arXiv subjects

Jiro Soda

Publications and source records attributed to Jiro Soda.

At least 73 records · Page 4Linked to original sources

arXiv2018

Cosmological imprints of string axions in plateau

We initiate a study on various cosmological imprints of string axions whose scalar potentials have plateau regions. In such cases, we show that a delayed onset of oscillation generically leads to a parametric resonance instability. In particular, for ultralight axions, the parametric resonance can enhance the power spectrum slightly below the Jeans scale, alleviating the tension with the Lyman $α$ forest observations. We also argue that a long-lasting resonance can lead to an emission of gravitational waves at the frequency bands which are detectable by gravitational wave interferometers and pulsar timing arrays.

astro-ph.CO↗

arXiv2018

Possible detection of nonclassical primordial gravitational waves with Hanbury Brown - Twiss interferometry

We consider possible detection of nonclassicality of primordial gravitational waves (PGWs) by applying Hanbury Brown - Twiss (HBT) interferometry to cosmology. We characterize the nonclassicality of PGWs in terms of sub-Poissonian statistics that can be measured by the HBT interferometry. We show that the presence of classical sources during inflation makes us possible to detect nonclassical PGWs with the HBT interferometry. We present two examples that realize the classical sources during inflation. It turns out that PGWs with frequencies higher than 10 kHz enable us to detect their nonclassicality.

hep-th↗

arXiv2018

Stability of Squashed Kaluza-Klein Black Holes

The stability of squashed Kaluza-Klein black holes is studied. The squashed Kaluza-Klein black hole looks like five dimensional black hole in the vicinity of horizon and four dimensional Minkowski spacetime with a circle at infinity. In this sense, squashed Kaluza-Klein black holes can be regarded as black holes in the Kaluza-Klein spacetimes. Using the symmetry of squashed Kaluza-Klein black holes, $SU(2)\times U(1)\simeq U(2)$, we obtain master equations for a part of the metric perturbations relevant to the stability. The analysis based on the master equations gives a strong evidence for the stability of squashed Kaluza-Klein black holes. Hence, the squashed Kaluza-Klein black holes deserve to be taken seriously as realistic black holes in the Kaluza-Klein spacetime.

hep-th↗

arXiv2018

Conversion of Gravitons into Dark Photons in Cosmological Dark Magnetic Fields

It is well known that gravitons can convert into photons, and vice versa, in the presence of cosmological magnetic fields. We study this conversion process in the context of atomic dark matter scenario. In this scenario, we can expect cosmological dark magnetic fields, which are free from the stringent constraint from the cosmic microwave observations. We find that gravitons can effectively convert into dark photons in the presence of cosmological dark magnetic fields. The graviton-dark photon conversion effect may open up a new window for ultra high frequency gravitational waves.

astro-ph.CO↗

arXiv2018

Gravitational wave forest from string axiverse

Axions predicted in string theory may have a scalar potential which has a much shallower potential region than the conventional cosine potential. We first show that axions which were located at such shallow potential regions generically undergo prominent resonance instabilities: the well-known narrow resonance and/or the flapping resonance, which has not been well investigated. We also study non-linear dynamics of axions caused by these resonance instabilities based on lattice simulation. We find that string axions in various mass ranges generate gravitational waves (GWs) with peaks at various frequencies determined by the mass scales, dubbed the GW forest. This may allow us to explore string axiverse through future multi-frequency GW observations. We also investigate GWs produced by the axion which accounts for present dark matter component.

astro-ph.CO↗

arXiv2018

Exploring the string axiverse and parity violation in gravity with gravitational waves

We show that the parametric resonance of gravitational waves occurs due to the axion coherent oscillation and the circular polarization of gravitational waves is induced by the Chern-Simons coupling. However, we have never observed these signatures in the data of gravitational waves. Using this fact, we give stringent constraints on the Chern-Simons coupling constant $\ell$ and the abundance of the light string axion. In particular, we improved the current bound $\ell\leq10^{8}\,\,{\rm km}$ by many orders of magnitude.

gr-qc↗

arXiv2018

Anisotropic Constant-roll Inflation

We study constant-roll inflation in the presence of a gauge field coupled to an inflaton. By imposing the constant anisotropy condition, we find new exact anisotropic constant-roll inflationary solutions which include anisotropic power-law inflation as a special case. We also numerically show that the new anisotropic solutions are attractors in the phase space.

