arXiv subjects

Jiro Soda

Publications and source records attributed to Jiro Soda.

At least 55 records · Page 3Linked to original sources

arXiv2020

$O(d,d;\mathbb{Z})$ invariant Fierz-Pauli massive gravity

We consider an $O(d,d;\mathbb{Z})$ invariant massive deformation of double field theory at the level of free theory. We study Kaluza-Klein reduction on $R^{1,n-1} \times T^{d}$ and derive the diagonalized second order action for each helicity mode. Imposing the absence of ghosts and tachyons, we obtain a class of consistency conditions which include the well known weak constraint in double field theory as a special case. Consequently, we find two-parameter sets of $O(d,d;\mathbb{Z})$ invariant Fierz-Pauli massive gravity theories.

hep-th↗

arXiv2020

Nano-Hz gravitational wave signature from axion dark matter

We calculate the accurate spectrum of the stochastic gravitational wave background from U(1) gauge fields produced by axion dark matter. The explosive production of gauge fields soon invalidates the applicability of the linear analysis and one needs nonlinear schemes. We make use of numerical lattice simulations to properly follow the nonlinear dynamics such as backreaction and rescattering which gives important contributions to the emission of gravitational waves. It turns out that the axion with the decay constant $f \sim 10^{16}$ GeV which gives the correct dark matter abundance predicts the circularly polarized gravitational wave signature detectable by SKA. We also show that the resulting gravitational wave spectrum has a potential to explain NANOGrav 12.5 year data.

astro-ph.CO↗

arXiv2020

Noise and decoherence induced by gravitons

We study quantum noise and decoherence induced by gravitons. We derive a Langevin equation of geodesic deviation in the presence of gravitons. The amplitude of noise correlations tells us that large squeezing is necessary to detect the noise. We also consider the decoherence of spatial superpositions of two massive particles caused by gravitons in the vacuum state and find that gravitons could give a relevant contribution to the decoherence. The decoherence induced by gravitons would offer new vistas to test quantum gravity in tabletop experiments.

hep-th↗

arXiv2020

Effective Field Theory of Anisotropic Inflation and Beyond

We develop an effective-field-theory (EFT) framework for inflation with various symmetry breaking pattern. As a prototype, we formulate anisotropic inflation from the perspective of EFT and construct an effective action of the Nambu-Goldstone bosons for the broken time translation and rotation symmetries. We also calculate the statistical anisotropy in the scalar two-point correlation function for concise examples of the effective action.

hep-th↗

arXiv2020

Stability of magnetic black holes in general nonlinear electrodynamics

We study the perturbative stability of magnetic black holes in a general class of nonlinear electrodynamics, where the Lagrangian is given by a general function of the field strength of electromagnetic field $F_{μν}$ and its Hodge dual $\widetilde{F}_{μν}$. We derive sufficient conditions for the stability of the black holes. We apply the stability conditions to Bardeen's regular black holes, black holes in Euler-Heisenberg theory, and black holes in Born-Infeld theory. As a result, we obtain a sufficient condition for the stability of Bardeen's black holes, which restricts $F_{μν}\widetilde{F}^{μν}$ dependence of the Lagrangian. We also show that black holes in Euler-Heisenberg theory are stable for a sufficiently small magnetic charge. Moreover, we prove the stability of black holes in the Born-Infeld electrodynamics even when including $F_{μν}\widetilde{F}^{μν}$ dependence.

gr-qc↗

arXiv2020

Pulsar timing residual induced by ultralight vector dark matter

We study the ultralight vector dark matter with a mass around $10^{-23}\,\mathrm{eV}$. The vector field oscillating coherently on galactic scales induces oscillations of the spacetime metric with a frequency around nHz, which is detectable by pulsar timing arrays. We find that the pulsar timing signal due to the vector dark matter has nontrivial angular dependence unlike the scalar dark matter and the maximal amplitude is three times larger than that of the scalar dark matter.

gr-qc↗

arXiv2020

Polarized initial states of primordial gravitational waves

Polarizations of primordial gravitational waves can be relevant when considering inflationary universe in modified gravity or when matter fields survive during inflation. Such polarizations have been discussed in the Bunch-Davies vacuum. Instead of taking into account dynamical generation of polarizations of gravitational waves, in this paper, we consider polarized initial states constructed from $SU(2)$ coherent states. We then evaluate the power spectrums of the primordial gravitational waves in the states.

