■Faculty | Department of Science / Graduate School of Science and Engineering Research |

■Position | Professor |

■Degree | |

■Commentator Guide | https://www.kindai.ac.jp/meikan/268-oota-nobuyoshi.html |

■URL | |

Last Updated :2020/04/06

- 2006 04 , - 現在, Professor, Faculty of Science and Engineering, Kindai University
- 2006 , - 現在, Faculty of Science and Engineering, Kindai University
- 1994 04 , - 2006 03 , Associate Professor, Faculty of Science, Osaka University
- 1990 12 , - 1994 03 , Associate Professor, College of general Education, Osaka University
- 1988 04 , - 1990 11 , Lecturer, College of general Education, Osaka University
- 1983 10 , - 1988 03 , Assistant Professor, College of general Education, Osaka University

- Natural sciences, Theoretical studies related to particle-, nuclear-, cosmic ray and astro-physics

- Asymptotic Safety, supergravity, Quantum gravity, black hole, inflation, supersymmetric theories, superstring theories

- Asymptotic safety and field parametrization dependence in the $f(R)$ truncation, G. P. De Brito, N. Ohta, A. D. Pereira, A. A. Tomaz, M. Yamada, Physical Review D, Physical Review D, 98(2), 026027-1 - 026027-25, Jul. 2018 , Refereed
- $f(R, R_{\mu\nu}^2)$ at one loop, N. Ohta, R. Percacci, A. D. Pereira, Physical Review D, Physical Review D, 97(2), 026007-1 - 026007-11, Apr. 2018 , Refereed
- Quantum Equivalence of $f(R)$ Gravity and Scalar-tensor Theories in the Jordan and Einstein Frames, Nobuyoshi Ohta, PTEP, PTEP, 2018(3), 033B02 (13pp), Mar. 2018 , Refereed
- The path integral of unimodular gravity, R. de León Ardón, N. Ohta, R. Percacci, Physical Review D, Physical Review D, 97(2), 026007, Jan. 2018 , Refereed
- Background Scale Independence in Quantum Gravity, Nobuyoshi Ohta, Progress of Theoretical and Experimantal Physics, Progress of Theoretical and Experimantal Physics, 2017, 033E02, Mar. 2017 , Refereed
- Renormalization Group Equation for f(R) gravity on hyperbolic spaces, Nobuyoshi Ohta, Kevin Falls, Phys. Rev. D, Phys. Rev. D, 94(8,Pt.A), 20, Oct. 2016 , Refereed
- Gauges and functional measures in quantum gravity I: Einstein theory, Nobuyoshi Ohta, Roberto Percacci, Antonio D. Pereira, JHEP, JHEP, 06, 115, Jun. 2016 , Refereed
- Renormalization Group Equation and scaling solutions for f(R) gravity in exponential parametrization, Nobuyoshi Ohta, Roberto Percacci, Gian Paolo Vacca, European Physical Journal C, European Physical Journal C, 76(2), 1 - 18, Jan. 2016 , Refereed
- Ultraviolet Fixed Points in Conformal Gravity and General Quadratic Theories, Nobuyoshi Ohta, Roberto Percacci, Classical and Quantum Gravity, Classical and Quantum Gravity, 33(3), Jan. 2016 , RefereedSummary:We study the beta functions for four-dimensional conformal gravity using two different parametrizations of metric fluctuation, linear split and exponential parametrization. We find that after imposing the traceless conditions, the beta functions are the same in four dimensions though the dependence on the dimensions are quite different. This indicates the universality of these results. We also examine the beta functions in general quadratic theory with the Einstein and cosmological terms for exponential parametrization, and find that it leads to results for beta functions of dimensionful couplings different from linear split, though the fact that there exists nontrivial fixed point remains the same

and the fixed points also remain the same. - A flow equation for f(R) gravity and some of its exact solutions, Nobuyoshi Ohta, Roberto Percacci, Gian Paolo Vacca, Phys. Rev. D, Phys. Rev. D, 92(6,Pt.A), 1 - 61501, Sep. 2015 , Refereed
- Covariant Approach to the No-Ghost Theorem in Massive Gravity, Nobuyoshi Ohta, JPS Conf. Proc., JPS Conf. Proc., 7, 010007-1 - 010007-5, 2015 , Refereed
- Asymptotically AdS Charged Black Holes in String Theory with Gauss-Bonnet Correction in Various Dimensions, Nobuyoshi Ohta, Takashi Torii, Phys. Rev. D, Phys. Rev. D, 88, 064002, Jul. 2013 , RefereedSummary:We study charged black hole solutions in Einstein-Maxwell-Gauss-Bonnet theory

with the dilaton field which is the low-energy effective theory of the

heterotic string. The spacetime is $D$-dimensional and assumed to be static and

plane symmetric with the $(D-2)$-dimensional constant curvature space and

asymptotically anti-de Sitter. By imposing the boundary conditions of the

existence of the regular black hole horizon and proper behavior at infinity

where the Breitenlohner-Freedman bound should be satisfied, we construct black

hole solutions numerically. We give the relations among the physi... - Charged Black Holes in String Theory with Gauss-Bonnet Correction in Various Dimensions, Nobuyoshi Ohta, Takashi Torii, Phys.Rev. D, Phys.Rev. D, 86(10,Pt.A), 104016, Aug. 2012 , RefereedSummary:We study charged black hole solutions in Einstein-Gauss-Bonnet theory with

the dilaton field which is the low-energy effective theory of the heterotic

string. The spacetime is D-dimensional and assumed to be static and spherically

symmetric with the $(D-2)$-dimensional constant curvature space and

asymptotically flat. The system of the basic equations is complex and the

solutions are obtained numerically. We identify the allowed parameter region

where the black hole solutions exist, and show configurations of the field

functions in D=4 -- 6 and 10. We also show the relations of the physical... - Beta Function and Asymptotic Safety in Three-dimensional Higher Derivative Gravity, Nobuyoshi Ohta, Class.Quant.Grav., Class.Quant.Grav., 29, 205012, May 2012 , RefereedSummary:We study the quantum properties of the three-dimensional higher derivative

gravity. In particular we calculate the running of the gravitational and

cosmological constants. The flow of these couplings shows that there exist both

Gaussian and nontrivial fixed points in the theory, thus confirming that the

theory is asymptotically safe. It is shown that the new massive gravity or

$f(R)$ gravity in three dimensions do not correspond to the fixed point within

the approximation that the coefficients of the higher curvature terms are not

subject to the flow. The fixed point value of the cosmologic... - Unitarity versus Renormalizability of Higher Derivative Gravity in 3D, Kenji Muneyuki, Nobuyoshi Ohta, Phys.Rev. D, Phys.Rev. D, 85(10,Pt.A), 101501, Jan. 2012 , RefereedSummary:It has been suggested that new massive gravity with higher order terms in the

curvature may be renormalizable and thus a candidate for renormalizable quantum

gravity. We show that three-dimensional gravity that contains quadratic scalar

curvature and Ricci tensor is renormalizable, but those theories with special

relation between their coefficients including new massive gravity are not. - Hawking Radiation and Tunneling Mechanism for a New Class of Black Holes in Einstein-Gauss-Bonnet Gravity, Kenji Muneyuki, Nobuyoshi Ohta, Eur.Phys.J. C, Eur.Phys.J. C, 72, 1858, Nov. 2011 , RefereedSummary:We study the Hawking radiation in a new class of black hole solutions in the

Einstein-Gauss-Bonnet theory. The black hole has been argued to have vanishing

mass and entropy, but finite Hawking temperature. To check if it really emits

radiation, we analyse the Hawking radiation using the original method of

quantization of scalar field in the black hole background and the quantum

tunneling method, and confirm that it emits radiation at the Hawking

temperature. A general formula is derived for the Hawking temperature and

backreaction in the tunneling approach. Physical implications of these re... - Accelerating Universes in String Theory via Field Redefinition, Kei-ichi Maeda, Nobuyoshi Ohta, Ryo Wakebe, Eur.Phys.J. C, Eur.Phys.J. C, 72, 1949, Nov. 2011 , RefereedSummary:We study cosmological solutions in the effective heterotic string theory with

$\alpha'$-correction terms in string frame. It is pointed out that the

effective theory has an ambiguity via field redefinition and we analyze

generalized effective theories due to this ambiguity. We restrict our analysis

to the effective theories which give equations of motion of second order in the

derivatives, just as "Galileon" field theory. This class of effective actions

contains two free coupling constants. We find de Sitter solutions as well as

the power-law expanding universes in our four-dimensional Eins... - A Complete Classification of Higher Derivative Gravity in 3D and Criticality in 4D, Nobuyoshi Ohta, Class.Quant.Grav., Class.Quant.Grav., 29, 015002, Sep. 2011 , RefereedSummary:We study the condition that the theory is unitary and stable in

three-dimensional gravity with most general quadratic curvature,

Lorentz-Chern-Simons and cosmological terms. We provide the complete

classification of the unitary theories around flat Minkowski and (anti-)de

Sitter spacetimes. The analysis is performed by examining the quadratic

fluctuations around these classical vacua. We also discuss how to understand

critical condition for four-dimensional theories at the Lagrangian level. - Intersecting black branes in strong gravitational waves, Li-Ming Cao, Oleg Evnin, Nobuyoshi Ohta, JHEP 1106:062,2011, JHEP 1106:062,2011, Apr. 2011 , RefereedSummary:We consider intersecting black branes with strong gravitational waves

propagating along their worldvolume in the context of supergravity theories.

Both near-horizon and space-filling gravitational wave modes are included in

our ansatz. The equations of motion (originally, partial differential

equations) are shown to reduce to ordinary differential equations, which

include a Toda-like system. For special arrangements of intersecting black

branes, the Toda-like system becomes integrable, permitting a more thorough

analysis of the gravitational equations of motion. - Extended black holes in strong gravitational waves, Li-Ming Cao, Oleg Evnin, Nobuyoshi Ohta, JHEP 1104:105,2011, JHEP 1104:105,2011, Jan. 24 2011 Summary:We describe a large class of solutions in pure gravity, dilaton gravity and

supergravity corresponding to extended higher-dimensional black holes with

strong (non-linear) gravitational waves propagating along their worldvolume.

For pure gravity, the extended black holes are higher-dimensional analogs of

the point-like Schwarzschild black hole in four dimensions. For supergravity,

they are non-extremal p-branes. The gravitational waves can be both

space-filling and localized around the worldvolume of the extended black holes.

The solutions we present contain a large number of arbitrary funct... - AdS Black Hole Solution in Dilatonic Einstein-Gauss-Bonnet Gravity, Kei-ichi Maeda, Nobuyoshi Ohta, Yukinori Sasagawa, Phys.Rev.D83:044051,2011, Phys.Rev.D83:044051,2011, Dec. 03 2010 Summary:We find that AdS spacetime with a non-trivial linear dilaton field is an

exact solution in the effective action of string theory, which is described by

gravity with the Gauss-Bonnet curvature terms coupled to a dilaton field in the

string frame. The AdS radius is determined by the spacetime dimensions and the

coupling constants of curvature corrections. We also construct the

asymptotically AdS black hole solutions with a linear dilaton field

numerically. We discuss the thermodynamical properties of those solutions.

