arXiv subjects

Masato Nozawa

Publications and source records attributed to Masato Nozawa.

At least 37 records · Page 2Linked to original sources

arXiv2015

Black holes in an expanding universe and supersymmetry

This paper analyzes the supersymmetric solutions to five and six-dimensional minimal (un)gauged supergravities for which the bilinear Killing vector constructed from the Killing spinor is null. We focus on the spacetimes which admit an additional ${\rm SO}(1,1)$ boost symmetry. Upon the toroidal dimensional reduction along the Killing vector corresponding to the boost, we show that the solution in the ungauged case describes a charged, nonextremal black hole in a Friedmann-Lemaître-Robertson-Walker (FLRW) universe with an expansion driven by a massless scalar field. For the gauged case, the solution corresponds to a charged, nonextremal black hole embedded conformally into a Kantowski-Sachs universe. It turns out that these dimensional reductions break supersymmetry since the bilinear Killing vector and the Killing vector corresponding to the boost fail to commute. This represents a new mechanism of supersymmetry breaking that has not been considered in the literature before.

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arXiv2015

Lovelock black holes with non-constant curvature horizon

This paper studies a class of $D=n+2(\ge 6)$ dimensional solutions to Lovelock gravity that is described by the warped product of a two-dimensional Lorentzian metric and an $n$-dimensional Einstein space. Assuming that the angular part of the stress-energy tensor is proportional to the Einstein metric, it turns out that the Weyl curvature of an Einstein space must obey two kinds of algebraic conditions. We present some exact solutions satisfying these conditions. We further define the quasilocal mass corresponding to the Misner-Sharp mass in general relativity. It is found that the quasilocal mass is constructed out of the Kodama flux and satisfies the unified first law and the monotonicity property under the dominant energy condition. Making use of the quasilocal mass, we show Birkhoff's theorem and address various aspects of dynamical black holes characterized by trapping horizons.

gr-qc↗

arXiv2015

Geometry of Killing spinors in neutral signature

We classify the supersymmetric solutions of minimal $N=2$ gauged supergravity in four dimensions with neutral signature. They are distinguished according to the sign of the cosmological constant and whether the vector field constructed as a bilinear of the Killing spinor is null or non-null. In neutral signature the bilinear vector field can be spacelike, which is a new feature not arising in Lorentzian signature. In the $Λ<0$ non-null case, the canonical form of the metric is described by a fibration over a three-dimensional base space that has $\text{U}(1)$ holonomy with torsion. We find that a generalized monopole equation determines the twist of the bilinear Killing field, which is reminiscent of an Einstein-Weyl structure. If, moreover, the electromagnetic field strength is self-dual, one gets the Kleinian signature analogue of the Przanowski-Tod class of metrics, namely a pseudo-hermitian spacetime determined by solutions of the continuous Toda equation, conformal to a scalar-flat pseudo-Kähler manifold, and admitting in addition a charged conformal Killing spinor. In the $Λ<0$ null case, the supersymmetric solutions define an integrable null Kähler structure. In the $Λ>0$ non-null case, the manifold is a fibration over a Lorentzian Gauduchon-Tod base space. Finally, in the $Λ>0$ null class, the metric is contained in the Kundt family, and it turns out that the holonomy is reduced to ${\rm Sim}(1)\times{\rm Sim}(1)$. There appear no self-dual solutions in the null class for either sign of the cosmological constant.