gr-qc↗

arXiv2020

Search for ultralight scalar dark matter with NANOGrav pulsar timing arrays

An ultralight scalar field is a candidate for the dark matter. The ultralight scalar dark matter with mass around $10^{-23}\,{\rm eV}$ induces oscillations of the pulse arrival time in the sensitive frequency range of the pulsar timing arrays. We search for the ultralight scalar dark matter using the North American Nanohertz Observatory for Gravitational Waves 11-year Data Set. We give the 95% confidence upper limit for the signal induced by the ultralight scalar dark matter. In comparison with the published Bayesian upper limits on the amplitude of the ultralight scalar dark matter obtained by Bayesian analysis using the Parkes Pulsar Timing Array 12-year data set (Porayko et al. 2018), we find three times stronger upper limit in the frequency range from $10^{-8.34}$ to $10^{-8.19}\,{ \rm Hz}$ which corresponds to the mass range from $9.45\times10^{-24}$ to $1.34\times10^{-23}\,{\rm eV}$. In terms of the energy density of the dark matter, we find that the energy density near the Earth is less than $7\,{\rm GeV/cm^3}$ in the range from $10^{-8.55}$ to $10^{-8.01}\,{ \rm Hz}$ (from $5.83\times10^{-24}$ to $2.02\times10^{-23}\,{\rm eV}$). The strongest upper limit on the the energy density is given by $2\,{\rm GeV/cm^3}$ at a frequency $10^{-8.28}\,{ \rm Hz}$ (corresponding to a mass $1.09\times10^{-23}\,{\rm eV}$). We find that the signal of the ultralight scalar dark matter can be explained by the solar system ephemeris effect. Also, we reveal that the model of the solar system ephemeris effect prefers parameters which are contrary to the expectation that noise will be reduced on all pulsars.

astro-ph.HE↗

arXiv2020

Constraining the gravitational coupling of axion dark matter at LIGO

The axion-gravity Chern-Simons coupling is well motivated but is relatively weakly constrained, partly due to difficult measurements of gravity. We study the sensitivity of LIGO measurements of chirping gravitational waves (GWs) on such coupling. When the frequency of the propagating GW matches with that of the coherent oscillation of axion dark matter field, the decay of axions into gravitons can be stimulated, resonantly enhancing the GW. Such a resonance peak can be detected at LIGO as a deviation from the chirping waveform. Since all observed GWs will undergo similar resonant enhancement from the Milky-Way (MW) axion halo, LIGO O1+O2 observations can potentially provide the strongest constraint on the coupling, at least for the axion mass $m_a = 5 \times 10^{-13} - 5 \times 10^{-12}$ eV. Along the course, we also emphasize the relevance of the finite coherence of axion fields and the ansatz separating forward and backward propagations of GWs. As a result, the parity violation of the Chern-Simons coupling is not observable from chirping GWs.

hep-ph↗

arXiv2020

Probing GHz Gravitational Waves with Graviton-magnon Resonance

A novel method for extending frequency frontier in gravitational wave observations is proposed. It is shown that gravitational waves can excite a magnon. Thus, gravitational waves can be probed by a graviton-magnon detector which measures resonance fluorescence of magnons. Searching for gravitational waves with a wave length $λ$ by using a ferromagnetic sample with a dimension $l$, the sensitivity of the graviton-magnon detector reaches spectral densities, around $5.4 \times 10^{-22} \times (\frac{l}{λ/2π})^{-2} \ [{\rm Hz}^{-1/2}]$ at 14 GHz and $8.6 \times 10^{-21} \times (\frac{l}{λ/2π})^{-2} \ [{\rm Hz}^{-1/2}]$ at 8.2 GHz, respectively.

gr-qc↗

arXiv2020

Gravitational Waves in Axion Dark Matter

Axion dark matter is interesting as it allows a natural coupling to the gravitational Chern-Simons term. In the presence of an axion background, the gravitational Chern-Simons term produces parity violating effects in the gravitational sector, in particular on the propagation of gravitational waves. Previously, it has been shown that the coherent oscillation of the axion field leads to a parametric amplification of gravitational waves with a specific frequency. In this paper we focus on the parity violating effects of the Chern-Simon coupling and show the occurrence of gravitational birefringence. We also find deviation from the speed of light of the velocity of the gravitational waves. We give constraints on the axion-Chern-Simons coupling constant and the abundance of the axion dark matter from the observation of GW170817 and GRB170817A.