Extending the model to the case with the even-order higher Lovelock curvatur... - Dark Energy and QCD Ghost, Nobuyoshi Ohta, Phys. Lett. B695:41-44,2011, Phys. Lett. B695:41-44,2011, Oct. 07 2010 Summary:It has been suggested that the dark energy that explains the observed

accelerating expansion of the universe may arise due to the contribution to the

vacuum energy of the QCD ghost in a time-dependent background. The argument

uses a four-dimensional simplified model. In this paper, we put the discussion

in more realistic model keeping all components of the QCD vector ghost and show

that indeed QCD ghost produces dark energy proportional to the Hubble parameter

$H\Lambda_{QCD}^3$ ($\Lambda_{QCD}$ is the QCD mass scale) which has the right

magnitude $\sim (3\times 10^{-3}$ eV)$^4$. - Cosmological intersecting brane solutions, Masato Minamitsuji, Nobuyoshi Ohta, Kunihito Uzawa, Phys.Rev.D82:086002,2010, Phys.Rev.D82:086002,2010, Jul. 11 2010 Summary:The recent discovery of an explicit dynamical description of p-branes makes

it possible to investigate the existence of intersection of such objects. We

generalize the solutions depending on the overall transverse space coordinates

and time to those which depend also on the relative transverse space and

satisfy new intersection rules. We give classification of these dynamical

intersecting brane solutions involving two branes, and discuss the application

of these solutions to cosmology and show that these give

Friedmann-Lemaitre-Robertson-Walker cosmological solutions. Finally, we

construct ... - Dynamical p-branes with a cosmological constant, Kei-ichi Maeda, Masato Minamitsuji, Nobuyoshi Ohta, Kunihito Uzawa, Phys.Rev.D82:046007,2010, Phys.Rev.D82:046007,2010, Jun. 11 2010 Summary:We present a class of dynamical solutions in a D-dimensional gravitational

theory coupled to a dilaton, a form field strength, and a cosmological

constant. We find that for any D due to the presence of a cosmological

constant, the metric of solutions depends on a quadratic function of the brane

world volume coordinates, and the transverse space cannot be Ricci flat except

for the case of 1-branes. We then discuss the dynamics of 1-branes in a

D-dimensional spacetime. For a positive cosmological constant, 1-brane

solutions with D>4 approach the Milne universe in the far-brane region. On the

... - Global Structure of Black Holes in String Theory with Gauss-Bonnet Correction in Various Dimensions, Nobuyoshi Ohta, Takashi Torii, Prog.Theor.Phys.124:207-225,2010, Prog.Theor.Phys.124:207-225,2010, Apr. 16 2010 Summary:We study global structures of black hole solutions in Einstein gravity with

Gauss-Bonnet term coupled to dilaton in various dimensions. In particular we

focus on the problem whether the singularity is weakened due to the

Gauss-Bonnet term and dilaton. We find that there appears the non-central

singularity between horizon and the center in many cases, where the metric does

not diverge but the Kretschmann invariant does diverge. Hence this is a

singularity, but we find the singularity is much milder than the Schwarzschild

solution and the non-dilatonic one. We discuss the origin of this "fat"... - Dynamical solutions in the 3-Form Field Background in the Nishino-Salam-Sezgin Model, Masato Minamitsuji, Nobuyoshi Ohta, Kunihito Uzawa, Mar. 31 2010 Summary:We investigate the dynamical 3-form flux compactifications and their

implications for cosmology in our brane world in the six-dimensional

Nishino-Salam-Sezgin model, which is usually referred to as the Salam-Sezgin

model. We take the background of the 3-form field acting on the internal space

and timelike dimensions without the U(1) gauge field strength. The first class

of solutions we discuss is the dynamical generalization of the static solutions

obtained recently. In this class, we find that the time evolution is restricted

only in the azimuthal dimension of the internal space and not in... - Notes on Entropy Force in General Spherically Symmetric Spacetimes, Rong-Gen Cai, Li-Ming Cao, Nobuyoshi Ohta, Phys.Rev.D81:084012,2010, Phys.Rev.D81:084012,2010, Feb. 05 2010 Summary:In a recent paper [arXiv:1001.0785], Verlinde has shown that the Newton

gravity appears as an entropy force. In this paper we show how gravity appears

as entropy force in Einstein's equation of gravitational field in a general

spherically symmetric spacetime. We mainly focus on the trapping horizon of the

spacetime. We find that when matter fields are absent, the change of entropy

associated with the trapping horizon indeed can be identified with an entropy

force. When matter fields are present, we see that heat flux of matter fields

also leads to the change of entropy. Applying arguments m... - Friedmann Equations from Entropic Force, Rong-Gen Cai, Li-Ming Cao, Nobuyoshi Ohta, Phys. Rev. D 81, 061501(R) (2010), Phys. Rev. D 81, 061501(R) (2010), Jan. 20 2010 Summary:In this note by use of the holographic principle together with the

equipartition law of energy and the Unruh temperature, we derive the Friedmann

equations of a Friedmann-Robertson-Walker universe. - Supersymmetric Intersecting Branes on the Waves, Kei-ichi Maeda, Nobuyoshi Ohta, Makoto Tanabe, Ryo Wakebe, JHEP 1004:013,2010, JHEP 1004:013,2010, Jan. 15 2010 Summary:We construct a general family of supersymmetric solutions in time- and

space-dependent wave backgrounds in general supergravity theories describing

single and intersecting p-branes embedded into time-dependent dilaton-gravity

plane waves of an arbitrary (isotropic) profile, with the brane world-volume

aligned parallel to the propagation direction of the wave. We discuss how many

degrees of freedom we have in the solutions. We also propose that these

solutions can be used to describe higher-dimensional time-dependent "black

holes", and discuss their property briefly. - Black Holes in Gravity with Conformal Anomaly and Logarithmic Term in Black Hole Entropy, Rong-Gen Cai, Li-Ming Cao, Nobuyoshi Ohta, JHEP 1004:082,2010, JHEP 1004:082,2010, Nov. 23 2009 Summary:We present a class of exact analytic and static, spherically symmetric black

hole solutions in the semi-classical Einstein equations with Weyl anomaly. The

solutions have two branches, one is asymptotically flat and the other

asymptotically de Sitter. We study thermodynamic properties of the black hole

solutions and find that there exists a logarithmic correction to the well-known

Bekenstein-Hawking area entropy. The logarithmic term might come from non-local

terms in the effective action of gravity theories. The appearance of the

logarithmic term in the gravity side is quite important in t... - Black Holes without Mass and Entropy in Lovelock Gravity, Rong-Gen Cai, Li-Ming Cao, Nobuyoshi Ohta, Phys.Rev.D81:024018,2010, Phys.Rev.D81:024018,2010, Nov. 02 2009 Summary:We present a class of new black hole solutions in $D$-dimensional Lovelock

gravity theory. The solutions have a form of direct product $\mathcal{M}^m

\times \mathcal{H}^{n}$, where $D=m+n$, $\mathcal{H}^n$ is a negative constant

curvature space, and are characterized by two integration constants. When $m=3$

and 4, these solutions reduce to the exact black hole solutions recently found

by Maeda and Dadhich in Gauss-Bonnet gravity theory. We study thermodynamics of

these black hole solutions. Although these black holes have a nonvanishing

Hawking temperature, surprisingly, the mass of these s... - Global solutions for higher-dimensional stretched small black holes, Chiang-Mei Chen, Dmitri V. Gal'tsov, Nobuyoshi Ohta, Dmitry G. Orlov, Oct. 19 2009 Summary:Small black holes in heterotic string theory have vanishing horizon area at

the supergravity level, but the horizon is stretched to the finite radius

$AdS_2 \times S^{D-2}$ geometry once higher curvature corrections are turned

on. This has been demonstrated to give good agreement with microscopic entropy

counting. Previous considerations, however, were based on the classical local

solutions valid only in the vicinity of the event horizon. Here we address the

question of global existence of extremal black holes in the $D$-dimensional

Einstein-Maxwell-Dilaton theory with the Gauss-Bonnet term... - Horizon Thermodynamics and Gravitational Field Equations in Horava-Lifshitz Gravity, Rong-Gen Cai, Nobuyoshi Ohta, Phys. Rev. D 81, 084061 (2010), Phys. Rev. D 81, 084061 (2010), Oct. 13 2009 Summary:We explore the relationship between the first law of thermodynamics and

gravitational field equation at a static, spherically symmetric black hole

horizon in Ho\v{r}ava-Lifshtiz theory with/without detailed balance. It turns

out that as in the cases of Einstein gravity and Lovelock gravity, the

gravitational field equation can be cast to a form of the first law of

thermodynamics at the black hole horizon. This way we obtain the expressions

for entropy and mass in terms of black hole horizon, consistent with those from

other approaches. We also define a generalized Misner-Sharp energy for st... - Generalized Misner-Sharp Energy in f(R) Gravity, Rong-Gen Cai, Li-Ming Cao, Ya-Peng Hu, Nobuyoshi Ohta, Phys.Rev.D80:104016,2009, Phys.Rev.D80:104016,2009, Oct. 13 2009 Summary:We study generalized Misner-Sharp energy in $f(R)$ gravity in a spherically

symmetric spacetime. We find that unlike the cases of Einstein gravity and

Gauss-Bonnet gravity, the existence of the generalized Misner-Sharp energy

depends on a constraint condition in the $f(R)$ gravity. When the constraint

condition is satisfied, one can define a generalized Misner-Sharp energy, but

it cannot always be written in an explicit quasi-local form. However, such a

form can be obtained in a FRW universe and for static spherically symmetric

solutions with constant scalar curvature. In the FRW universe, ... - Black Hole Solutions in String Theory with Gauss-Bonnet Curvature Correction, Kei-ichi Maeda, Nobuyoshi Ohta, Yukinori Sasagawa, Phys.Rev.D80:104032,2009, Phys.Rev.D80:104032,2009, Aug. 28 2009 Summary:We present the black hole solutions and analyse their properties in the

superstring effective field theory with the Gauss-Bonnet curvature correction

terms. We find qualitative differences in our results from those obtained in

the truncated model in the Einstein frame. The main difference in our model

from the truncated one is that the existence of a turning point in the

mass-area curve, the mass-entropy curve, and the mass-temperature curve in five

and higher dimensions, where we expect a change of stability. We also find a

mass gap in our model, where there is no black hole solution. In f... - Black Holes in the Dilatonic Einstein-Gauss-Bonnet Theory in Various Dimensions IV - Topological Black Holes with and without Cosmological Term, Nobuyoshi Ohta, Takashi Torii, Prog.Theor.Phys.122:1477-1500,2010, Prog.Theor.Phys.122:1477-1500,2010, Aug. 27 2009 Summary:We study black hole solutions in the Einstein gravity with Gauss-Bonnet term,

the dilaton and a positive "cosmological constant" in various dimensions.

Physically meaningful black holes with a positive cosmological term are

obtained only for those in static spacetime with $(D-2)$-dimensional hyperbolic

space of negative curvature and $D>4$. We construct such black hole solutions

of various masses numerically in $D=5,6$ and 10 dimensional spacetime and

discuss their properties. In spite of the positive cosmological constant the

spacetime approach anti-de Sitter spacetime asymptotically. The ... - Thermodynamics of Black Holes in Horava-Lifshitz Gravity, Rong-Gen Cai, Li-Ming Cao, Nobuyoshi Ohta, Phys.Lett.B679:504-509,2009, Phys.Lett.B679:504-509,2009, May 07 2009 Summary:By using the canonical Hamiltonian method, we obtain the mass and entropy of

the black holes with general dynamical coupling constant $\lambda$ in Ho\v{r}

ava-Lifshitz Gravity. Regardless of whether the horizon is sphere, plane or

hyperboloid, we find these black holes are thermodynamically stable in some

parameter space and unstable phase also exists in other parameter space. The

relation between the entropy and horizon area of the black holes has an

additional coefficient depending on the coupling constant $\lambda$, compared

to the $\lambda=1$ case. For $\lambda=1$, the well-known coeffi... - Topological Black Holes in Horava-Lifshitz Gravity, Rong-Gen Cai, Li-Ming Cao, Nobuyoshi Ohta, Phys.Rev.D80:024003,2009, Phys.Rev.D80:024003,2009, Apr. 23 2009 Summary:We find topological (charged) black holes whose horizon has an arbitrary

constant scalar curvature $2k$ in Ho\v{r}ava-Lifshitz theory. Without loss of

generality, one may take $k=1,0$ and -1. The black hole solution is

asymptotically AdS with a nonstandard asymptotic behavior. Using the

Hamiltonian approach, we define a finite mass associated with the solution. We

discuss the thermodynamics of the topological black holes and find that the

black hole entropy has a logarithmic term in addition to an area term. We find

a duality in Hawking temperature between topological black holes in

Ho\v{r}... - Dynamics of intersecting brane systems -- Classification and their applications --, Kei-ichi Maeda, Nobuyoshi Ohta, Kunihito Uzawa, JHEP 0906:051,2009, JHEP 0906:051,2009, Mar. 31 2009 Summary:We present dynamical intersecting brane solutions in higher-dimensional

gravitational theory coupled to dilaton and several forms. Assuming the forms

of metric, form fields, and dilaton field, we give a complete classification of

dynamical intersecting brane solutions with/without M-waves and Kaluza-Klein

monopoles in eleven-dimensional supergravity. We apply these solutions to

cosmology and black holes. It is shown that these give FRW cosmological

solutions and in some cases Lorentz invariance is broken in our world. If we

regard the bulk space as our universe, we may interpret them as bla... - Supersymmetric Intersecting Branes in Time-dependent Backgrounds, Kei-ichi Maeda, Nobuyoshi Ohta, Makoto Tanabe, Ryo Wakebe, JHEP 0906:036,2009, JHEP 0906:036,2009, Mar. 19 2009 Summary:We construct a fairy general family of supersymmetric solutions in time- and

space-dependent backgrounds in general supergravity theories. One class of the

solutions are intersecting brane solutions with factorized form of time- and

space-dependent metrics, the second class are brane solutions in pp-wave

backgrounds carrying spacetime-dependence, and the final class are the

intersecting branes with more nontrivial spacetime-dependence, and their

intersection rules are given. Physical properties of these solutions are

discussed, and the relation to existing literature is also briefly mention... - Black Holes in the Dilatonic Einstein-Gauss-Bonnet Theory in Various Dimensions III -- Asymptotically AdS Black Holes with $k=\pm 1$ --, Nobuyoshi Ohta, Takashi Torii, Prog.Theor.Phys.121:959-981,2009, Prog.Theor.Phys.121:959-981,2009, Feb. 24 2009 Summary:We study black hole solutions in the Einstein-Gauss-Bonnet gravity with the

dilaton and a negative ``cosmological constant''. We derive the field equations

for the static spherically symmetric ($k=1$) and hyperbolically symmetric

($k=-1$) spacetime in general $D$ dimensions. The system has some scaling

symmetries which are used in our analysis of the solutions. We find exact

solutions, i.e., regular AdS solution for $k=1$ and a massless black hole

solution for $k=-1$. Nontrivial asymptotically AdS solutions are obtained

numerically in D=4 -- 6 and 10 dimensional spacetimes. For spherically

... - Shear Viscosity from Gauss-Bonnet Gravity with a Dilaton Coupling, Rong-Gen Cai, Zhang-Yu Nie, Nobuyoshi Ohta, Ya-Wen Sun, Phys.Rev.D79:066004,2009, Phys.Rev.D79:066004,2009, Jan. 11 2009 Summary:We calculate the shear viscosity of field theories with gravity duals of