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arXiv2015

On the integrability of Einstein-Maxwell-(A)dS gravity in presence of Killing vectors

We study some symmetry and integrability properties of four-dimensional Einstein-Maxwell gravity with nonvanishing cosmological constant in the presence of Killing vectors. First of all, we consider stationary spacetimes, which lead, after a timelike Kaluza-Klein reduction followed by a dualization of the two vector fields, to a three-dimensional nonlinear sigma model coupled to gravity, whose target space is a noncompact version of $\mathbb{C}\text{P}^2$ with SU(2,1) isometry group. It is shown that the potential for the scalars, that arises from the cosmological constant in four dimensions, breaks three of the eight SU(2,1) symmetries, corresponding to the generalized Ehlers and the two Harrison transformations. This leaves a semidirect product of a one-dimensional Heisenberg group and a translation group $\mathbb{R}^2$ as residual symmetry. We show that, under the additional assumptions that the three-dimensional manifold is conformal to a product space $\mathbb{R}\timesΣ$, and all fields depend only on the coordinate along $\mathbb{R}$, the equations of motion are integrable. This generalizes the results of Leigh et al. in arXiv:1403.6511 to the case where also electromagnetic fields are present. In the second part of the paper we consider the purely gravitational spacetime admitting a second Killing vector that commutes with the timelike one. We write down the resulting two-dimensional action and discuss its symmetries. If the fields depend only on one of the two coordinates, the equations of motion are again integrable, and the solution turns out to be one constructed by Krasinski many years ago.

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arXiv2015

Hairy black holes in N=2 gauged supergravity

We construct black holes with scalar hair in a wide class of four-dimensional N=2 Fayet-Iliopoulos gauged supergravity theories that are characterized by a prepotential containing one free parameter. Considering the truncated model in which only a single real scalar survives, the theory is reduced to an Einstein-scalar system with a potential, which admits at most two AdS critical points and is expressed in terms of a real superpotential. Our solution is static, admits maximally symmetric horizons, asymptotically tends to AdS space corresponding to an extremum of the superpotential, but is disconnected from the Schwarzschild-AdS family. The condition under which the spacetime admits an event horizon is addressed for each horizon topology. It turns out that for hyperbolic horizons the black holes can be extremal. In this case, the near-horizon geometry is AdS_2 x H^2, where the scalar goes to the other, non-supersymmetric, critical point of the potential. Our solution displays fall-off behaviours different from the standard one, due to the fact that the mass parameter $m^2=-2/\ell^2$ at the supersymmetric vacuum lies in a characteristic range $m^2_{BF}\le m^2\le m^2_{\rm BF}+\ell^{-2}$ for which the slowly decaying scalar field is also normalizable. Nevertheless, we identify a well-defined mass for our spacetime, following the prescription of Hertog and Maeda. Quite remarkably, the product of all horizon areas is not given in terms of the asymptotic cosmological constant alone, as one would expect in absence of electromagnetic charges and angular momentum. Our solution shows qualitatively the same thermodynamic behaviour as the Schwarzschild-AdS black hole, but the entropy is always smaller for a given mass and AdS curvature radius. We also find that our spherical black holes are unstable against radial perturbations.

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arXiv2015

Inflation in maximal gauged supergravities

We discuss the dynamics of multiple scalar fields and the possibility of realistic inflation in the maximal gauged supergravity. In this paper, we address this problem in the framework of recently discovered 1-parameter deformation of ${\rm SO}(4,4)$ and ${\rm SO}(5,3)$ dyonic gaugings, for which the base point of the scalar manifold corresponds to an unstable de Sitter critical point. In the gauge-field frame where the embedding tensor takes the value in the sum of the {\bf 36} and {\bf 36'} representations of ${\rm SL}(8)$, we present a scheme that allows us to derive an analytic expression for the scalar potential. With the help of this formalism, we derive the full potential and gauge coupling functions in analytic forms for the ${\rm SO}(3)\times {\rm SO}(3)$-invariant subsectors of ${\rm SO}(4,4)$ and ${\rm SO}(5,3)$ gaugings, and argue that there exist no new critical points in addition to those discovered so far. For the ${\rm SO}(4,4)$ gauging, we also study the behavior of 6-dimensional scalar fields in this sector near the Dall'Agata-Inverso de Sitter critical point at which the negative eigenvalue of the scalar mass square with the largest modulus goes to zero as the deformation parameter approaches a critical value. We find that when the deformation parameter is taken sufficiently close to the critical value, inflation lasts more than 60 e-folds even if the initial point of the inflaton allows an $O(0.1)$ deviation in Planck units from the Dall'Agata-Inverso critical point. It turns out that the spectral index $n_s$ of the curvature perturbation at the time of the 60 e-folding number is always about 0.96 and within the $1σ$ range $n_s=0.9639\pm0.0047$ obtained by Planck, irrespective of the value of the $η$ parameter at the critical saddle point. The tensor-scalar ratio predicted by this model is around $10^{-3}$ and is close to the value in the Starobinsky model.

