arXiv subjects

Takahiro Tanaka

Publications and source records attributed to Takahiro Tanaka.

At least 109 records · Page 6Linked to original sources

arXiv2015

Effective theory of Black Holes in the 1/D expansion

The gravitational field of a black hole is strongly localized near its horizon when the number of dimensions D is very large. In this limit, we can effectively replace the black hole with a surface in a background geometry (eg Minkowski or Anti-deSitter space). The Einstein equations determine the effective equations that this 'black hole surface' (or membrane) must satisfy. We obtain them up to next-to-leading order in 1/D for static black holes of the Einstein-(A)dS theory. To leading order, and also to next order in Minkowski backgrounds, the equations of the effective theory are the same as soap-film equations, possibly up to a redshift factor. In particular, the Schwarzschild black hole is recovered as a spherical soap bubble. Less trivially, we find solutions for 'black droplets', ie black holes localized at the boundary of AdS, and for non-uniform black strings.

hep-th↗

arXiv2015

Synergy between ground and space based gravitational wave detectors for estimation of binary coalescence parameters

We study the advantage of the co-existence of future ground and space based gravitational wave detectors, in estimating the parameters of a binary coalescence. Using the post-Newtonian waveform for the inspiral of non-spinning neutron star-black hole pairs in circular orbits, we study how the estimates for chirp mass, symmetric mass ratio, and time and phase at coalescence are improved by combining the data from different space-ground detector pairs. Since the gravitational waves produced by binary coalescence also provide a suitable domain where we can study strong field gravity, we also study the deviations from general relativity using the parameterized post-Einsteinian framework. As an example, focusing on the Einstein telescope and DECIGO pair, we demonstrate that there exists a sweet spot range of sensitivity in the pre-DECIGO phase where the best enhancement due to the synergy effect can be obtained for the estimates of the post-Newtonian waveform parameters as well as the modification parameters to general relativity.

gr-qc↗

arXiv2015

Detectability of bigravity with graviton oscillations using gravitational wave observations

The gravitational waveforms in the ghost-free bigravity theory exhibit deviations from those in general relativity. The main difference is caused by graviton oscillations in the bigravity theory. We investigate the prospects for the detection of the corrections to gravitational waveforms from coalescing compact binaries due to graviton oscillations and for constraining bigravity parameters with the gravitational wave observations. We consider the bigravity model discussed by the De Felice-Nakamura-Tanaka subset of the bigravity model, and the phenomenological model in which the bigravity parameters are treated as independent variables. In both models, the bigravity waveform shows strong amplitude modulation, and there can be a characteristic frequency of the largest peak of the amplitude, which depends on the bigravity parameters. We show that there is a detectable region of the bigravity parameters for the advanced ground-based laser interferometers, such as Advanced LIGO, Advanced Virgo, and KAGRA. This region corresponds to the effective graviton mass of $μ\geq 10^{-17}~{\rm cm}^{-1}$ for $\tilde{c}-1 \geq 10^{-19}$ in the phenomenological model, while $μ\geq 10^{-16.5}~{\rm cm}^{-1}$ for $κξ_c^2 \geq 10^{0.5}$ in the De Felice-Nakamura-Tanaka subset of the bigravity model, respectively, where $\tilde{c}$ is the propagation speed of the massive graviton and $κξ_c^2$ corresponds to the corrections to the gravitational constant in general relativity. These regions are not excluded by existing solar system tests. We also show that, in the case of $1.4-1.4M_{\rm sun}$ binaries at the distance of $200~{\rm Mpc}$, $\logμ^2$ is determined with an accuracy of ${\cal O}$(0.1)\% at the 1$σ$ level for a fiducial model with $μ^2=10^{-33}~{\rm cm}^{-2}$ in the case of the phenomenological model.

gr-qc↗

arXiv2014

Tunnelling with wormhole creation

The description of quantum tunnelling in the presence of gravity shows subtleties in some cases. Here we discuss wormhole production in the context of the spherically symmetric thin-shell approximation. By presenting a fully consistent treatment based on canonical quantization, we solve a controversy present in literature.

gr-qc↗

arXiv2014

Gravitational Self-Force Correction to the Innermost Stable Circular Equatorial Orbit of a Kerr Black Hole