hep-th↗

arXiv2018

Schwinger Pair Production by Electric Field Coupled to Inflaton

We analytically investigate the Schwinger pair production in the de Sitter background by using {\em the uniform asymptotic approximation method}, and show that the equation of motion in general has two turning points, and the nature of these points could be single, double, real or complex, depending on the choice of the free parameters involved in the theory. Different natures of these points lead to different electric currents. In particular, when $β\equiv m^2/H^2-9/4$ is positive, both turning points are complex, and the electric current due to the Schwinger process is highly suppressed, where $m$ and $H$ denote, respectively, the mass of the particle and the Hubble parameter. For the turning points to be real, it is necessary to have $β< 0$, and the more negative of $β$, the easier to produce particles. In addition, when $β< 0$, we also study the particle production when the electric field $E$ is very weak. We find that the electric current in this case is proportional to $E^{1/2 - \sqrt{|β|}}$, which is strongly enhanced in the weak electric field limit when $m < \sqrt{2} H$.

gr-qc↗

arXiv2017

Primordial Gravitational Waves Induced by Magnetic Fields in an Ekpyrotic Scenario

Both inflationary and ekpyrotic scenarios can account for the origin of the large scale structure of the universe. It is often said that detecting primordial gravitational waves is the key to distinguish both scenarios. We show that this is not true if the gauge kinetic function is present in the ekpyrotic scenario. In fact, primordial gravitational waves sourced by the gauge field can be produced in an ekpyrotic universe. We also study scalar fluctuations sourced by the gauge field and show that it is negligible compared to primordial gravitational waves. This comes from the fact that the fast roll condition holds in ekpyrotic models.

hep-th↗

arXiv2017

Electromagnetic waves propagating in the string axiverse

It is widely believed that axions are ubiquitous in string theory and could be the dark matter. The peculiar features of the axion dark matter are coherent oscillations and a coupling to the electromagnetic field through the Chern-Simons term. In this paper, we study consequences of these two features of the axion with the mass in a range from $10^{-13}\,{\rm eV}$ to $10^{3}\,{\rm eV}$. First, we study the parametric resonance of electromagnetic waves induced by the coherent oscillation of the axion. As a result of the resonance, the amplitude of the electromagnetic waves is enhanced and the circularly polarized monochromatic waves will be generated. Second, we study the velocity of light in the background of the axion dark matter. In the presence of the Chern-Simons term, the dispersion relation is modified and the speed of light will oscillate in time. It turns out that the change of speed of light would be difficult to observe. We argue that the future radio wave observations of the resonance can give rise to a stronger constraint on the coupling constant and/or the density of the axion dark matter.

hep-th↗

arXiv2017

Electromagnetic Memory Effect Induced by Axion Dark Matter

Memory effects of gravitational waves from astronomical events or primordial universe might have the information of new physics. It is intriguing to observe that the memory effect exists in electrodynamics as a net momentum kick, while the memory effect in gravity appears as a net relatively displacement. In particular, Winicour has shown that the B-mode memory, which characterizes parity odd global distribution of memory, does not exist. We study the memory effect in axion electrodynamics and find that the B-mode memory effect can exist provided the existence of coherently oscillating axion background field. Moreover, we examine the detectability of the axion dark matter using this effect. We also argue the existence of the B-mode gravitational memory effect in the presence of the axion dark matter.

gr-qc↗

arXiv2017

Photon-Axion Conversion, Magnetic Field Configuration, and Polarization of Photons

We study the evolution of photon polarization during the photon-axion conversion process with focusing on the magnetic field configuration dependence. Most previous studies have been carried out in a conventional model where a network of magnetic domains is considered and each domain has a constant magnetic field. We investigate a more general model where a network of domains is still assumed, but each domain has a helical magnetic field. We find that the asymptotic behavior does not depend on the configuration of magnetic fields. Remarkably, we analytically obtain the asymptotic values of the variance of polarization in the conventional model. When the helicity is small, we show that there appears the damped oscillating behavior in the early stage of evolution. Moreover, we see that the constraints on the axion coupling and the cosmological magnetic fields using polarization observations are affected by the magnetic field configuration. This is because the different transient behavior of polarization dynamics is caused by the different magnetic field configuration. Recently, [C. Wang and D. Lai, J. Cosmol. Astropart. Phys. 06 (2016) 006.] claimed that the photon-axion conversion in helical model behaves peculiarly. However, our helical model gives much closer predictions to the conventional discontinuous magnetic field configuration model.