Gauss-Bonnet gravity with a non-trivial dilaton using AdS/CFT. We find that the

dilaton filed has a non-trivial contribution to the ratio of shear viscosity

over entropy density and after imposing causal constraint for the boundary

field theory, the new lower bound $4/25\pi$, obtained from pure Gauss-Bonnet

gravity, may have a small violation. - Black Holes in the Dilatonic Einstein-Gauss-Bonnet Theory in Various Dimensions II -- Asymptotically AdS Topological Black Holes --, Zong-Kuan Guo, Nobuyoshi Ohta, Takashi Torii, Prog.Theor.Phys.121:253-273,2009, Prog.Theor.Phys.121:253-273,2009, Nov. 19 2008 Summary:We study asymptotically AdS topological black hole solutions with k=0 (plane

symmetric) in the Einstein gravity with Gauss-Bonnet term, the dilaton and a

"cosmological constant" in various dimensions. We derive the field equations

for suitable ansatz for general D dimensions. We determine the parameter

regions including dilaton couplings where such solutions exist and construct

black hole solutions of various masses numerically in D=4,5,6 and 10

dimensional spacetime with (D-2)-dimensional hypersurface of zero curvature. - Black Holes in the Dilatonic Einstein-Gauss-Bonnet Theory in Various Dimensions I -- Asymptotically Flat Black Holes --, Zong-Kuan Guo, Nobuyoshi Ohta, Takashi Torii, Prog.Theor.Phys.120:581-607,2008, Prog.Theor.Phys.120:581-607,2008, Jun. 16 2008 Summary:We study spherically symmetric, asymptotically flat black hole solutions in

the low-energy effective heterotic string theory, which is the Einstein gravity

with Gauss-Bonnet term and the dilaton, in various dimensions. We derive the

field equations for suitable ansatz for general D dimensions and construct

black hole solutions of various masses numerically in D=4,5,6 and 10

dimensional spacetime with (D-2)-dimensional hypersurface with positive

constant curvature. A detailed comparison with the non-dilatonic solutions is

made. We also examine the thermodynamic properties of the solutions. I... - Generic estimates for magnetic fields generated during inflation including Dirac-Born-Infeld theories, Kazuharu Bamba, Nobuyoshi Ohta, Shinji Tsujikawa, Phys.Rev.D78:043524,2008, Phys.Rev.D78:043524,2008, May 26 2008 Summary:We estimate the strength of large-scale magnetic fields produced during

inflation in the framework of Dirac-Born-Infeld (DBI) theories. This analysis

is sufficiently general in the sense that it covers most of conformal symmetry

breaking theories in which the electromagnetic field is coupled to a scalar

field. In DBI theories there is an additional factor associated with the speed

of sound, which allows a possibility to lead to an extra amplification of the

magnetic field in a ultra-relativistic region. We clarify the conditions under

which seed magnetic fields to feed the galactic dynamo m... - Cosmological Evolution of Dirac-Born-Infeld Field, Zong-Kuan Guo, Nobuyoshi Ohta, JCAP0804:035,2008, JCAP0804:035,2008, Mar. 07 2008 Summary:We investigate the cosmological evolution of the system of a

Dirac-Born-Infeld field plus a perfect fluid. We analyze the existence and

stability of scaling solutions for the AdS throat and the quadratic potential.

We find that the scaling solutions exist when the equation of state of the

perfect fluid is negative and in the ultra-relativistic limit. - Entropy Function and Universality of Entropy-Area Relation for Small Black Holes, Rong-Gen Cai, Chiang-Mei Chen, Kei-ichi Maeda, Nobuyoshi Ohta, Da-Wei Pang, Phys.Rev.D77:064030,2008, Phys.Rev.D77:064030,2008, Dec. 27 2007 Summary:We discuss the entropy-area relation for the small black holes with higher

curvature corrections by using the entropy function formalism and field

redefinition method. We show that the entropy $S_{BH}$ of small black hole is

proportional to its horizon area $A$. In particular we find a universal result

that $S_{BH}=A/2G$, the ratio is two times of Bekenstein-Hawking entropy-area

formula in many cases of physical interest. In four dimensions, the universal

relation is always true irrespective of the coefficients of the higher-order

terms if the dilaton couplings are the same, which is the ca... - Accelerating Cosmologies in the Einstein-Gauss-Bonnet Theory with Dilaton, Kazuharu Bamba, Zong-Kuan Guo, Nobuyoshi Ohta, Prog.Theor.Phys.118:879-892,2007, Prog.Theor.Phys.118:879-892,2007, Jul. 30 2007 Summary:We study cosmological solutions in the low-energy effective heterotic string

theory, which is the Einstein gravity with Gauss-Bonnet term and the dilaton.

We show that the field equations are cast into an autonomous system for flat

internal and external spaces, and derive all the fixed points in the system. We

also examine the time evolution of the solutions and whether the solutions can

give (transient) accelerated expansion of our four-dimensional space in the

Einstein frame. - Deconfinement Transition of AdS/QCD at ${\cal O}(α'^3)$, Rong-Gen Cai, Nobuyoshi Ohta, Phys.Rev.D76:106001,2007, Phys.Rev.D76:106001,2007, Jul. 14 2007 Summary:We study the confinement/deconfinement phase transition of holographic

AdS/QCD models by using Ricci flat $AdS_5$ black holes up to ${\cal

O}(\alpha'^3)$, which corresponds to the $\lambda$ expansion correction in the

dual field theory to $\lambda^{-3/2}$, where $\lambda$ is the 't Hooft coupling

constant. We consider two cases: one is the hard-wall AdS/QCD model where a

small radius region of the $AdS_5$ is removed; the other is the case where one

of spatial coordinates for the $AdS_5$ space is compactified, resulting in

Witten's QCD model in 2+1 dimensions. We find that in the hard-wall A... - Probing the Coupling between Dark Components of the Universe, Zong-Kuan Guo, Nobuyoshi Ohta, Shinji Tsujikawa, Phys.Rev.D76:023508,2007, Phys.Rev.D76:023508,2007, Feb. 01 2007 Summary:We place observational constraints on a coupling between dark energy and dark

matter by using 71 Type Ia supernovae (SNe Ia) from the first year of the

five-year Supernova Legacy Survey (SNLS), the cosmic microwave background (CMB)

shift parameter from the three-year Wilkinson Microwave Anisotropy Probe

(WMAP), and the baryon acoustic oscillation (BAO) peak found in the Sloan

Digital Sky Survey (SDSS). The interactions we study are (i) constant coupling

delta and (ii) varying coupling delta(z) that depends on a redshift z, both of

which have simple parametrizations of the Hubble parameter t... - The D2 Susy Zoo, Dongsu Bak, Nobuyoshi Ohta, Paul K. Townsend, JHEP 0703:013,2007, JHEP 0703:013,2007, Dec. 12 2006 Summary:We present new supersymmetric solutions of the Dirac-Born-Infeld equations

for time-independent D2-branes, including a 1/2 supersymmetric `dyonic'

D2-brane and various 1/4 supersymmetric configurations that include `twisted'

supertubes, superfunnels with arbitrary planar cross-section, asymptotically

planar D2-branes, and non-singular intersections of `magnetic' D2-branes. Our

analysis is exhaustive for D2-branes in three space dimensions. - Realizing Scale-invariant Density Perturbations in Low-energy Effective String Theory, Zong-Kuan Guo, Nobuyoshi Ohta, Shinji Tsujikawa, Phys.Rev.D75:023520,2007, Phys.Rev.D75:023520,2007, Nov. 01 2006 Summary:We discuss the realization of inflation and resulting cosmological

perturbations in the low-energy effective string theory. In order to obtain

nearly scale-invariant spectra of density perturbations and a suppressed

tensor-to-scalar ratio, it is generally necessary that the dilaton field $\phi$

is effectively decoupled from gravity together with the existence of a slowly

varying dilaton potential. We also study the effect of second-order corrections

to the tree-level action which are the sum of a Gauss-Bonnet term coupled to

$\phi$ and a kinetic term $(\nabla \phi)^4$. We find that it is po... - Supersymmetric Intersecting Branes in Time-dependent Backgrounds, Nobuyoshi Ohta, Kamal L. Panigrahi, Phys.Rev.D74:126003,2006, Phys.Rev.D74:126003,2006, Oct. 01 2006 Summary:We construct a family of supersymmetric solutions in time-dependent

backgrounds in supergravity theories. One class of the solutions are

intersecting brane solutions and another class are brane solutions in pp-wave

backgrounds, and their intersection rules are also given. The relation to

existing literature is also discussed. An example of D1-D5 with linear null

dilaton together with its possible dual theory is briefly discussed. - Addendum to "Hyperbolic Space Cosmologies", Chiang-Mei Chen, Pei-Ming Ho, Ishwaree P. Neupane, Nobuyoshi Ohta, John E. Wang, JHEP0611:044,2006, JHEP0611:044,2006, Sep. 06 2006 Summary:In our earlier paper [JHEP 0310 (2003) 058], we considered higher dimensional

cosmological models with hyperbolic spaces. In particular the eternal

accelerating expansion was obtained by studying small perturbation around the

critical non-accelerated solution for $D > 10$. In this addendum, we show that

there is also such a solution in the critical case $D = 10$. - On the No-ghost Theorem in String Theory, Kazuyuki Furuuchi, Nobuyoshi Ohta, Prog.Theor.Phys. 116 (2006) 601-604, Prog.Theor.Phys. 116 (2006) 601-604, Jul. 17 2006 Summary:We give a simple proof of the no-ghost theorem in the critical bosonic string

theory by using a similarity transformation. - Supersymmetric Rotating Black Hole in a Compactified Spacetime, Kei-ichi Maeda, Nobuyoshi Ohta, Makoto Tanabe, Phys.Rev. D74 (2006) 104002, Phys.Rev. D74 (2006) 104002, Jul. 13 2006 Summary:We construct a supersymmetric rotating black hole with asymptotically flat

four-dimensional spacetime times a circle, by superposing an infinite number of

BMPV black hole solutions at the same distance in one direction. The near

horizon structure is the same as that of the five-dimensional BMPV black hole.