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arXiv2014

Positive mass theorem in extended supergravities

Following the Witten-Nester formalism, we present a useful prescription using Weyl spinors towards the positivity of mass. As a generalization of arXiv:1310.1663, we show that some "positivity conditions" must be imposed upon the gauge connections appearing in the supercovariant derivative acting on spinors. A complete classification of the connection fulfilling the positivity conditions is given. It turns out that these positivity conditions are indeed satisfied for a number of extended supergravity theories. It is shown that the positivity property holds for the Einstein-complex scalar system, provided that the target space is Hodge-Kahler and the potential is expressed in terms of the superpotential. In the Einstein-Maxwell-dilaton theory with a dilaton potential, the dilaton coupling function and the superpotential are fixed by the positive mass property. We also explore the $N=8$ gauged supergravity and demonstrate that the positivity of the mass holds independently of the gaugings and the deformation parameters.

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arXiv2014

Modeling scalar fields consistent with positive mass

This paper explores the conditions under which modified gravitational theories admit the positive mass. Following Witten's spinor argument, it is argued that a single condition should be imposed upon a gauge connection in the super-covariant derivative. Under this condition, we present a simple formula for the divergence of the Nester tensor in Einstein's gravity with general source. Applying this prescription to the Einstein-scalar system, we find that for a certain class of the gauge connection, a special kind of non-canonical scalar-field theory admits the positivity property in addition to the ordinary canonical scalar-field system. In both cases the scalar potential can be written in terms of a superpotential. In the non-canonical case we obtain the most general "BPS" solutions which preserve at least half of the supersymmetry.

gr-qc↗

arXiv2013

Supersymmetry of the C-metric and the general Plebanski-Demianski solution

We derive the necessary and sufficient conditions under which the general Plebanski-Demianski (PD) solution of Einstein-Maxwell theory with a negative cosmological constant admits Killing spinors. We consider in detail two different scaling limits of the PD metric. The first of these limits removes the acceleration parameter, and leads to the Carter-Plebanski solution. In this case, the integrability conditions for Killing spinors were obtained by Alonso-Alberca, Meessen and Ortin in hep-th/0003071, and we show that these are not only necessary, but also sufficient for the existence of Killing spinors. This fills also a gap in hep-th/9808097, where the integrability conditions for supersymmetry of the Kerr-Newman-AdS black hole were worked out, but the Killing spinor was not constructed explicitely. The second scaling limit eliminates the rotation parameter, and leads to the cosmological C-metric, which describes accelerated black holes in AdS. Also in this case, the supersymmetry conditions are obtained, and it is shown that they follow from the ones of the general PD solution by scaling the parameters appropriately. In all cases, we determine the three-dimensional base space that appears in the classification scheme of hep-th/0307022, and prove that for the 1/2-supersymmetric Reissner-Nordstrom-AdS spacetime, this base is unique. A Wick-rotation of our results leads to gravitational instantons that generalize the ones constructed recently by Martelli, Passias and Sparks in arXiv:12124618 to U(1)xU(1) symmetry. These instantons are shown to admit an integrable almost complex structure. Finally, our work may open the possibility to systematically construct generalizations of the PD metric that include scalar fields with a potential in matter-coupled gauged supergravity.

