take the role of dark matter. This scenario typically predicts very light pseudo NG boson dark matter.

hep-ph↗

arXiv2025

Abelian-Higgs vortices in the oscillating axion background

We study the dynamics of Abelian-Higgs vortices in the background of a coherently oscillating axion field. We show that the electric field is induced in the magnetic core of the vortex due to the axion-photon conversion. Moreover, because the electromagnetic field is confined in the vortex and excluded from the superconducting bulk regions due to the Meissner effect, the vortex tube can be regarded as a cylindrical cavity, and our numerical analysis shows that the resonant cavity mode (TM010 mode) can be efficiently enhanced in this tube. We also focus on the interaction of two vortices in the oscillating axion background, resulting in attractive or repulsive forces, even in the case with the BPS limit. These new features open up a new possibility for the axion dark matter search using superconducting devices.

hep-ph↗

arXiv2024

A Bound on Light Dark Photon Dark Matter

We derive a bound on dark photon dark matter scenarios where the dark photon mass is generated through the Higgs mechanism, based on the requirement that symmetry breaking must occur sufficiently early in the universe. We emphasize that dark photon production occurs successfully when the dark Higgs field remains in the symmetric phase due to non-thermal trapping effects. For renormalizable Higgs potentials, our bound reads $$\frac{m_{\gamma'}}{q_H e_H}\;\gg \;60\,{\rm eV}\left(\frac{2\pi}{\lambda}\right)^{1/4}$$ where $m_{\gamma'}$ is the dark photon mass, $e_H$ is the gauge coupling, $q_H$ is the charge of the dark Higgs boson, and $\lambda$ is the Higgs quartic coupling}. This constraint holds independently of any complications arising from the Schwinger effect and vortex formation in the Higgsed phase. For more general Higgs potentials such as the Coleman-Weinberg type potential, our bound yields different forms. We argue that late-time symmetry breaking of the dark U(1) symmetry satisfying our bound has only a mild impact on both the abundance and momentum distribution of dark photon dark matter, and therefore does not pose any serious problem for the dark photon dark matter scenario.

hep-ph↗

arXiv2024

Non-Gaussianity from primordial black holes

We study the effects of non-Gaussianity from primordial black holes (PBHs). The formation of PBHs is in general a rare event and the number of PBHs fluctuates following the Poisson distribution function, which is independent from the pre-existing inflationary adiabatic fluctuations. Such fluctuations can dominate over the adiabatic mode on small scales. We focus on the non-Gaussianity of matter density fluctuations induced by the Poisson fluctuation of PBHs and discuss the potentially observable consequences such as the skewness, kurtosis and the scale-dependent bias.

astro-ph.CO↗

arXiv2023

Stability of domain walls with inflationary fluctuations under potential bias, and gravitational wave signatures

A recent study has shown that domain walls with inflationary initial fluctuations exhibit remarkable stability against population bias due to long-range correlations, challenging the claims of prior research. In this paper, we study the dynamics of these domain walls in the presence of potential bias and show that they collapse with a lifetime several times longer than that due to thermal fluctuations. This is interpreted as a difference in the average distance between domain walls, leading us to derive a new formula for the domain wall lifetime which depends on the area parameter in a qualitatively different way from previous studies. In addition, we compute the spectrum of gravitational waves generated by such domain walls and find that both the peak frequency and the peak abundance are lowered in a manner that depends on the area parameter. Based on these findings, we also determine the necessary degree of vacuum degeneracy for axion domain walls to explain the isotropic cosmic birefringence.

hep-ph↗

arXiv2023

Primordial Origin of Supermassive Black Holes from Axion Bubbles

We study a modification of the primordial black hole (PBH) formation model from axion bubbles. We assume that the Peccei-Quinn scalar rolls down in the radial direction from a large field value to the potential minimum during inflation, which suppresses the axion fluctuations and weakens the clustering of PBHs on large scales. We find that the modified model can produce a sufficient number of PBHs that seed the supermassive black holes while avoiding the observational constraints from isocurvature perturbations and angular correlation of the high-redshift quasars.

astro-ph.CO↗

arXiv2023

Gravitational Waves from Domain Wall Collapse, and Application to Nanohertz Signals with QCD-coupled Axions

We study for the first time the gravitational waves generated during the collapse of domain walls, incorporating the potential bias in the lattice simulations. The final stages of domain wall collapse are crucial for the production of gravitational waves, but have remained unexplored due to computational difficulties. As a significant application of this new result, we show that the observed NANOGrav, EPTA, PPTA, and CPTA data, which indicate stochastic gravitational waves in the nanohertz regime, can be attributed to axion domain walls coupled to QCD. In our model, non-perturbative effects of QCD induce a temperature-dependent bias around the QCD crossover, inducing the rapid collapse of the domain walls. We use sophisticated lattice simulations that account for the temperature-dependent bias to measure the gravitational waves resulting from the domain wall annihilation. We also discuss the future prospects for accelerator-based searches for the axion and the potential for the formation and detection of primordial black holes.