The rotation of this black hole can exceed the Kerr bound in general relativity

($q\equiv a/G_4 M=1$), if the size is small. - Time-Dependent Supersymmetric Solutions in M-Theory and the Compactification-Decompactification Transition, Hideo Kodama, Nobuyoshi Ohta, Prog.Theor.Phys.116:295-318,2006, Prog.Theor.Phys.116:295-318,2006, May 18 2006 Summary:We show that the diagonal light-like solution with 16 supersymmetries in

eleven-dimensional supergravity derived in our previous paper (hep-th/0509173)

can be generalised to non-diagonal solutions preserving the same number of

supersymmetries. This class of solutions contains a subclass equivalent to the

class of solutions found by Bin Chen that are dependent on the

spatial-coordinates. Utilising these solutions, we construct toroidally

compactified solutions that smoothly connect a static compactified region with

a dynamically decompactifying region along a null hypersurface. - Black Holes in Pure Lovelock Gravities, Rong-Gen Cai, Nobuyoshi Ohta, Phys.Rev. D74 (2006) 064001, Phys.Rev. D74 (2006) 064001, Apr. 12 2006 Summary:The Lovelock gravity is a fascinating extension of general relativity, whose

action consists of the dimensionally extended Euler densities. Compared to

other higher order derivative gravity theories, the Lovelock gravity is

attractive since it has a lot of remarkable features such as that there are no

more than second order derivatives with respect to metric in its equations of

motion, and that the theory is free of ghost. Recently in the study of black

string and black brane in the Lovelock gravity, a special class of Lovelock

gravity is considered, which is named pure Lovelock gravity, wh... - Matrix String Description of Cosmic Singularities in a Class of Time-dependent Solutions, Takayuki Ishino, Nobuyoshi Ohta, Phys.Lett.B638:105-109,2006, Phys.Lett.B638:105-109,2006, Mar. 28 2006 Summary:A large class of time-dependent solutions with 1/2 supersymmetry were found

previously. These solutions involve cosmic singularities at early time. In this

paper, we study if matrix string description of the singularities in these

solutions with backgrounds is possible and present several examples where the

solutions can be described well in the perturbative picture. - Mass and Thermodynamics of Kaluza-Klein Black Holes with Squashed Horizons, Rong-Gen Cai, Li-Ming Cao, Nobuyoshi Ohta, Phys.Lett.B639:354-361,2006, Phys.Lett.B639:354-361,2006, Mar. 26 2006 Summary:Recently a five-dimensional Kaluza-Klein black hole solution with squashed

horizon has been found in hep-th/0510094. The black hole spacetime is

asymptotically locally flat and has a spatial infinity $S^1 \hookrightarrow

S^{2}$. By using "boundary counterterm" method and generalized Abbott-Deser

method, we calculate the mass of this black hole. When an appropriate

background is chosen, the generalized Abbott-Deser method gives the same mass

as the "boundary counterterm" method. The mass is found to satisfy the first

law of black hole thermodynamics. The thermodynamic properties of the

Kaluz... - Parametrizations of the Dark Energy Density and Scalar Potentials, Zong-Kuan Guo, Nobuyoshi Ohta, Yuan-Zhong Zhang, Mod.Phys.Lett.A22:883-890,2007, Mod.Phys.Lett.A22:883-890,2007, Mar. 04 2006 Summary:We develop a theoretical method of constructing the scalar (quintessence or

phantom) potential directly from the dimensionless dark energy function X(z),

the dark energy density in units of its present value. We apply our method to

two parametrizations of the dark energy density, the quiessence-Lambda ansatz

and the generalized Chaplygin gas model, and discuss some features of the

constructed potentials. - Inflation from Superstring/M-Theory Compactification with Higher Order Corrections II -- Case of Quartic Weyl Terms --, Kenta Akune, Kei-ichi Maeda, Nobuyoshi Ohta, Phys.Rev.D73:103506,2006, Phys.Rev.D73:103506,2006, Feb. 23 2006 Summary:We present a detailed study of inflationary solutions in M-theory with higher

order quantum corrections. We first exhaust all exact and asymptotic solutions

of exponential and power-law expansions in this theory with quartic curvature

corrections, and then perform a linear perturbation analysis around fixed

points for the exact solutions in order to see which solutions are more generic

and give interesting cosmological models. We find an interesting solution in

which the external space expands exponentially and the internal space is static

both in the original and Einstein frames. This may ... - Holography and D3-branes in Melvin Universes, Rong-Gen Cai, Nobuyoshi Ohta, Phys.Rev.D73:106009,2006, Phys.Rev.D73:106009,2006, Jan. 08 2006 Summary:Recently,in a paper hep-th/0511197, it was found that non-commutative super

Yang-Mills (NCSYM) theory with space-dependent noncommutativity can be

formulated as a decoupling limit of open strings ending on D3-branes wrapping a

Melvin universe supported by a flux of the NSNS B-field. Under S-duality, we

show that this theory turns into a noncommutative open string (NCOS) theory

with space-dependent space-time noncommutativity and effective space-dependent

string scale. It is an NCOS theory with both space-dependent space-space and

space-time noncommutativities under more general $SL(2,\mathb... - M-theory Supertubes with Three and Four Charges, Dongsu Bak, Kyungyu Kim, Nobuyoshi Ohta, JHEP0601:072,2006, JHEP0601:072,2006, Nov. 04 2005 Summary:Using the covariant M5-brane action, we construct configurations

corresponding to supertubes with three and four charges. We derive the BPS

equations and study the full structure of the solutions. In particular, we find

new solutions involving arbitrariness in field strengths. - Time-dependent Solutions with Null Killing Spinor in M-theory and Superstrings, Takayuki Ishino, Hideo Kodama, Nobuyoshi Ohta, Phys.Lett.B631:68-73,2005, Phys.Lett.B631:68-73,2005, Sep. 22 2005 Summary:Imposing the condition that there should be a null Killing spinor with all

the metrics and background field strengths being functions of the light-cone

coordinates, we find general 1/2 BPS solutions in D=11 supergravity, and

discuss several examples. In particular we show that the linear dilaton

background is the most general supersymmetric solution without background under

the additional requirement of flatness in the string frame. We also give the

most general solutions for flat spacetime in the string frame with RR or NS-NS

backgrounds, and they are characterized by a single function. - Intersecting Non-extreme p-Branes and Linear Dilaton Background, Chiang-Mei Chen, Dmitri V. Gal'tsov, Nobuyoshi Ohta, Phys.Rev. D72 (2005) 044029, Phys.Rev. D72 (2005) 044029, Jun. 26 2005 Summary:We construct the general static solution to the supergravity action

containing gravity, the dilaton and a set of antisymmetric forms describing the

intersecting branes delocalized in the relative transverse dimensions. The

solution is obtained by reducing the system to a set of separate Liouville

equations (the intersection rules implying the separability); it contains the

maximal number of free parameters corresponding to the rank of the differential

equations. Imposing the requirement of the absence of naked singularities, we

show that the general configurations are restricted to two and ... - Parametrization of Quintessence and Its Potential, Zong-Kuan Guo, Nobuyoshi Ohta, Yuan-Zhong Zhang, Phys.Rev.D72:023504,2005, Phys.Rev.D72:023504,2005, May 13 2005 Summary:We develop a theoretical method of constructing the quintessence potential

directly from the effective equation of state function $w(z)$, which describes

the properties of the dark energy. We apply our method to four parametrizations

of equation of state parameter and discuss the general features of the

resulting potentials. In particular, it is shown that the constructed

quintessence potentials are all in the form of a runaway type. - Accelerating Cosmologies and Inflation from M/Superstring Theories, Nobuyoshi Ohta, Int.J.Mod.Phys. A20 (2005) 1-40, Int.J.Mod.Phys. A20 (2005) 1-40, Nov. 25 2004 Summary:We review the recent developments in obtaining accelerating cosmologies

and/or inflation from higher-dimensional gravitational theories, in particular

superstring theories in ten dimensions and M-theory in eleven dimensions. We

first discuss why it is difficult to obtain inflationary behavior in the

effective low-energy theories of superstring/M-theory, i. e. supergravity

theories. We then summarize interesting solutions including S-branes that give

rise to accelerating cosmologies and inflationary solutions in M-theory with

higher order corrections. Other approaches to inflation in the str... - Inflation from Superstring/M Theory Compactification with Higher Order Corrections I, Kei-ichi Maeda, Nobuyoshi Ohta, Phys.Rev. D71 (2005) 063520, Phys.Rev. D71 (2005) 063520, Nov. 09 2004 Summary:We study time-dependent solutions in M and superstring theories with higher

order corrections. We first present general field equations for theories of

Lovelock type with stringy corrections in arbitrary dimensions. We then exhaust

all exact and asymptotic solutions of exponential and power-law expansions in

the theory with Gauss-Bonnet terms relevant to heterotic strings and in the

theories with quartic corrections corresponding to the M-theory and type II

superstrings. We discuss interesting inflationary solutions that can generate

enough e-foldings in the early universe. - A Geometric Look on the Microstates of Supertubes, Dongsu Bak, Yoshifumi Hyakutake, Seok Kim, Nobuyoshi Ohta, Nucl.Phys. B712 (2005) 115-138, Nucl.Phys. B712 (2005) 115-138, Jul. 30 2004 Summary:We give a geometric interpretation of the entropy of the supertubes with

fixed conserved charges and angular momenta in two different approaches using

the DBI action and the supermembrane theory. By counting the geometrically

allowed microstates, it is shown that both the methods give consistent result

on the entropy. In doing so, we make the connection to the gravity microstates

clear. - Inflation from M-Theory with Fourth-order Corrections and Large Extra Dimensions, Kei-ichi Maeda, Nobuyoshi Ohta, Phys.Lett. B597 (2004) 400-407, Phys.Lett. B597 (2004) 400-407, May 24 2004 Summary:We study inflationary solutions in the M-theory. Including the fourth-order

curvature correction terms, we find three generalized de Sitter solutions, in

which our 3-space expands exponentially. Taking one of the solutions, we

propose an inflationary scenario of the early universe. This provides us a

natural explanation for large extra dimensions in a brane world, and suggests

some connection between the 60 e-folding expansion of inflation and TeV gravity

based on the large extra dimensions. - Phase Moduli Space of Supertubes, Dongsu Bak, Yoshifumi Hyakutake, Nobuyoshi Ohta, Nucl.Phys. B696 (2004) 251-262, Nucl.Phys. B696 (2004) 251-262, Apr. 15 2004 Summary:We study possible deformations of BPS supertubes keeping their conserved

charges fixed. We show that there is no flat direction to closed supertubes of

circular cross section with uniform electric and magnetic fields, and also to

open planar supertubes. We also find that there are continuously infinite flat

deformations to supertubes of general shape under certain conditions. - Complete Intersecting Non-Extreme p-Branes, Yan-Gang Miao, Nobuyoshi Ohta, Phys.Lett. B594 (2004) 218-226, Phys.Lett. B594 (2004) 218-226, Apr. 12 2004 Summary:We give general intersecting brane solutions without assuming any restriction

on the metric in supergravity coupled to a dilaton and antisymmetric tensor

fields in arbitrary dimensions $D$. The result is a general class of

intersecting brane solutions which interpolate the non-extreme solutions of

type 1 and 2. We also discuss the relation of our solutions to the known single

brane solution. - Dilatonic Repulsons and Confinement via the AdS/CFT Correspondence, Dongsu Bak, Michael Gutperle, Shinji Hirano, Nobuyoshi Ohta, Phys.Rev. D70 (2004) 086004, Phys.Rev. D70 (2004) 086004, Mar. 25 2004 Summary:We study a class of dilatonic deformations of asymptotically AdS_5 X S^5

geometry analytically and numerically. The spacetime is non-supersymmetric and

suffers from a naked singularity. We propose that the causality bound may serve

as a criterion for such a geometry with a naked singularity to still make sense

in the AdS/CFT correspondence. We show that the static string, the one

corresponding to a large Wilson loop in the dual gauge theory, reveals

confinement in a certain range of parameters of our solutions, where the

singularity exhibits the repulsion that can well cloak the singularity... - Hyperbolic Space Cosmologies, Chiang-Mei Chen, Pei-Ming Ho, Ishwaree P. Neupane, Nobuyoshi Ohta, John E. Wang, JHEP 0310:058,2003, JHEP 0310:058,2003, Jun. 30 2003 Summary:We present a systematic study of accelerating cosmologies obtained from

M/string theory compactifications of hyperbolic spaces with time-varying

volume. A set of vacuum solutions where the internal space is a product of

hyperbolic manifolds is found to give qualitatively the same accelerating

four-dimensional FLRW universe behavior as a single hyperbolic space. We also

examine the possibility that our universe is a hyperbolic space and provide

exact Milne type solutions, as well as intersecting S-brane solutions. When

both the usual 4D spacetime and the m-dimensional internal space are

hype... - Intersecting branes in pp-wave spacetime, Nobuyoshi Ohta, Kamal L. Panigrahi, Sanjay, Nucl.Phys. B674 (2003) 306-328; Erratum-ibid. B748 (2006) 309-332, Nucl.Phys. B674 (2003) 306-328; Erratum-ibid. B748 (2006) 309-332, Jun. 20 2003 Summary:We derive intersecting brane solutions in pp-wave spacetime by solving the

supergravity field equations explicitly. The general intersection rules are

presented. We also generalize the construction to the non-extremal solutions.

Both the extremal and non-extremal solutions presented here are asymptotic to

BFHP plane waves. We find that these solutions do not have regular horizons. - Uniqueness of M-theory PP-Wave Background with Extra Supersymmetries, Nobuyoshi Ohta, Makoto Sakaguchi, Phys.Rev. D69 (2004) 066006, Phys.Rev. D69 (2004) 066006, May 24 2003 Summary:We examine Killing spinor equations of the general eleven-dimensional pp-wave

backgrounds, which contain a scalar H(x^m,x^-) in the metric and a three-form

\xi(x^m,x^-) in the flux. Considering non-harmonic extra Killing spinors, we

show that if the backgrounds admit at least one extra Killing spinor in

addition to the standard 16 Killing spinors, they can be reduced to the form

with H=A_{mn}(x^-)x^mx^n and \xi(x^-) modulo coordinate transformations. We

further examine the cases in which the extra Killing spinor is characterized by

a set of Cartan matrices. The super-isometry algebras of th... - A Study of Accelerating Cosmologies from Superstring/M theories, Nobuyoshi Ohta, Prog.Theor.Phys.110:269-283,2003, Prog.Theor.Phys.110:269-283,2003, Apr. 21 2003 Summary:We study aspects of the accelerating cosmologies obtained from the

compactification of vacuum solution and S2-branes of superstring/M theories.