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arXiv2013

Classification and stability of vacua in maximal gauged supergravity

This article presents a systematic study of critical points for the SL(8, R)-type gauging in four dimensional maximal gauged supergravity. We determine all the possible vacua for which the origin of the moduli space becomes a critical point. We formulate a new tool which enables us to find analytically the mass spectrum of the corresponding vacua in terms of eigenvalues of the embedding tensor. When the cosmological constant is nonvanishing, it turns out that many vacua obtained by the dyonic embedding admit a single deformation parameter of the theory, in agreement with the results of the recent paper by Dall'Agata, Inverso and Trigiante [arXiv:1209.0760]. Nevertheless, it is shown that the resulting mass spectrum is independent of the deformation parameter and can be classified according to the unbroken gauge symmetry at the vacua, rather than the underlying gauging. We also show that the generic Minkowski vacua exhibit instability.

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arXiv2011

Parity Violation in Graviton Non-gaussianity

We study parity violation in graviton non-gaussianity generated during inflation. We develop a useful formalism to calculate graviton non-gaussianity. Using this formalism, we explicitly calculate the parity violating part of the bispectrum for primordial gravitational waves in the exact de Sitter spacetime and prove that no parity violation appears in the non-gaussianity. We also extend the analysis to slow-roll inflation and find that the parity violation of the bispectrum is proportional to the slow-roll parameter. We argue that parity violating non-gaussianity can be tested by the CMB. Our results are also useful for calculating three-point function of the stress tensor in the non-conformal field theory through the gravity/field theory correspondence.

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arXiv2011

Black hole solutions in string theory

Supersymmetric solutions of supergravity have been of particular importance in the advances of string theory. This article reviews the current status of black hole solutions in higher-dimensional supergravity theories. We discuss primarily the gravitational aspects of supersymmetric black holes and their relatives in various dimensions. Supersymmetric solutions and their systematic derivation are reviewed with prime examples. We also study the stationary or dynamically intersecting branes in ten and eleven-dimensions, which provide a number of interesting black objects via the dimensional reduction and duality transformations.

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arXiv2011

On the Bogomol'nyi bound in Einstein-Maxwell-dilaton gravity

It has been shown that the 4-dimensional Einstein-Maxwell-dilaton theory allows a Bogomol'nyi-type inequality for an arbitrary dilaton coupling constant $α$, and that the bound is saturated if and only if the (asymptotically flat) spacetime admits a nontrivial spinor satisfying the gravitino and the dilatino Killing spinor equations. The present paper revisits this issue and argues that the dilatino equation fails to ensure the dilaton field equation unless the solution is purely electric/magnetic, or the dilaton coupling constant is given by $α=0, \sqrt 3$, corresponding to the Brans-Dicke-Maxwell theory and the Kaluza-Klein reduction of 5-dimensional vacuum gravity, respectively. A systematic classification of the supersymmetric solutions reveals that the solution can be rotating if and only if the solution is dyonic or the coupling constant is given by $α=0, \sqrt 3$. This implies that the theory with $α\ne 0, \sqrt 3$ cannot be embedded into supergravity except for the static truncation. Physical properties of supersymmetric solutions are explored from various points of view.

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arXiv2010

Cosmological rotating black holes in five-dimensional fake supergravity

In recent series of papers, we found an arbitrary dimensional, time-evolving and spatially-inhomogeneous solutions in Einstein-Maxwell-dilaton gravity with particular couplings. Similar to the supersymmetric case the solution can be arbitrarily superposed in spite of non-trivial time-dependence, since the metric is specified by a set of harmonic functions. When each harmonic has a single point source at the center, the solution describes a spherically symmetric black hole with regular Killing horizons and the spacetime approaches asymptotically to the Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology. We discuss in this paper that in 5-dimensions this equilibrium condition traces back to the 1st-order "Killing spinor" equation in "fake supergravity" coupled to arbitrary U(1) gauge fields and scalars. We present a 5-dimensional, asymptotically FLRW, rotating black-hole solution admitting a nontrivial "Killing spinor," which is a spinning generalization of our previous solution. We argue that the solution admits nondegenerate and rotating Killing horizons in contrast with the supersymmetric solutions. It is shown that the present pseudo-supersymmetric solution admits closed timelike curves around the central singularities. When only one harmonic is time-dependent, the solution oxidizes to 11-dimensions and realizes the dynamically intersecting M2/M2/M2-branes in a rotating Kasner universe. The Kaluza-Klein type black holes are also discussed.