For a self-gravitating particle of mass μin orbit around a Kerr black hole of mass M >> μ, we compute the O(μ/M) shift in the frequency of the innermost stable circular equatorial orbit (ISCEO) due to the conservative piece of the gravitational self-force acting on the particle. Our treatment is based on a Hamiltonian formulation of the dynamics in terms of geodesic motion in a certain locally-defined effective smooth spacetime. We recover the same result using the so-called first law of binary black-hole mechanics. We give numerical results for the ISCEO frequency shift as a function of the black hole's spin amplitude, and compare with predictions based on the post-Newtonian approximation and the effective one-body model. Our results provide an accurate strong-field benchmark for spin effects in the general relativistic two-body problem.

gr-qc↗

arXiv2014

A simple diagnosis of non-smoothness of black hole horizon: Curvature singularity at horizons in extremal Kaluza-Klein black holes

We propose a simple method to prove non-smoothness of a black hole horizon. The existence of a $C^1$ extension across the horizon implies that there is no $C^{N + 2}$ extension across the horizon if some components of $N$-th covariant derivative of Riemann tensor diverge at the horizon in the coordinates of the $C^1$ extension. In particular, the divergence of a component of the Riemann tensor at the horizon directly indicates the presence of a curvature singularity. By using this method, we can confirm the existence of a curvature singularity for several cases where the scalar invariants constructed from the Riemann tensor, e.g., the Ricci scalar and the Kretschmann invariant, take finite values at the horizon. As a concrete example of the application, we show that the Kaluza-Klein black holes constructed by Myers have a curvature singularity at the horizon if the spacetime dimension is higher than five.

gr-qc↗

arXiv2014

Viable cosmology in bimetric theory

We study cosmological perturbations in bimetric theory with two fluids each of which is coupled to one of the two metrics. Focusing on a healthy branch of background solutions, we clarify the stability of the cosmological perturbations. For this purpose, we extend the condition for the absence of the so-called Higuchi ghost, and show that the condition is guaranteed to be satisfied on the healthy branch. We also calculate the squared propagation speeds of perturbations and derive the conditions for the absence of the gradient instability. To avoid the gradient instability, we find that the model parameters are weakly constrained.

hep-th↗

arXiv2014

Loops in inflationary correlation functions

We review the recent progress regarding the loop corrections to the correlation functions in the inflationary universe. A naive perturbation theory predicts that loop corrections generated during inflation suffer from various infrared (IR) pathologies. Introducing an IR cutoff by hand is not only unsatisfactory but also does not fix the problem of secular growth, which may ruin the predictive power of inflation models if the inflation lasts sufficiently long. We discuss the origin of the IR divergences and explore the regularity conditions of loop corrections for the adiabatic perturbation, the iso-curvature perturbation, and the tensor perturbation, in turn. These three kinds of perturbations have qualitative differences, but in discussing the IR regularity there is a feature common to all cases, which is the importance of the proper identification of observable quantities. Genuinely observable quantities should respect the gauge invariance from the view point of a local observer. Interestingly, we find that the requirement of the IR regularity restricts the allowed quantum states.

hep-th↗

arXiv2014

Strong restriction on inflationary vacua from the local gauge invariance III: Infrared regularity of graviton loops

It has been claimed that the super Hubble modes of the graviton generated during inflation can make loop corrections diverge. Even if we introduce an infrared (IR) cutoff at a comoving scale as an ad hoc but a practical way for the regularization, we encounter the secular growth, which may lead to the breakdown of perturbative expansion for a sufficiently long lasting inflation. In this paper, we show that the IR pathology concerning the graviton can be attributed to the presence of residual gauge degrees of freedom in the local observable universe as in the case of the adiabatic curvature perturbation. We will show that choosing the Euclidean vacuum as the initial state ensures the invariance under the above-mentioned residual gauge transformations. We will also show that as long as we consider a gauge invariant quantity in the local universe, we encounter neither the IR divergence nor the secular growth. The argument in this paper applies to general single field models of inflation up to a sufficiently high order in perturbation.