Parameter dependence of the resulting expansion of our universe and internal

space is examined for all cases. We find that accelerating expansions are

obtained also from spherical internal spaces, albeit the solution enters into

contracting phase eventually. The relation between the models of SM2- and

SD2-branes are also discussed, and a potential problem with SD2-brane model is

noted. - Accelerating Cosmologies from S-Branes, Nobuyoshi Ohta, Phys.Rev.Lett.91:061303,2003, Phys.Rev.Lett.91:061303,2003, Mar. 27 2003 Summary:We point out that the recently proposed model of a flat 4-dimensional

universe with accelerated expansion in string/M-theory is a special case of

time-dependent solutions that the author found under the name of ``S-branes.''

We also show that similar accelerating models can be obtained from S-branes if

the internal space is chosen to be hyperbolic or flat spaces. - Null-Brane Solutions in Supergravities, Nobuyoshi Ohta, Phys.Lett. B559 (2003) 270-278, Phys.Lett. B559 (2003) 270-278, Feb. 18 2003 Summary:We find a new class of time-dependent brane solutions in supergravities in

arbitrary dimensions $D$. These are general intersecting light-like branes

(null-branes), and their superposition and intersection rules are obtained.

This is achieved by directly solving bosonic field equations for supergravity

coupled to a dilaton and antisymmetric tensor fields. We discuss their possible

significance. - Parent Actions, Dualities and New Weyl-invariant Actions of Bosonic p-branes, Yan-Gang Miao, Nobuyoshi Ohta, JHEP 0304:010,2003, JHEP 0304:010,2003, Jan. 29 2003 Summary:By using the systematic approach of parent action method, we derive one

Weyl-noninvariant and two Weyl-invariant actions of bosonic $p$-branes ($p\geq

2$) starting from the Nambu-Goto action, and establish the duality symmetries

in this set of four actions. Moreover, we discover a new bosonic $p$-brane

action (including the string theory) and deduce two corresponding

Weyl-invariant formulations by proposing a new special parent action. We find

that the same duality symmetries as those mentioned above exist in this new set

of actions. The new $p$-brane actions are also briefly analyzed. - Intersection Rules for S-Branes, Nobuyoshi Ohta, Phys.Lett. B558 (2003) 213-220, Phys.Lett. B558 (2003) 213-220, Jan. 15 2003 Summary:We give a model-independent derivation of general intersecting rules for

spacelike branes (S-branes) in arbitrary dimensions $d$. This is achieved by

directly solving bosonic field equations for supergravity coupled to a dilaton

and antisymmetric tensor fields with minimal ans\"{a}tze. We compare the

results with those in eleven-dimensional supergravity and other solutions. - NCOS and D-branes in Time-dependent Backgrounds, Rong-Gen Cai, Jian-Xin Lu, Nobuyoshi Ohta, Phys.Lett.B551:178-186,2003, Phys.Lett.B551:178-186,2003, Oct. 22 2002 Summary:We study noncommutative open string (NCOS) theories realized in string theory

with time-dependent backgrounds. Starting from a noncommutative Yang-Mills

theory (NCYM) with a constant space-space noncommutativity but in a

time-dependent background and making an S-dual transformation, we show that the

resulting theory is an NCOS also in a time-dependent background but now with a

time-dependent time-space noncommutativity and a time-dependent string scale.

The corresponding dual gravity description is also given. A general SL(2,Z)

transformation on the NCYM results in an NCOS with a time-depen... - Supersymmetric Brane-Antibrane Systems: Matrix Model Description, Stability and Decoupling Limits, Dongsu Bak, Nobuyoshi Ohta, Mohammad M, Sheikh-Jabbari, JHEP 0209:048,2002, JHEP 0209:048,2002, May 26 2002 Summary:After reviewing the supertubes and super brane-antibrane systems in the

context of matrix model, we look for more general higher-dimensional

configurations. For D3-bar{D3}, we find a non-trivial configuration with E cdot

B not equal to 0 and describe the worldvolume gauge theory. We present the

string probe of D3-bar{D3} system and study the decoupling limits leading to

either noncommutative Super-Yang-Mills or NCOS theories with eight

supercharges. - Supertubes and Supercurves from M-Ribbons, Yoshifumi Hyakutake, Nobuyoshi Ohta, Phys.Lett. B539 (2002) 153-161, Phys.Lett. B539 (2002) 153-161, Apr. 19 2002 Summary:We construct 1/4 BPS configurations, `M-ribbons', in M-theory on T^2, which

give the supertubes and supercurves in type IIA theory upon dimensional

reduction. These M-ribbons are generalized so as to be consistent with the

SL(2,Z) modular transformation on T^2. In terms of the type IIB theory, the

generalized M-ribbons are interpreted as an SL(2,Z) duality family of super

D-helix. It is also shown that the BPS M-ribbons must be straight in one

direction. - Moduli Space and Scattering of D0-Branes in Noncommutative Super Yang-Mills Theory, Masashi Hamanaka, Yasuyuki Imaizumi, Nobuyoshi Ohta, Phys.Lett. B529 (2002) 163-170, Phys.Lett. B529 (2002) 163-170, Dec. 07 2001 Summary:We study the moduli space of the D0-brane system on Dp-branes realized in the

noncommutative super Yang-Mills theory. By examining the fluctuations around

the solitonic solutions generated by solution generating technique, we confirm

the interpretation of the moduli as the positions of D0-branes on Dp-branes.

Low-energy scattering process is also examined for two D0-branes. We find that

the D0-branes scatter at right angle for head-on collision in the D0-D4 system.

For D0-D6 and D0-D8 systems we find special solutions which reduce to the D0-D4

case, giving the same behavior. This suggests t... - Supersymmetric D2 anti-D2 Strings, Dongsu Bak, Nobuyoshi Ohta, Phys.Lett. B527 (2002) 131-141, Phys.Lett. B527 (2002) 131-141, Dec. 05 2001 Summary:We consider the flat supersymmetric D2 and anti-D2 system, which follows from

ordinary noncommutative D2 anti-D2 branes by turning on an appropriate

worldvolume electric field describing dissolved fundamental strings. We study

the strings stretched between D2 and anti-D2 branes and show explicitly that

the would-be tachyonic states become massless. We compute the string spectrum

and clarify the induced noncommutativity on the worldvolume. The results are

compared with the matrix theory description of the worldvolume gauge theories. - On decoupled theories in (5+1) dimensions from (F,D1,NS5,D5) supergravity configuration, R. -G. Cai, I. Mitra, N. Ohta, S. Roy, JHEP 0112:029,2001, JHEP 0112:029,2001, Nov. 03 2001 Summary:It is well-known that ($N$, $M$) 5-branes of type IIB supergravity form a

non-threshold bound state with ($N'$, $M'$) strings called the (F, D1, NS5, D5)

bound state where the strings lie along one of the spatial directions of the

5-branes (hep-th/9905056). By taking low energy limits in appropriate ways on

this supergravity configuration, we obtain the supergravity duals of various

decoupled theories in (5+1) dimensions corresponding to noncommutative open

string (NCOS) theory, open D-string (OD1) theory and open D5-brane (OD5)

theory. We then study the $SL(2, Z)$ transformation properties... - Supersymmetry, Spectrum and Fate of D0-Dp Systems with B-field, Akira Fujii, Yasuyuki Imaizumi, Nobuyoshi Ohta, Nucl.Phys. B615 (2001) 61-81, Nucl.Phys. B615 (2001) 61-81, May 09 2001 Summary:It has been shown that D0-D$p$ $(p=2, 4, 6, 8)$ systems can be BPS in the

presence of $B$-field even if they are not otherwise. We review the number of

remaining supersymmetries, the open string ground state spectrum and the

construction of the D0-D$p$ systems as solitonic solutions in the

noncommutative super Yang-Mills theory. We derive the complete mass spectrum of

the fluctuations to discuss the stability of the systems. The results are found

to agree with the analysis in the string picture. In particular, we show that

supersymmetry is enhanced in D0-D8 depending on the $B$-fields and i... - Bekenstein Bound, Holography and Brane Cosmology in Charged Black Hole Backgrounds, Rong-Gen Cai, Yun Soo Myung, Nobuyoshi Ohta, Class.Quant.Grav. 18 (2001) 5429-5440, Class.Quant.Grav. 18 (2001) 5429-5440, May 08 2001 Summary:We obtain a Bekenstein entropy bound for the charged objects in arbitrary

dimensions ($D\ge 4$) using the D-bound recently proposed by Bousso. With the

help of thermodynamics of CFTs corresponding to AdS Reissner-Norstr\"om (RN)

black holes, we discuss the relation between the Bekenstein and

Bekenstein-Verlinde bounds. In particular we propose a Bekenstein-Verlinde-like

bound for the charged systems. In the Einstein-Maxwell theory with a negative

cosmological constant, we discuss the brane cosmology with positive tension

using the Binetruy-Deffayet-Langlois approach. The resulting

Friedman-... - M-brane interpolations and (2,0) Renormalization Group flow, E. Bergshoeff, R-G. Cai, N. Ohta, P. K. Townsend, Phys.Lett. B495 (2000) 201-206, Phys.Lett. B495 (2000) 201-206, Sep. 19 2000 Summary:We obtain the M5-M2-MW bound state solutions of 11-dimensional supergravity

corresponding to the 1/2 supersymmetric vacua of the M5-brane equations with

constant background fields. In the `near-horizon' case the solution

interpolates between the $adS_7\times S^4$ Kaluza-Klein vacuum and D=11

Minkowski spacetime via a Domain Wall spacetime. We discuss implications for

renormalization group flow of (2,0) D=6 field theories. - Noncommutative Gauge Dynamics from Brane Configurations with Background B field, Nobuyoshi Ohta, Dan Tomino, Prog.Theor.Phys. 105 (2001) 287-299, Prog.Theor.Phys. 105 (2001) 287-299, Sep. 04 2000 Summary:We study D=3 field theories on the D3-branes stretched between two NS5-branes

with NS B-field background. The theory is a noncommutative gauge theory. The

mirror symmetry and S-duality of the theory are discussed. A new feature is

that the mirror of the noncommutative gauge theory is not a field theory but an

open string decoupled from the closed string. We also consider brane creation

phenomena and use the result to discuss the analogue of the Seiberg duality. A

noncommutative soliton is interpreted as a D1-brane induced on D3-brane. - Lorentz Transformation and Light-Like Noncommutative SYM, Rong-Gen Cai, Nobuyoshi Ohta, JHEP 0010 (2000) 036, JHEP 0010 (2000) 036, Aug. 15 2000 Summary:We show that combining the spatial noncommutative SYM limit and Lorentz

transformation, one can obtain a well-behaved light-like noncommutative SYM

limit. The light-like noncommutative SYM is unitary. When the boost velocity is

finite, the resulting theory with space-time noncommutativity is unitary as

well. The light-like noncommutative SYM limit can also be approached by

combining the noncommutative open string theory limit and Lorentz

transformation. Along this line, we obtain the supergravity dual for the

light-like noncommutative SYM, which is the same as the one acquired using a

diffe... - (F1, D1, D3) Bound State, Its Scaling Limits and SL(2,Z) Duality, Rong-Gen Cai, Nobuyoshi Ohta, Prog.Theor.Phys. 104 (2000) 1073-1087, Prog.Theor.Phys. 104 (2000) 1073-1087, Jul. 14 2000 Summary:We discuss the properties of the bound state (F1, D1, D3) in IIB supergravity

in three different scaling limits and the $SL(2,{\bf Z})$ transformation of the

resulting theories. In the simple decoupling limit with finite electric and

magnetic components of NS $B$ field, the worldvolume theory is the ${\cal N}$=4

super Yang-Mills (SYM) and the supergravity dual is still the $AdS_5 \times

S^5$. In the large magnetic field limit with finite electric field, the theory

is the noncommutative super Yang-Mills (NCSYM), and the supergravity dual is

the same as that without the electric background. W... - Noncommutative and Ordinary Super Yang-Mills on (D$(p-2)$, D$p$) Bound States, Rong-Gen Cai, Nobuyoshi Ohta, JHEP 0003 (2000) 009, JHEP 0003 (2000) 009, Jan. 31 2000 Summary:We study properties of (D$(p-2)$, D$p$) nonthreshold bound states ($2 \le p