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arXiv2010

Black Hole in the Expanding Universe with Arbitrary Power-Law Expansion

We present a time-dependent and spatially inhomogeneous solution that interpolates the extremal Reissner-Nordström (RN) black hole and the Friedmann-Lemaître-Robertson-Walker (FLRW) universe with arbitrary power-law expansion. It is an exact solution of the $D$-dimensional Einstein-"Maxwell"-dilaton system, where two Abelian gauge fields couple to the dilaton with different coupling constants, and the dilaton field has a Liouville-type exponential potential. It is shown that the system satisfies the weak energy condition. The solution involves two harmonic functions on a $(D-1)$-dimensional Ricci-flat base space. In the case where the harmonics have a single-point source on the Euclidean space, we find that the spacetime describes a spherically symmetric charged black hole in the FLRW universe, which is characterized by three parameters: the steepness parameter of the dilaton potential $n_T$, the U$(1)$ charge $Q$, and the "nonextremality" $τ$. In contrast with the extremal RN solution, the spacetime admits a nondegenerate Killing horizon unless these parameters are finely tuned. The global spacetime structures are discussed in detail.

gr-qc↗

arXiv2010

Black Hole in the Expanding Universe from Intersecting Branes

We study physical properties and global structures of a time-dependent, spherically symmetric solution obtained via the dimensional reduction of intersecting M-branes. We find that the spacetime describes a maximally charged black hole which asymptotically tends to the Friedmann-Lemaître-Robertson-Walker (FLRW) universe filled by a stiff matter. The metric solves the field equations of Einstein-Maxwell-dilaton system, in which four Abelian gauge fields couple to the dilation with different coupling constants. The spacetime satisfies the dominant energy condition and is characterized by two parameters, $Q$ and $τ$, related to the Maxwell charge and the relative ratio of black-hole horizon radii, respectively. In spite of the nontrivial time-dependence of the metric, it turns out that the black hole event horizon is a Killing horizon. This unexpected symmetry may be ascribed to the fact that the 11-dimensional (11D) brane configurations are supersymmetric in the static limit. Finally, combining with laws of trapping horizon, we discuss the thermodynamic properties of the black hole. It is shown that the horizon possesses a nonvanishing temperature, contrary to the extremal Reissner-Nordstrøm (RN) solution.

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arXiv2009

Stability of branes trapped by d-dimensional black holes

The system of extended objects interplaying with a black hole describes or mimics various gravitational phenomena. In this brief paper, we report the results of stability analysis of codimension-one Dirac-Nambu-Goto branes rest at the equatorial plane of $d$-dimensional spherical black holes, including the Schwarzschild and Schwarzschild-(anti-)de Sitter black holes. For the Schwarzschild and Schwarzschild-anti-de Sitter backgrounds the stability of branes is shown analytically by means of a deformation technique. In contrast, for the Schwarzschild-de Sitter background we demonstrate with the help of numerics that the brane is unstable (only) against the s-wave sector of perturbations.

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arXiv2008

Quasinormal modes of black holes localized on the Randall-Sundrum 2-brane

We investigate conformal scalar, electromagnetic, and massless Dirac quasinormal modes of a brane-localized black hole. The background solution is the four-dimensional black hole on a 2-brane that has been constructed by Emparan, Horowitz, and Myers in the context of a lower dimensional version of the Randall-Sundrum model. The conformally transformed metric admits a Killing tensor, allowing us to obtain separable field equations. We find that the radial equations take the same form as in the four-dimensional "braneless" Schwarzschild black hole. The angular equations are, however, different from the standard ones, leading to a different prediction for quasinormal frequencies.

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