astro-ph.CO↗

arXiv2017

Nonlinear resonant oscillation of gravitational potential induced by ultralight axion in $f(R)$ gravity

We study the ultralight axion dark matter with mass around $10^{-22}$ eV in $f(R)$ gravity which might resolve the dark energy problem. In particular, we focus on the fact that the pressure of the axion field oscillating in time produces oscillations of gravitational potentials. We show that the oscillation of the gravitational potential is sensitive to the model of gravity. Remarkably, we find that the detectability of the oscillation through the gravitational wave detectors can be significantly enhanced due to the nonlinear resonance between the ultralight axion and the scalaron.

astro-ph.CO↗

arXiv2017

MHz Gravitational Waves from Short-term Anisotropic Inflation

We reveal the universality of short-term anisotropic inflation. As a demonstration, we study inflation with an exponential type gauge kinetic function which is ubiquitous in models obtained by dimensional reduction from higher dimensional fundamental theory. It turns out that an anisotropic inflation universally takes place in the later stage of conventional inflation. Remarkably, we find that primordial gravitational waves with a peak amplitude around $10^{-26}$ ~ $10^{-27}$ are copiously produced in high-frequency bands 10MHz~100MHz. If we could detect such gravitational waves in future, we would be able to probe higher dimensional fundamental theory.

hep-th↗

arXiv2017

Detecting ultralight axion dark matter wind with laser interferometers

The ultralight axion with mass around $10^{-22}$ eV is known as a candidate of dark matter. A peculiar feature of the ultralight axion is oscillating pressure in time, which produces oscillation of gravitational potentials. Since the solar system moves through the dark matter halo at the velocity of about $v \sim 300 \, \text{km} / \text{s} = 10^{-3}$, there exists axion wind, which looks like scalar gravitational waves for us. Hence, there is a chance to detect ultralight axion dark matter with a wide mass range by using laser interferometer detectors. We calculate the detector signal induced by the oscillating pressure of the ultralight axion field, which would be detected by future laser interferometer experiments. We also argue that the detector signal can be enhanced due to the resonance in modified gravity theory explaining the dark energy.

astro-ph.CO↗

arXiv2017

Infinite violation of Bell inequalities in inflation

We study the violation of Bell-Mermin-Klyshko (BMK) inequalities in initial quantum states of scalar fields in inflation. We show that the Bell inequality is maximally violated by the Bunch-Davies vacuum which is a two-mode squeezed state of a scalar field. However, we find that the violation of the BMK inequalities does not increase with the number of modes to measure. We then consider a non-Bunch-Davies vacuum expressed by a four-mode squeezed state of two scalar fields. Remarkably, we find that the violation of the BMK inequalities increases exponentially with the number of modes to measure. This indicates that some evidence that our universe has a quantum mechanical origin may survive in CMB data even if quantum entanglement decays exponentially afterward due to decoherence.

hep-th↗

arXiv2016

Quantum discord in de Sitter space

We study quantum discord between two free modes of a massive scalar field in a maximally entangled state in de Sitter space. We introduce two observers, one in a global chart and the other in an open chart of de Sitter space, and the observers determine the quantum discord created by each detecting one of the modes. This situation is analogous to the relationship between an observer in a Minkowski chart and another in one of the two Rindler charts in flat space. We find that the state becomes less entangled as the curvature of the open chart gets larger. In particular, for the cases of a massless, and a conformally coupled scalar field, the entanglement vanishes in the limit of infinite curvature. However, we find that the quantum discord never disappears even in the limit that entanglement disappears.

hep-th↗

arXiv2016

Oscillating Chiral Tensor Spectrum from Axionic Inflation

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 the tensor spectrum can occur due to the enhancement of chiral gravitational waves sourced by the gauge field. We expect that this feature will be a new probe to axion phenomenologies in early universe through the chiral gravitational waves.

astro-ph.CO↗