\le 6 $) in the dual gravity description. These bound states can be viewed as

D$p$-branes with a nonzero NS $B$ field of rank two. We find that in the

decoupling limit, the thermodynamics of the $N_p$ coincident D$p$-branes with

$B$ field is the same not only as that of $N_p$ coincident D$p$-branes without

$B$ field, but also as that of the $N_{p-2}$ coincident D$(p-2)$-branes with

two smeared coordinates and no $B$ field, for $N_{p-2}/N_p=

\tilde{V}_2/[(2\pi)^2 \tilde{b}]$ with $\tilde{V}_2$ being the area of the t... - Surface counterterms and boundary stress-energy tensors for asymptotically non-anti-de Sitter spaces, Rong-Gen Cai, Nobuyoshi Ohta, Phys.Rev. D62 (2000) 024006, Phys.Rev. D62 (2000) 024006, Dec. 02 1999 Summary:For spaces which are not asymptotically anti-de Sitter where the asymptotic

behavior is deformed by replacing the cosmological constant by a dilaton scalar

potential, we show that it is possible to have well-defined boundary

stress-energy tensors and finite Euclidean actions by adding appropriate

surface counterterms. We illustrate the method by the examples of domain-wall

black holes in gauged supergravities, three-dimensional dilaton black holes and

topological dilaton black holes in four dimensions. We calculate the boundary

stress-energy tensor and Euclidean action of these black config... - On the Thermodynamics of Large N Noncommutative Super Yang-Mills Theory, Rong-Gen Cai, Nobuyoshi Ohta, Phys.Rev. D61 (2000) 124012, Phys.Rev. D61 (2000) 124012, Oct. 12 1999 Summary:We study the thermodynamics of the large N noncommutative super Yang-Mills

theory in the strong 't Hooft coupling limit in the spirit of AdS/CFT

correspondence. It has already been noticed that some thermodynamic quantities

of near-extremal D3-branes with NS B fields, which are dual gravity

configurations of the noncommutative ${\cal N}$=4 super Yang-Mills theory, are

the same as those without B fields. In this paper, (1) we examine the

$\alpha'^3 R^4$ corrections to the free energy and find that the part of the

tree-level contribution remains unchanged, but the one-loop and the

non-perturb... - Spectrum of Maxwell-Chern-Simons Theory Realized on Type IIB Brane, Takuhiro Kitao, Nobuyoshi Ohta, Nucl.Phys. B578 (2000) 215-238, Nucl.Phys. B578 (2000) 215-238, Aug. 02 1999 Summary:We study the 3D field theory on one D3-brane stretched between (r,s) and

(p,q)5-branes. The boundary conditions are determined from the analysis of NS5

and D5 charges of the two 5-branes. We carry out the mode expansions for all

the fields and identify the field theory as Maxwell-Chern-Simons theory. We

examine the mass spectrum to determine the conditions for unbroken

supersymmetry (SUSY) in this field theory and compare the results with those

from the brane configurations. The spectrum is found to be invariant under the

Type IIB SL(2,{\bf Z})-transformation. We also discuss the theory wit... - Maxwell Chern-Simons Solitons from Type IIB String Theory, Bum-Hoon Lee, Hyuk-jae Lee, Nobuyoshi Ohta, Hyun Seok Yang, Phys.Rev. D60 (1999) 106003, Phys.Rev. D60 (1999) 106003, Apr. 27 1999 Summary:We study various three-dimensional supersymmetric Maxwell Chern-Simons

solitons by using type IIB brane configurations. We give a systematic

classification of soliton spectra such as topological BPS vortices and

nontopological vortices in $\cn=2,3$ supersymmetric Maxwell Chern-Simons system

via the branes of type IIB string theory. We identify the brane configurations

with the soliton spectra of the field theory and obtain a nice agreement with

field theory aspects. We also discuss possible brane constructions for BPS

domain wall solutions. - Spacetime Superalgebra in AdS_4 \times S^7 via Supermembrane Probe, Kazuyuki Furuuchi, Nobuyoshi Ohta, Jian-Ge Zhou, Phys.Rev. D60 (1999) 047901, Phys.Rev. D60 (1999) 047901, Feb. 17 1999 Summary:The spacetime superalgebra via the supermembrane probe in the background of

AdS_4 \times S^7 is discussed to the lowest order in the spinor coordinate

$\t$. To obtain the correct spacetime superalgebras, all $\t^2$ order

corrections for supervielbein and super 3-form gauge potential have to be

included. The central extension of the superalgebra OSp(8|4) of the super

isometries for AdS_4 \times S^7 is found. - Thermalization of Poincaré Vacuum State and Fermion Emission from AdS_3 Black Holes in Bulk-Boundary Correspondence, Nobuyoshi Ohta, Jian-Ge Zhou, JHEP 9812 (1998) 023, JHEP 9812 (1998) 023, Nov. 06 1998 Summary:The greybody factors for spin 1/2 particles in the BTZ black holes are

discussed from 2D CFT in bulk-boundary correspondence. It is found that the

initial state of spin 1/2 particle in the BTZ black holes can be described by

the Poincar\'{e} vacuum state in boundary 2D CFT, and the nonlinear coordinate

transformation causes the thermalization of the Poincar\'{e} vacuum state. For

special case, our results for the greybody factors agree with the semiclassical

calculation. - AdS$_3$/CFT Correspondence, Poincaré Vacuum State and Greybody Factors in BTZ Black Holes, H. J, W. Müller-Kirsten, Nobuyoshi Ohta, Jian-Ge Zhou, Phys.Lett. B445 (1999) 287-295, Phys.Lett. B445 (1999) 287-295, Sep. 28 1998 Summary:The greybody factors in BTZ black holes are evaluated from 2D CFT in the

spirit of AdS$_3$/CFT correspondence. The initial state of black holes in the

usual calculation of greybody factors by effective CFT is described as

Poincar\'{e} vacuum state in 2D CFT. The normalization factor which cannot be

fixed in the effective CFT without appealing to string theory is shown to be

determined by the normalized bulk-to-boundary Green function. The relation

among the greybody factors in different dimensional black holes is exhibited.

Two kinds of $(h,{\bar h})=(1,1)$ operators which couple with the b... - Three-Dimensional Gauge Dynamics from Brane Configurations with (p,q)-Fivebrane, Takuhiro Kitao, Kazutoshi Ohta, Nobuyoshi Ohta, Nucl.Phys. B539 (1999) 79-106, Nucl.Phys. B539 (1999) 79-106, Aug. 19 1998 Summary:We study three-dimensional gauge dynamics by using type IIB superstring brane

configurations, which can be obtained from the M-theory configuration of

M2-branes stretched between two M5-branes with relative angles. Our

construction of brane configurations includes (p,q)5-brane and gives a

systematic classification of possible three-dimensional gauge theories. The

explicit identification of gauge theories are made and their mirror symmetry is

discussed. As a new feature, our theories include interesting

Maxwell-Chern-Simons system whose vacuum structure is also examined in detail,

obtaining ... - More on the Similarity between D=5 Simple Supergravity and M Theory, S. Mizoguchi, N. Ohta, Phys.Lett. B441 (1998) 123-132, Phys.Lett. B441 (1998) 123-132, Jul. 16 1998 Summary:It has been known that D=5 simple supergravity resembles D=11 supergravity in

many respects. We present their further resemblances in (1) the duality groups

upon dimensional reduction, and (2) the worldsheet structure of the solitonic

string of the D=5 supergravity. We show that the D=3, G_{2(+2)}/SO(4) (bosonic)

nonlinear sigma model is obtained by using Freudenthal's construction in

parallel to the derivation of the D=3, E_{8(+8)}/SO(16) sigma model from D=11

supergravity. The zero modes of the string solution with unbroken (4,0)

supersymmetry consist of three (non-chiral) scalars, four M... - Fermionic Zero Mode and String Creation between D4-Branes at Angles, Takuhiro Kitao, Nobuyoshi Ohta, Jian-Ge Zhou, Phys.Lett. B428 (1998) 68-74, Phys.Lett. B428 (1998) 68-74, Jan. 20 1998 Summary:We study the creation of a fundamental string between D4-branes at angles in

string theory. It is shown that $R(-1)^{F}$ part of the one-loop potential of

open string changes its sign due to the change of fermionic zero-mode vacua

when the branes cross each other. As a result the effective potential is

independent of the angles when supersymmetry is partially unbroken, and leads

to a consistent picture that a fundamental string is created in the process. We

also discuss the s-rule in the configuration. The same result is obtained from

the one-loop potential for the orthogonal D4-branes with... - Euclidean Path Integral, D0-Branes and Schwarzschild Black Holes in Matrix Theory, Nobuyoshi Ohta, Jian-Ge Zhou, Nucl.Phys. B522 (1998) 125-136, Nucl.Phys. B522 (1998) 125-136, Jan. 07 1998 Summary:The partition function in Matrix theory is constructed by Euclidean path

integral method. The D0-branes, which move around in the finite region with a

typical size of Schwarzschild radius, are chosen as the background. The mass

and entropy of the system obtained from the partition function contain the

parameters of the background. After averaging the mass and entropy over the

parameters, we find that they match the properties of 11D Schwarzschild black

holes. The period $\b$ of Euclidean time can be identified with the reciprocal

of the boosted Hawking temperature. The entropy $S$ is shown ... - Creation of Fundamental String in M(atrix) Theory, Nobuyoshi Ohta, Takashi Shimizu, Jian-Ge Zhou, Phys.Rev.D57:2040-2044,1998, Phys.Rev.D57:2040-2044,1998, Oct. 29 1997 Summary:The potential between two D4-branes at angles with partially unbroken

supersymmetry is computed, and is used to discuss the creation of a fundamental

string when two such D4-branes cross each other in M(atrix) theory. The

effective Lagrangian is shown to be anomalous at 1-loop approximation, but it

can be modified by bare Chern-Simons terms to preserve the invariance under the

large gauge transformation. The resulting effective potential agrees with that

obtained from the string calculations. The result shows that a fundamental

string is created in order to cancel the repulsive force betwee... - Supersymmetry of M-Branes at Angles, Nobuyoshi Ohta, Paul K. Townsend, Phys.Lett. B418 (1998) 77-84, Phys.Lett. B418 (1998) 77-84, Oct. 16 1997 Summary:We determine the possible fractions of supersymmetry preserved by two

intersecting M-5-branes. These include the fractions 3/32 and 5/32 which have

not occurred previously in intersecting brane configurations. Both occur in

non-orthogonal pointlike intersections of M-5-branes but 5/32 supersymmetry is

possible only for specific fixed angles. - Realization of D4-Branes at Angles in Super Yang-Mills Theory, Nobuyoshi Ohta, Jian-Ge Zhou, Phys.Lett. B418 (1998) 70-76, Phys.Lett. B418 (1998) 70-76, Sep. 09 1997 Summary:An explicit solution in super Yang-Mills theory, which after T-duality

describes two sets of D4-branes at angles, is constructed. The gauge

configuration possesses 3/16 unbroken supersymmetry for the equal magnitudes of

field strengths, and can be considered as the counterpart of the solution of

D=11 supergravity with the same amount of supersymmetry in the solutions given

by Gauntlett et al. The energy of the Born-Infeld action of the gauge

configuration gives further evidence for the geometrical interpretation as two

sets of D4-branes at angles. The energy of super Yang-Mills theory is sh... - A Note on Consistent Anomalies, L. Baulieu, C. Laroche, N. Ohta, M. Picco, Phys.Lett. B414 (1997) 77-84, Phys.Lett. B414 (1997) 77-84, Jul. 31 1997 Summary:Within a BRST formulation, we determine the expressions of the consistent

anomaly for superstrings with extended worldsheet supersymmetries of rank N. We

consider the O(N) superconformal algebras up to N=4, as well as the `small N=4'

superalgebra. This is done using a superfield formalism, allowing to recover

previous results that were expressed in components. Moreover, we identify the

`small N=4' algebra as the constrained `large N=4' via a self-duality like

condition in superspace. - Low Energy Theorems in N=1 Supersymmetric Theory, Kiyoshi Higashijima, Muneto Nitta, Kazutoshi Ohta, Nobuyoshi Ohta, Prog.Theor.Phys. 98 (1997) 1165-1178, Prog.Theor.Phys. 98 (1997) 1165-1178, Jun. 30 1997 Summary:In N=1 supersymmetric theories, quasi Nambu-Goldstone (QNG) bosons appear in

addition to ordinary Nambu-Goldstone (NG) bosons when the global symmetry G

breaks down spontaneously. We investigate two-body scattering amplitudes of

these bosons in the low-energy effective Lagrangian formalism. They are

expressed by the curvature of Kahler manifold. The scattering amplitudes of QNG

bosons are shown to coincide with those of NG bosons though the effective

Lagrangian contains an arbitrary function, and those with odd number of QNG

bosons all vanish. - Towards the Classification of Non-Marginal Bound States of M-branes and Their Construction Rules, Nobuyoshi Ohta, Jian-Ge Zhou, Int.J.Mod.Phys. A13 (1998) 2013-2046, Int.J.Mod.Phys. A13 (1998) 2013-2046, Jun. 23 1997 Summary:We present a systematic analysis of possible bound states of M-brane

solutions (including waves and monopoles) by using the solution generating

technique of reduction of M-brane to 10 dimensions, use of T-duality and then

lifting back to 11 dimensions. We summarize a list of bound states for one- and

two-charge cases including tilted brane solutions. Construction rules for these

non-marginal solutions are also discussed. - Intersection Rules for Non-Extreme $p$-Branes, Nobuyoshi Ohta, Phys.Lett. B403 (1997) 218-224, Phys.Lett. B403 (1997) 218-224, Feb. 24 1997 Summary:We give a model-independent derivation of general intersecting rules for

non-extreme $p$-branes in arbitrary dimensions $D$. This is achieved by

directly solving bosonic field equations for supergravity coupled to a dilaton

and antisymmetric tensor fields with minimal ans\"{a}tze. We compare the

results with those in eleven-dimensional supergravity. Supersymmetry is

recovered in the extreme limit if the backgrounds are taken to be independent.