hep-ph↗

arXiv2023

Dark Photon Dark Matter from Cosmic Strings and Gravitational Wave Background

Dark photon dark matter may be produced by the cosmic strings in association with the dark U(1) gauge symmetry breaking. We perform three-dimensional lattice simulations of the Abelian-Higgs model and follow the evolution of cosmic strings. In particular, we simulate the case of (very) light vector boson and find that such vector bosons are efficiently produced by the collapse of small loops while the production is inefficient in the case of heavy vector boson. We calculate the spectrum of the gravitational wave background produced by the cosmic string loops for the light vector boson case and find characteristic features in the spectrum, which can serve as a probe of the dark photon dark matter scenario. In particular, we find that the current ground-based detectors may be sensitive to such gravitational wave signals and also on-going/future pulsar timing observations give stringent constraint on the dark photon dark matter scenario.

hep-ph↗

arXiv2023

Nanohertz gravitational waves from cosmic strings and dark photon dark matter

The recent observations by pulsar timing array (PTA) experiments suggest the existence of stochastic gravitational wave background in the nano-Hz range. It can be a hint for the new physics and cosmic string is one of the promising candidate. In this paper, we study the implication of the PTA result for cosmic strings and dark photon dark matter produced by the decay of cosmic string loops. It can simultaneously explain the PTA result and present dark matter abundance for the dark photon mass m~10^{-6}--10^{-4}eV. Implications for the gravitational wave detection with multi-frequency bands are also discussed.

hep-ph↗

arXiv2023

Stochastic gravitational wave background from early dark energy

We study the production of stochastic gravitational wave background from early dark energy (EDE) model. It is caused by resonant amplification of scalar field fluctuations, which easily takes place for typical EDE potential based on the string axion or $α$-attractor model. The resultant spectrum of gravitational wave background is computed by performing 3D lattice simulations. We show that, specifically in some class of generalized $α$-attractor EDE model, a significant amount of gravitational waves can be produced via tachyonic instability with a peak around femto-Hz frequency range. Models predicting such gravitational waves can be constrained by the cosmic microwave background observations.

astro-ph.CO↗

arXiv2023

Clustering of Primordial Black Holes from QCD Axion Bubbles

We study the clustering of primordial black holes (PBHs) and axion miniclusters produced in the model proposed to explain the LIGO/Virgo events or the seeds of the supermassive black holes (SMBHs) in arXiv:2006.13137. It is found that this model predicts large isocurvature perturbations due to the clustering of PBHs and axion miniclusters, from which we obtain stringent constraints on the model parameters. Specifically, for the axion decay constant $f_a=10^{16}~\mathrm{GeV}$, which potentially accounts for the seeds of the SMBHs, the PBH fraction in dark matter should be $f_\mathrm{PBH}\lesssim7\times 10^{-10}$. Assuming that the mass of PBHs increases by more than a factor of $\mathcal{O}(10)$ due to accretion, this is consistent with the observed abundance of SMBHs. On the other hand, for $f_a=10^{17}~\mathrm{GeV}$ required to produce PBHs of masses detected in the LIGO/Virgo, the PBH fraction should be $f_\mathrm{PBH}\lesssim6\times 10^{-8}$, which may be too small to explain the LIGO/Virgo events, although there is a significant uncertainty in calculating the merger rate in the presence of clustering.

astro-ph.CO↗

arXiv2023

Resonant production of dark photons from axion without a large coupling

Dark photons could be produced resonantly by the oscillating axion field in the early universe. This resonant production mechanism has been used in various contexts, including dark photon dark matter and primordial magnetic field production. However, for this resonant production to work in an expanding universe, a large axion-dark photon coupling is required, which is not easy to realize in terms of model building and requires the introduction of many charged fermions and/or the complex clockwork mechanism. In this paper, we present a new scenario that efficiently produces dark photons from the axion with a much smaller coupling. This is possible by modifying the dynamics of axion and significantly delaying the onset of oscillations, as in the so-called trapped misalignment mechanism. As a specific example, we consider models in which dark photon production occurs efficiently despite the small axion-dark photon coupling by temporally trapping an axion in a wrong minimum and releasing it after the Hubble parameter becomes much smaller than the axion mass. In this scenario, it is expected that the polarization asymmetry of dark photons and gravitational waves generated from dark photons will be significantly reduced.

hep-ph↗

arXiv2023

Viable Vector Coherent Oscillation Dark Matter

We construct a viable model of the vector coherent oscillation dark matter. The vector boson is coupled to the inflaton through the kinetic function so that the effective Hubble mass term is cancelled out. In order to avoid strong constraints from isocurvature perturbation and statistically anisotropic curvature perturbation, the inflaton is arranged so that it does not contribute to the observed large scale curvature perturbation and we introduce a curvaton. We found viable vector coherent oscillation dark matter scenario for the wide vector mass range from $10^{-21}\,{\rm eV}$ to $1\,{\rm eV}$.

hep-ph↗