hep-th↗

arXiv2014

A viable explanation of the CMB dipolar statistical anisotropy

The presence of a dipolar statistical anisotropy in the spectrum of cosmic microwave background (CMB) fluctuations was reported by the Wilkinson Microwave Anisotropy Probe (WMAP), and has recently been confirmed in the Planck 2013 analysis of the temperature anisotropies. At the same time, the Planck 2013 results report a stringent bound on the amplitude of the local-type non-Gaussianity. We show that the non-linear effect of the dipolar anisotropy generates not only a quadrupole moment in the CMB but also a local-type non-Gaussianity. Consequently, it is not easy to build models having a large dipolar modulation and at the same time a sufficiently small quadrupole and level of local bispectral anisotropy to agree with the present data. In particular, most models proposed so far are almost excluded, or are at best marginally consistent with observational data. We present a simple alternative scenario that may explain the dipolar statistical anisotropy while satisfying the observational bounds on both the quadrupole moment and local-type non-Gaussianity.

astro-ph.CO↗

arXiv2014

Mapping de Rham-Gabadadze-Tolley bigravity into braneworld setup

We discuss whether or not bigravity theory can be embedded into the braneworld setup. As a candidate, we consider Dvali-Gabadadze-Porrati two-brane model with the Goldberger-Wise radion stabilization. We will show that we can construct a ghost free model whose low energy spectrum is composed of a massless graviton and a massive graviton with a small mass. As is expected, the behavior of this effective theory is shown to be identical to de Rham-Gabadadze-Tolley bigravity. Unfortunately, this correspondence breaks down at a relatively low energy due to the limitation of the adopted stabilization mechanism.

hep-th↗

arXiv2014

Schwinger effect in de Sitter space

We consider Schwinger pair production in 1+1 dimensional de Sitter space, filled with a constant electric field $E$. This can be thought of as a model for describing false vacuum decay beyond the semiclassical approximation, where pairs of a quantum field $ϕ$ of mass $m$ and charge $e$ play the role of vacuum bubbles. We find that the adiabatic "in" vacuum associated with the flat chart develops a space-like expectation value for the current $J$, which manifestly breaks the de Sitter invariance of the background fields. We derive a simple expression for $J(E)$, showing that both "upward" and "downward" tunneling contribute to the build-up of the current. For heavy fields, with $m^2\gg eE,H^2$, the current is exponentially suppressed, in agreement with the results of semiclassical instanton methods. Here $H$ is the inverse de Sitter radius. On the other hand, light fields with $ m \ll H$ lead to a phenomenon of infrared hyperconductivity, where a very small electric field $mH \lesssim eE \ll H^2$ leads to a very large current $J \sim H^3 /E$. We also show that all Hadamard states for $ϕ$ necessarily break de Sitter invariance. Finally, we comment on the role of initial conditions, and "persistence of memory" effects.

hep-th↗

arXiv2013

Rest frame of bubble nucleation

Vacuum bubbles nucleate at rest with a certain critical size and subsequently expand. But what selects the rest frame of nucleation? This question has been recently addressed in [1] in the context of Schwinger pair production in 1+1 dimensions, by using a model detector in order to probe the nucleated pairs. The analysis in [1] showed that, for a constant external electric field, the adiabatic "in" vacuum of charged particles is Lorentz invariant, and in this case pairs tend to nucleate preferentially at rest with respect to the detector. Here, we sharpen this picture by showing that the typical relative velocity between the frame of nucleation and that of the detector is at most of order Δv ~ S_E^{-1/3} << 1. Here, S_E >> 1 is the action of the instanton describing pair creation. The bound Δv coincides with the minimum uncertainty in the velocity of a non-relativistic charged particle embedded in a constant electric field. A velocity of order Δv is reached after a time interval of order Δt ~ S_E^{-1/3} r_0 << r_0 past the turning point in the semiclassical trajectory, where r_0 is the size of the instanton. If the interaction takes place in the vicinity of the turning point, the semiclassical description of collision does not apply. Nonetheless, we find that even in this case there is still a strong asymmetry in the momentum transferred from the nucleated particles to the detector, in the direction of expansion after the turning point. We conclude that the correlation between the rest frame of nucleation and that of the detector is exceedingly sharp.

hep-th↗

arXiv2013

Evolution of the Carter constant for a resonant inspiral into a Kerr black hole: I. The scalar case