Consistency with non-supersymmetric solutions is also discussed. Finally the

general formulae for the ADM mass, entropy and Hawking temperature are g... - Non-Extreme Black Holes from Intersecting M-Branes, Nobuyoshi Ohta, Takashi Shimizu, Int.J.Mod.Phys.A13:1305-1328,1998, Int.J.Mod.Phys.A13:1305-1328,1998, Jan. 21 1997 Summary:We investigate the possibility of extending non-extreme black hole solutions

made of intersecting M-branes to those with two non-extreme deformation

parameters, similar to Reissner-Nordstr{\o}m solutions. General analysis of

possible solutions is carried out to reduce the problem of solving field

equations to a simple algebraic one for static spherically-symmetric case in

$D$ dimensions. The results are used to show that the extension to

two-parameter solutions is possible for $D=4,5$ dimensions but not for higher

dimensions, and that the area of horizon always vanishes in the extreme limit... - Worldsheets with Extended Supersymmetry, Laurent Baulieu, Nobuyoshi Ohta, Phys.Lett. B391 (1997) 295-304, Phys.Lett. B391 (1997) 295-304, Sep. 26 1996 Summary:We determine the equations which govern the gauge symmetries of worldsheets

with local supersymmetry of arbitrary rank $(N,N')$, and their possible

anomalies. Both classical and ghost conformally invariant multiplets of the

left or right sector are assembled into the components of a single

$O(N)$-superfield. The component with ghost number zero of this superfield is

the $N$-supersymmetric generalization of the Beltrami differential. In a

Lagrangian approach, and after gauge-fixing, it becomes the super-moduli of

Riemann surfaces coupled to local supersymmetry of rank $N$. It is also the

sou... - All the Exact Solutions of Generalized Calogero-Sutherland Models, Nobuyoshi Ohta, Jour. Phys. Soc. Japan. 65 (1996)3769, Jour. Phys. Soc. Japan. 65 (1996)3769, May 13 1996 Summary:A collective field method is extended to obtain all the explicit solutions of

the generalized Calogero-Sutherland models that are characterized by the roots

of all the classical groups, including the solutions corresponding to spinor

representations for $B_N$ and $D_N$ cases. - Exact Solutions of Generalized Calogero-Sutherland Models -- BC_N and C_N cases, Masamichi Kojima, Nobuyoshi Ohta, Nucl.Phys. B473 (1996) 455-468, Nucl.Phys. B473 (1996) 455-468, Mar. 13 1996 Summary:Using a collective field method, we obtain explicit solutions of the

generalized Calogero-Sutherland models that are characterized by the roots of

the classical groups $B_N$ and $C_N$. Starting from the explicit wave functions

for $A_{N-1}$ type expressed in terms of the singular vectors of the $W_N$

algebra, we give a systematic method to construct wave functions and derive

energy eigenvalues for other types of theories. - UNIVERSAL STRING AND SMALL $N=4$ SUPERSTRING, Nobuyoshi Ohta, Takashi Shimizu, Phys.Lett.B355:127-134,1995, Phys.Lett.B355:127-134,1995, Apr. 20 1995 Summary:It was previously shown that most of the superstrings can be obtained from

those with higher world-sheet supersymmetry as spontaneously broken phases. In

this paper, we show that the small $N=4$ superstring, which was left out of

this hierarchy of the universal string, can be obtained from the large $N=4$

strings. We also show that the $N=2$ string is a special vacuum of small $N=4$

string. Thus all the known superstring theories can be derived from a universal

string by spontaneous breakdown of supersymmetry. - Thermodynamics of Quantum Fields in Black Hole Backgrounds, S. P. de Alwis, N. Ohta, Phys.Rev.D52:3529-3542,1995, Phys.Rev.D52:3529-3542,1995, Apr. 07 1995 Summary:We discuss the relation between the micro-canonical and the canonical

ensemble for black holes, and highlight some problems associated with extreme

black holes already at the classical level. Then we discuss the contribution of

quantum fields and demonstrate that the partition functions for scalar and

Dirac (Majorana) fields in static space-time backgrounds, can be expressed as

functional integrals in the corresponding optical space, and point out that the

difference between this and the functional integrals in the original metric is

a Liouville-type action. The optical method gives both th... - Note on W3 Realizations of the Bosonic String, Fiorenzo Bastianelli, Nobuyoshi Ohta, Phys. Lett. B348 (1995) 411, Phys. Lett. B348 (1995) 411, Nov. 22 1994 Summary:In order to investigate to what extent string theories are different vacua of

a general string theory (the ``universal string"), we discuss realizations of

the bosonic string as particular background of certain types of $W$-strings.

Our discussions include linearized $W_3^{lin}$, non-critical $W_3$, linearized

$W_3^{(2)lin}$ and critical $W_3^{(2)}$ realizations of the bosonic string. - Embeddings for Non-Critical Superstrings, Nathan Berkovits, Nobuyoshi Ohta, Phys.Lett. B334 (1994) 72-78, Phys.Lett. B334 (1994) 72-78, May 22 1994 Summary:It was previously shown that at critical central charge, $N$-extended

superstrings can be embedded in $(N+1)$-extended superstrings. In other words,

$(N=0,c=26)\to (N=1,c=15)\to (N=2,c=6)\to (N=3,c=0) \to (N=4,c=0) $. In this

paper, we show that similar embeddings are also possible for $N$-extended

superstrings at non-critical central charge. For any $x$, the embedding is

$(N=0,c=26+x) \to (N=1,c=15+x) \to (N=2,c=6+x) \to (N=3,c=x) \to (N=4,c=x)$. As

was conjectured by Vafa, the $(N=2,c=9) \to (N=3,c=3)$ embedding can be used to

prove that $N=0$ topological strings are special vaccua of N=1... - A Hierarchy of Superstrings, Fiorenzo Bastianelli, Nobuyoshi Ohta, Jens Lyng Petersen, Phys.Rev.Lett. 73 (1994) 1199-1202, Phys.Rev.Lett. 73 (1994) 1199-1202, Mar. 24 1994 Summary:We construct a hierarchy of supersymmetric string theories by showing that

the general N-extended superstrings may be viewed as a special class of the

(N+1)-extended superstrings. As a side result, we find a twisted (N+2)

superconformal algebra realized in the N-extended string. - The BRST Operator for the Large $N=4$ Superconformal Algebra, Fiorenzo Bastianelli, Nobuyoshi Ohta, Phys.Rev. D50 (1994) 4051-4059, Phys.Rev. D50 (1994) 4051-4059, Feb. 21 1994 Summary:We review the detailed structure of the large $N=4$ superconformal algebra,

and construct its BRST operator which constitutes the main object for analyzing

$N=4$ strings. We then derive the general condition for the nilpotency of the

BRST operator and show that there exists a line of critical $N=4$ string

theories. - Toward the Universal Theory of Strings, Fiorenzo Bastianelli, Nobuyoshi Ohta, Jens Lyng Petersen, Phys.Lett. B327 (1994) 35-39, Phys.Lett. B327 (1994) 35-39, Feb. 09 1994 Summary:We show that the $N=2$ superstrings may be viewed as a special class of the

$N=4$ superstrings and demonstrate their equivalence. This allows us to realize

all known string theories based on linear algebras and with $N<4$

supersymmetries as special choices of the vacua in the $N=4$ superstring. - $N=1$ from $N=2$ Superstrings, Nobuyoshi Ohta, Jens Lyng Petersen, Phys.Lett.B325:67-70,1994, Phys.Lett.B325:67-70,1994, Dec. 24 1993 Summary:We give a simple proof that a particular class of $N=2$ superstrings are

equivalent to the $N=1$ superstrings. This is achieved by constructing a

similarity transformation which transforms the $N=2$ BRST operators into a

direct sum of the BRST operators for the $N=1$ string and topological sectors. - Bosonization of a Topological Coset Model and Non-Critical String Theory, Nobuyoshi Ohta, Hisao Suzuki, Mod.Phys.Lett. A9 (1994) 541-548, Mod.Phys.Lett. A9 (1994) 541-548, Oct. 27 1993 Summary:We analyze the relation between a topological coset model based on super

$SL(2,R)/U(1)$ coset and non-critical string theory by using free field

realization. We show that the twisted $N=2$ algebra of the coset model can be

naturally transformed into that of non-critical string. The screening operators

of the coset models can be identified either with those of the minimal matters

or with the cosmological constant operator. We also find that another screening

operator, which is intrinsic in our approach, becomes the BRST nontrivial state

of ghost number $0$ (generator of the ground ring for $... - BRST Analysis of Physical States in Two-Dimensional Black Hole, Katsumi Itoh, Hiroshi Kunitomo, Nobuyoshi Ohta, Makoto Sakaguchi, Phys.Rev. D48 (1993) 3793-3808, Phys.Rev. D48 (1993) 3793-3808, May 31 1993 Summary:We study the BRST cohomology for $SL(2,R)/U(1)$ coset model, which describes

an exact string black hole solution. It is shown that the physical spectrum

could contain not only the extra discrete states corresponding to those in

$c=1$ two-dimensional gravity but also many additional new states with ghost

number $N_{FP}= -1 \sim 2$. We also discuss characters for nonunitary

representations and the relation of our results to other approaches. - Interactions of Discrete States with Nonzero Ghost Number \\ in $c=1$ $2D$ Gravity, Nobuyoshi Ohta, Hisao Suzuki, Mod.Phys.Lett. A7 (1992) 2723-2730, Mod.Phys.Lett. A7 (1992) 2723-2730, May 28 1992 Summary:We study the interactions of the discrete states with nonzero ghost number in

$c=1$ two-dimensional ($2D$) quantum gravity. By using the vertex operator

representations, it is shown that their interactions are given by the structure

constants of the group of the area preserving diffeomorphism similar to those

of vanishing ghost number. The effective action for these states is also worked

out. The result suggests the whole system has a BRST-like symmetry. - Brst Cohomology and Physical States in 2D Supergravity Coupled to ${\hat c} \leq 1$ Matter, Katsumi Itoh, Nobuyoshi Ohta, Nucl.Phys. B377 (1992) 113-142, Nucl.Phys. B377 (1992) 113-142, Oct. 04 1991 Summary:We study the BRST cohomology for two-dimensional supergravity coupled to

$\hat c \leq 1$ superconformal matter in the conformal gauge. The

super-Liouville and superconformal matters are represented by free scalar

fields $\phi^L$ and $\phi^M$ and fermions $\psi^L$ and $\psi^M$, respectively,

with suitable background charges, and these are coupled in such a way that the

BRST charge is nilpotent. The physical states of the full theory are determined

for NS and R sectors. It is shown that there are extra states with ghost number

$N_{FP}=0,\pm 1$ for discrete momenta other than the degree of fre...