gr-qc↗

arXiv2020

Stability of Axion Dark Matter-Photon Conversion

It is known that a coherently oscillating axion field is a candidate of the dark matter. In the presence of the oscillating axion, the photon can be resonantly produced through the parametric amplification. In the universe, there also exist cosmological magnetic fields which are coherent electromagnetic fields. In the presence of magnetic fields, an axion can be converted into a photon, and vice versa. Thus, it is interesting to investigate what happens for the axion-photon system in the presence of both the axion dark matter and the magnetic fields. This system can be regarded as a coupled system of the axion and the photon whose equations contain the Mathieu type terms. We find that the instability condition is changed in the presence of magnetic fields in contrast to the conventional Mathieu equation. The positions of bifurcation points between stable and unstable are shifted and new instability bands appear. This is because the resonantly amplified axion can be converted to photon, and vice versa.

hep-ph↗

arXiv2020

Arrival Time Differences of Lensed Massive Gravitational Waves

It is of fundamental importance to know the mass of gravitons. A simple method for constraining the graviton mass is to compare the arrival time of light and that of gravitational waves provided that both waves are simultaneously emitted from the same source. To date, from observations of gravitational waves by the LIGO, the upper bound on the graviton mass $m_g$ is given by $m_g\lesssim 5.0 \times 10^{-23}$eV. However, when we compare the arrival time of light and gravitational waves, lensing effects could be important for some cases. Moreover, in many cases, the wavelength of gravitational waves is comparable with the gravitational radius of a lens object. Hence, we calculate arrival time differences between electromagnetic waves and massive gravitational waves by taking into account the effect of the gravitational wave optics. Here we take two lens models, a point mass lens and a singular isothermal sphere lens. We find that the lensing changes the arrival time difference of two waves by more than a second for the massive gravitational waves detectable by the LISA.

gr-qc↗

arXiv2019

Birth of de Sitter Universe from time crystal

We show that a simple sub-class of Horndeski theory can describe a time crystal Universe. The time crystal Universe can be regarded as a baby Universe nucleated from a flat space, which is mediated by an extension of Giddings-Strominger instanton in a 2-form theory dual to the Horndeski theory. Remarkably, when a cosmological constant is included, de Sitter Universe can be created by tunneling from the time crystal Universe. It gives rise to a past completion of an inflationary Universe.

hep-th↗

arXiv2019

Accelerating Universe with a stable extra dimension in cuscuton gravity

We study Kaluza-Klein cosmology in cuscuton gravity and find an exact solution describing an accelerating 4-dimensional universe with a stable extra dimension. A cuscuton which is a non-dynamical scalar field is responsible for the accelerating expansion and a vector field makes the extra dimensional space stable. Remarkably, the accelerating universe in our model is not exactly de Sitter.

gr-qc↗

arXiv2019

Searching for Bispectrum of Stochastic Gravitational Waves with Pulsar Timing Arrays

We study how to probe bispectra of stochastic gravitational waves with pulsar timing arrays. The bispectrum is a key to probe the origin of stochastic gravitational waves. In particular, the shape of the bispectrum carries valuable information of inflation models. We show that an appropriate filter function for three point correlations enables us to extract a specific configuration of momentum triangles in bispectra. We also calculate the overlap reduction functions and discuss strategy for detecting the bispectrum with multiple pulsars.

gr-qc↗

arXiv2019

Quasinormal modes of $p$-forms in spherical black holes

We study the quasinormal modes of $p$-form fields in spherical black holes in $D$-dimensions. Using the spherical symmetry of the black holes and gauge symmetry, we show the $p$-form field can be expressed in terms of the coexact $p$-form and the coexact $(p-1)$-form on the sphere $S^{D-2}$. These variables allow us to find the master equations. By utilizing the S-deformation method, we explicitly show the stability of $p$-form fields in the spherical black hole spacetime. Moreover, using the WKB approximation, we calculate the quasinormal modes of the $p$-form fields in $D(\leq10)$-dimensions.

hep-th↗

arXiv2019

Dressed Power-law Inflation with Cuscuton

We study dressed inflation with a cuscuton and find a novel exact power-law solution. It is well known that the conventional power-law inflation is inconsistent with the Planck data. In contrast to this standard lore, we find that power-law inflation with a cuscuton can be reconciled with the Planck data. Moreover, we argue that the cuscuton generally ameliorates inflation models so that predictions are consistent with observations.

astro-ph.CO↗