We discuss the inspiral of a small body around a Kerr black hole. When the time scale of the radiation reaction is sufficiently longer than its orbital period, the leading order orbital evolution is described only by the knowledge of the averaged evolution of the constants of motion, i.e., the energy, azimuthal angular momentum and the Carter constant. Although there is no conserved current composed of the perturbation field corresponding to the Carter constant, it has been shown that the averaged rate of change of the Carter constant can be given by a simple formula, when there exists a simultaneous turning point of the radial and polar oscillations. However, an inspiralling orbit may cross a "resonance" point, where the frequencies of the radial and polar orbital oscillations are in a rational ratio. At the resonance point, one cannot find a simultaneous turning point in general. Hence, even for the averaged rate of change of the Carter constant, a direct computation of the 'self-force", which is still challenging especially in the case of the Kerr background, seems to be necessary. In this paper, we present a method of computing the averaged rate of change of the Carter constant in a relatively simple manner at the "resonance" point.

gr-qc↗

arXiv2013

Observation of finite excess noise in the voltage-biased quantum Hall regime as a precursor for breakdown

We performed noise measurements in a two-dimensional electron gas to investigate the nonequilibrium quantum Hall effect (QHE) state. While excess noise is perfectly suppressed around the zero-biased QHE state reflecting the dissipationless electron transport of the QHE state, considerable finite excess noise is observed in the breakdown regime of the QHE. The noise temperature deduced from the excess noise is found to be of the same order as the energy gap between the highest occupied Landau level and the lowest empty one. Moreover, unexpected finite excess noise is observed at a finite source-drain bias voltagesmaller than the onset voltage of the QHE breakdown, which indicates finite dissipation in the QHE state and may be related to the prebreakdown of the QHE.

cond-mat.mes-hall↗

arXiv2013

Possible existence of viable models of bi-gravity with detectable graviton oscillations by gravitational wave detectors

We discuss graviton oscillations based on the ghost free bi-gravity theory. We point out that this theory possesses a natural cosmological background solution which is very close to the case of general relativity. Furthermore, interesting parameter range of the graviton mass, which can be explored by the observations of gravitational waves, is not at all excluded by the constraint from the solar system tests. Therefore the graviton oscillation with possible inverse chirp signal would be an interesting scientific target of KAGRA, adv LIGO, adv Virgo and GEO.

gr-qc↗

arXiv2013

Strong restriction on inflationary vacua from the local gauge invariance II: Infrared regularity and absence of the secular growth in Euclidean vacuum

We investigate the initial state of the inflationary universe. In our recent publications, we showed that requesting the gauge invariance in the local observable universe to the initial state guarantees the infrared (IR) regularity of loop corrections in a general single clock inflation. Following this study, in this paper, we show that choosing the Euclidean vacuum ensures the gauge invariance in the local universe and hence the IR regularity of loop corrections. It has been suggested that loop corrections to inflationary perturbations may yield the secular growth, which can lead to the break down of the perturbative analysis in an extremely long term inflation. The absence of the secular growth has been claimed by picking up only the IR contributions, which we think is incomplete because the non-IR modes which are comparable to or smaller than the Hubble scale potentially can contribute to the secular growth. We prove the absence of the secular growth without neglecting these non-IR modes to a certain order in the perturbative expansion. We also discuss how the regularity of the n-point functions for the genuinely gauge invariant variable constrains the initial states of the inflationary universe. These results apply in a fully general single field model of inflation.

hep-th↗

arXiv2013

Strong restriction on inflationary vacua from the local gauge invariance I: Local gauge invariance and infrared regularity

The primordial perturbation is widely accepted to be generated through the vacuum fluctuation of the scalar field which drives inflation. It is, however, not completely clear what is the natural vacuum in the inflationary universe particularly in the presence of non-linear interactions. In this series of papers, we will address this issue, focusing on the condition required for the removal of the divergence from the infrared (IR) contribution to loop diagrams. We show that requesting the gauge invariance in the local observable universe guarantees the IR regularity of the loop corrections beginning with a simple initial state. In our previous works, the IR regularity condition was discussed using the slow roll expansion, which restricts the background evolution of the inflationary universe. We will show more generally that requesting the gauge invariance/the IR regularity leads to non-trivial constraints on the allowed quantum states.

hep-th↗