- Fundamental Physics, OHTA Nobuyoshi, 単著, 2011 05
- handbook of Particle Physics, OHTA Nobuyoshi, 分担執筆, Superstring Theories, 2010 10
- Supersymmetric Theories, OHTA Nobuyoshi, 共著, 2006 11
- Encyclopedia on Physics, OHTA Nobuyoshi, 分担執筆, Particle Physics, Asakura, 2005 10

- Gauge and Measure in Quantum Gravity, OHTA Nobuyoshi, The 2017 Autumn Meeting of the Phyiscal Soceity, 2017 09 13
- Gauge and Parametrization dependence in Renormalization group approach to Quantum Gravity, OHTA Nobuyoshi, YITP Workshop Strings and Fields 2017, 2017 08 , Yukawa Institute, Kyoto University
- Renormalization Group Approach to Quantum Gravity, OHTA Nobuyoshi, The 2017 Annual meeting of the Division of Gravitation and Relativistic Astrophysics of the Chinese Physical Society / The Fifth Galileo-Xu Guangqai Meeting, 2017 06 , 招待有り, Division of Gravitation and Relativistic Astrophysics of the Chinese Physical Society
- Renormalization Group Equations for f(R) Gravity and Some of Their Solutions, OHTA Nobuyoshi, 6th CST-MISC Joint Symposium on Particle Physics, 2016 10 , 招待有り, Kyoto Sangyo Univ. and Nihon Univ
- Renormalization Group Equation for f(R) gravity on a hyperbolic space, OHTA Nobuyoshi, International Workshop for String theory and Cosmology 2016, 2016 08 18 , 招待有り, APCTP
- Renormalization Group Equation for f(R) gravity on a hyperbolic space, OHTA Nobuyoshi, Strings and Fields 2016, 2016 08 11 , Yukawa Institute, Kyoto University
- Renormalization Group Equation for f(R) gravity on a hyperbolic space, OHTA Nobuyoshi, 2016 Annual Meeting and Member Representative Conference of the Division of Gravitation and Relativistic Astrophysics of Chinese Physical Society, 2016 06 , 招待有り, Division of Gravitation and Relativistic Astrophysics of the Chinese Physical Society
- Asymptotic Safety and Quantum Gravity, OHTA Nobuyoshi, 5th CST-MISC Joint Symposium on Particle Physics, 2015 10 , 招待有り
- Cosmic Acceleration with a Negative Cosmological Constant in Higher Dimensions, OHTA Nobuyoshi, The 19th annual International Conference on Particle Physics and Cosmology, 2015 09
- Asymptotic Safety and Quantum Gravity, OHTA Nobuyoshi, The second International Conference on Hot Topics in General Relativity and Gravitation, 2015 08
- Cosmic Acceleration with a Negative Cosmological Constant in Higher Dimensions, OHTA Nobuyoshi, 4th Bangkok workshop on high energy physics, 2015 01
- Cosmic Acceleration with a Negative Cosmological Constant in Higher Dimensions, OHTA Nobuyoshi, Hangzhou workshop on Gravitation and Cosmology, 2014 09 , 招待有り
- Quantum Gravity and Asymptotic Safety in Higher Derivative Gravity in Diverse Dimensions, OHTA Nobuyoshi, KITPC workshop "Quantum Gravity, Black Holes and Strings'', 2014 05 , 招待有り
- Covariant Approach to the No-Ghost Theorem in Massive Gravity, OHTA Nobuyoshi, 2014 03 27
- Covariant Approach to the No-Ghost Theorem in Massive Gravity, OHTA Nobuyoshi, CST-MISC Joint International Symposium on Particle Physics, 2014 03 16 , 招待有り
- Asymptotic safety in higher derivative gravity in diverse dimensions, OHTA Nobuyoshi, 3rd Bangkok workshop on high energy physics, 2014 01
- Higher derivative gravity and Asymptotic safety in diverse dimensions, OHTA Nobuyoshi, International Workshop on "Theoretical Particle Physics 2013", 2013 11 , 招待有り
- Asymptotic safety in higher derivative gravity in arbitrary dimensions, OHTA Nobuyoshi, 2013 09
- Asymptotic Safety in 3D and 4D Higher Derivative Gravity, 2nd Mediterranean Conference on Classical and Quantum Gravity, 2013 06 , 招待有り, 2nd Mediterranean Conference on Classical and Quantum GravitySummary:We report our recent study of asymptotic safety in 3D and 3D higher derivative gravity. We show that these theories are asymptotic safe, meaning that the theories have nontrivial ultraviolet fixed points. This opens the possibility that the theories makes sense as quantum gravity.
- Asymptotic Safety in Higher Derivative Gravity -- New Massive Gravity or D=3 --, Texas A&M University, 2013 04 , 招待有り, Texas A&M UniversitySummary:We review our recent work on the asymptotic safety in higher derivative gravity in 3 dimensions, and show that the theory is asymptotic safe. Unfortunately it is also shown that the new massive gravity does not correspond to the fixed point in the high energy limit.
- Asymptotic Safety in Higher Derivative Gravity, Asia-Pacific School/Workshop on Gravitation and Cosmology 2013, 2013 02 , Asia-Pacific School/Workshop on Gravitation and Cosmology 2013
- Unitarity versus Renormalizability of Higher Derivative Gravity, International conference on New Trends in Field Theories, 2012 11 , International conference on New Trends in Field Theories
- Beta Function and Asymptotic Safety in Three-dimensional Higher Derivative Gravity, International workshp on ''Theoretical Particle Physics 2012'', 2012 09 , 招待有り, International workshp on ''Theoretical Particle Physics 2012''
- Covariant Approach to the No-ghost Theorem in Massive Gravity, Molecule workshop ''Nonlinear Massive Gravity'', 2012 07 , 招待有り, Molecule workshop ''Nonlinear Massive Gravity''
- Accelerating Universes in String Theory via Field Redefinition, Workshop at Isaac Newton Institute for Mathematical Sciences ''String Phenomenology 2012'', 2012 06 , Workshop at Isaac Newton Institute for Mathematical Sciences ''String Phenomenology 2012''
- Unitarity versus Renormalizability of Higher Derivative Gravity, CQUEST Spring Workshop on Higher Spins and String Geometry, Sogang University, 2012 03 , CQUEST Spring Workshop on Higher Spins and String Geometry, Sogang University
- On the Unitarity and Renormalizability of Higher Derivative Gravity in 3D and higher, Xth International Conference on Gravitation, Astrophysics and Cosmology, 2011 12 , Xth International Conference on Gravitation, Astrophysics and Cosmology
- On the Unitarity and Renormalizability of Higher Derivative Gravity in 3D, 2011 11 Summary:We show that three-dimensional gravity that contains quadratic scalar curvature and a Ricci tensor is renormalizable, but those theories with a special relation between their coefficients, including new massive gravity, are not.
- Effects of the higher order term of the dilaton field and the charge to the stringy black holes, TORII Takashi, OHTA Nobuyoshi, 2010 09
- Cosmological intersecting brane solutions, 2010 09
- Accelerating Cosmologies in Dilatonic Einstein-Gauss-Bonnet Gravity in the String Frame, 2010 09
- Supersymmetric Intersecting Branes in Time-dependent Backgrounds, 2009 09
- Black holes in the Dilatonic Einstein-Gauss-Bonnet Theory in Various Dimensions II, 2009 03
- Black holes in the Dilatonic Einstein-Gauss-Bonnet Theory in Various Dimensions, 2008 09
- Cosmological Problems and String Theory, 2008 03 Summary:There are several important cosmological problems that cannot be addressed within the framework of general relativity. Some of them are expected to be resolved by considering quantum gravity. The leading candidate for such a theory is the superstring. In this talk, I discuss what superstring theories can tell us about these problems and recent status. These include how to obtain inflationary solutions and how to describe cosmology near big bang singularity.
- Accelerating Cosmology from Einstein-Gauss-Bonnet Theory with Dilaton, International Conference on String Theory and Cosmology, 2007 10 , International Conference on String Theory and CosmologySummary:We study cosmological solutions in low-energy effective heterotic string theory, which consists of the Einstein gravity with the Gauss-Bonnet term and a dilaton. We show that the field equations take the form of an autonomous system for flat internal and external spaces, and we derive all the fixed points in the system. We also examine the time evolution of the solutions and the question of whether the solutions can describe (transient) accelerated expansion of our four-dimensional space in the Einstein frame.
- Accelerating Cosmologies in the Einstein-Gauss-Bonnet Theory with Dilaton, Scientific Program on Gravity and Cosmology, 2007 09 , Scientific Program on Gravity and CosmologySummary:We study cosmological solutions in low-energy effective heterotic string theory, which consists of the Einstein gravity with the Gauss-Bonnet term and a dilaton. We show that the field equations take the form of an autonomous system for flat internal and external spaces, and we derive all the fixed points in the system. We also examine the time evolution of the solutions and the question of whether the solutions can describe (transient) accelerated expansion of our four-dimensional space in the Einstein frame.
- Supertubes and superfunnels, 2007 03
- Time-dependent Supersymmetric Solutions in M-theory and singularities of the Universe, 2006 09
- Black Hole Solutions in Supergravity with String Corrections, 2006 09
- Time-dependent Supersymmetric Solution in M-theory and the Cosmic Singularity, Cambridge-Mitchell Texas Conference, 2006 08 , Cambridge-Mitchell Texas Conference
- Lectures on Strings, Branes and Black Holes, APCTP/CQUeST Field Theory Winter School at Pohang, 2006 02 , APCTP/CQUeST Field Theory Winter School at Pohang
- Time-dependent Solutions with Null Killing Spinor and Singularities of the Universe, Asia-pasific Center for Theoretical physics, 2005 12 , Asia-pasific Center for Theoretical physics
- Accelerating Cosmologies and Inflation from M-theory, Taiwan National University, 2005 09 , Taiwan National University
- Superstrings and Branes, Shanghai Institute for Advanced Study, 2005 08 , Shanghai Institute for Advanced Study
- Supertube, 2005 03
- Accelerating Cosmologies and Inflation from M-theory, University of barcelona, 2004 12 , University of barcelona
- Accelerating Cosmologies and Inflation from M-theory, Taro Tamura Symposium, 2004 11 , Taro Tamura Symposium
- Accelerating Cosmologies and Inflation from M-theory, 2004 09
- A Geometric Look on Microstates of Supertubes, 2004 09

- Can We Obtain Inflationary Universe from Superstrings and M-Theory?, Ohta Nobuyoshi, Butsuri, 60, 8, 616, 623, 2005 08 05 , 10.11316/butsuri1946.60.616, http://ci.nii.ac.jp/naid/110002078861Summary:According to the standard cosmology, our universe is believed to have undergone huge expansion called inflation during its early evolution after Big Bang. This behavior must be explained within general relativity and particle physics, but there is a no-go theorem which forbids such solutions in superstrings and M-theory. Here we summarize how the no-go theorem is proved after explaining the standard inflationary scinario, and then review the recent developments in the inflationary cosmologies from superstrings and M-theory, overcoming the no-go theorem.
- Introduction to branes and M-theory for relativists and cosmologists, Nobuyoshi Ohta, Prog.Theor.Phys.Suppl.148:1-28,2003, 2002 05 09 , 10.1143/PTPS.148.1, http://arxiv.org/abs/gr-qc/0205036v4Summary:We review the recent developments in superstrings. We start with a brief

summary of various consistent superstring theories and discuss T-duality which

necessarily leads to the presence of D-branes. The properties of D-branes are

summarized and we discuss how these suggest the existence of 11-dimensional

quantum theory, M-theory, which is believed to give rise to various

superstrings as perturbative expansions around particular backgrounds in the

theory. We also discuss the interpretation of brane solutions as black holes in

string theories and statistical explanation of Bekenstein-Hawking ... - On the Entropy of Quantum Fields in Black Hole Backgrounds, S. P. de Alwis, N. Ohta, 1994 12 03 , http://arxiv.org/abs/hep-th/9412027v2Summary:We show that the partition function for a scalar field in a static spacetime

background can be expressed as a functional integral in the corresponding

optical space, and point out that the difference between this and the

functional integral in the original metric is a Liouville type action. A

general formula for the free energy is derived in the high temperature

approximation and applied to various cases. In particular we find that

thermodynamics in the extremal Reissner-Nordstr\"om space has extra

singularities that make it ill-defined. - Discrete States in Two-Dimensional Quantum Gravity, Nobuyoshi Ohta, 1992 06 03 , http://arxiv.org/abs/hep-th/9206012v2Summary:Minor misprints corrected.
- Spectrum of Two-Dimensional (Super)Gravity, Katsumi Itoh, Nobuyoshi Ohta, Prog.Theor.Phys.Suppl.110:97-116,1992, 1992 01 17 , 10.1143/PTPS.110.97, http://arxiv.org/abs/hep-th/9201034v1Summary:We review the BRST analysis of the system of a (super)conformal matter

coupled to 2D (super)gravity. The spectrum and its operator realization are

reported. In particular, the operators associated with the states of nonzero

ghost number are given. We also discuss the ground ring structure of the

super-Liouville coupled to ${\hat c}=1$ matter. In appendices, hermiticities,

states for $c<1$ conformal matter coupled to gravity and the proof for the

spectrum are discussed. - Supergravity, Ohta Nobuyoshi, 38, 9, 709, 716, 1983 09 05 , http://ci.nii.ac.jp/naid/110002075337

- Ministry of Education, Culture, Sports, Science and Technology, Grants-in-Aid for Scientific Research(基盤研究(C)), Study of higher-dimensional black holes and their singularities by superstrings, (抄録なし)
- Ministry of Education, Culture, Sports, Science and Technology, Grants-in-Aid for Scientific Research(基盤研究(C)), Research on time-dependent solutions and application to cosmology in superstring theories, We have carried out research related to quantum gravity and its consequences, which is one of the greatest problems in elementary particle physics and gravity. Since the most promising candidate for such a theory, the superstring, is defined in higher dimensions, extra dimensions must be compactified in order to give a realistic description of our world. In this research, we have investigated time-dependent inflationary solutions systematically to obtain realistic models for our universe in the low-energy effective theory of superstring, and clarified the possibility of describing big-bang ...
- Ministry of Education, Culture, Sports, Science and Technology, Grants-in-Aid for Scientific Research(基盤研究(C)), Quantum study of ground state and black holes in superstring theories, This research project achieved the following results.(1) It is important to study noncommutative theories realized on branes in string theories in order to gain deep understanding of quantum gravity. We have studied theories with noncommutativity not only between space and space coordinates but also space and time, and have given foundation to the nonperturbative analysis of these theories. Furthermore we explicitly examined the transformation properties of 3-dimensional theories under duality. We have also studied supersymmetry and fluctuation spectrum around D-branes realized as solitonic...