arXiv subjects

Ahmad Ghodsi

Publications and source records attributed to Ahmad Ghodsi.

At least 37 records · Page 2Linked to original sources

arXiv2012

Shear tensor and dynamics of relativistic accretion disks around rotating black holes

In this paper we solve the hydrodynamical equations of optically thin, steady state accretion disks around Kerr black holes. Here, fully general relativistic equations are used. We use a new method to calculate the shear tensor in the LNRF (Locally Non-Rotating Frame), BLF (Boyer-Lindquist Frame) and FRF (Fluid Rest Frame). We show that two components of shear tensor in the FRF are nonzero (in previous works only one nonzero component was assumed). We can use these tensors in usual transonic solutions and usual causal viscosity, but we derive solutions analytically by some simplifications. Then we can calculate the four velocity and density in all frames such as the LNRF, BLF and FRF.

astro-ph.GA↗

arXiv2012

On asymptotically AdS-like solutions of three dimensional massive gravity

In this paper we have added Maxwell, Maxwell-Chern-Simons and gravitational Chern-Simons terms to Born-Infeld extended new massive gravity and we have found different types of (non)extremal charged black holes. For each black hole we find mass, angular momentum, entropy and temperature. Since our solutions are asymptotically AdS or warped-AdS, we infer central charges of dual CFTs by using Cardy's formula. Computing conserved charges associated to asymptotic symmetry transformations confirms calculation of central charges. For CFTs dual to asymptotically AdS solutions we find left central charges from Cardy's formula, while conserved charge approach gives both left and right central charges. For CFTs dual to asymptotically warped-AdS solutions, left and right central charges are equal when we have Maxwell-Chern-Simons term but they have different values when gravitational Chern-Simons term is included.

hep-th↗

arXiv2011

Black Holes in Born-Infeld Extended New Massive Gravity

In this paper we find different types of black holes for the Born-Infeld extended new massive gravity. Our solutions include (un)charged warped (A)dS black holes for four and six derivative expanded action. We also look at the black holes in unexpanded BI action. In each case we calculate the entropy, angular momentum and mass of the black holes. We also find the central charges for the CFT duals.

hep-th↗

arXiv2011

Toroidal solutions in Horava Gravity

Recently a new four-dimensional non relativistic renormalizable theory of gravity was proposed by Horava. This gravity reduces to Einstein gravity at large distances. In this paper by using the new action for gravity we present different toroidal solutions to the equations of motion. Our solutions describe the near horizon geometry with slow rotating parameter.

hep-th↗

arXiv2010

Charged Black Holes in New Massive Gravity

We construct charged black hole solutions to three-dimensional New Massive Gravity (NMG), by adding electromagnetic Maxwell and Chern-Simons actions. We find charged black holes in the form of warped AdS_3 and "log" solutions in specific critical point. The entropy, mass and angular momentum of these black holes are computed.

hep-th↗

arXiv2010

The RN/CFT Correspondence

Recently it has been shown in 0901.0931 [hep-th] that the approach to extremality for the non-extremal Reissner-Nordstrom black hole is not continuous. The non-extremal RN black hole splits into two spacetimes at the extremality: an extremal black hole and a disconnected $AdS_2\times S^2$ space which has been called the "compactification solution". As a possible resolution for understanding the entropy of extremal RN black hole, it has been speculated that the entropy of the non-extremal black hole may be carried by the latter solution. By uplifting the four dimensional "compactification solution" with electric charge $Q_e$ to a five dimensional solution, we show that this solution is dual to a CFT with central charge $c=6Q_e^3$. The Cardy formula then shows that the microscopic entropy of the CFT is the same as the macroscopic entropy of the "compactification solution".

hep-th↗

arXiv2010

Extremal rotating solutions in Horava Gravity

Recently a new four-dimensional non relativistic renormalizable theory of gravity was proposed by Horava. In this paper we have found different near horizon geometries in Horava gravity. We find the rotating solutions in a special range of parameters of the theory. This range allows us to include up to six derivative terms in the equations of motion. We have found our solutions as perturbed solutions over spherical symmetric background with small rotating parameter.

hep-th↗

arXiv2009

Non-relativistic D3-brane in the presence of higher derivative corrections

Using alpha'^3 terms of type IIB supergravity action we study higher order corrections to the non-relativistic non-extremal D3-brane. Utilizing the corrected solution we evaluate corrections to temperature, entropy and shear viscosity. We also compute the eta/s ratio which although within the range of validity of the supergravity approximation and in the lowest order of the correction the universal bound is respected, there is a possibility for a violation of the bound when higher terms in the expansion are taken into account.

hep-th↗

arXiv2008

More on entropy function formalism for non-extremal branes

We find R^4 correction to the non-extremal D1D5P solution of the supergravity by exactly solving the differential equations of motion and by using the entropy function formalism. In both cases, we find the same correction to the supergravity solution. We then calculate the correction to the entropy using the free energy method and the entropy function formalism. The results are the same.

hep-th↗

arXiv2008

Hydrodynamics of N=6 Superconformal Chern-Simons Theories at Strong Coupling

Using the duality conjecture between N=6 supersymmetric U(N)_k \times U(N)_{-k} Chern-Simons theory and M-theory on AdS_4 \times S^7/Z_{k}, we calculate the corrections to the shear viscosity of the field theory at temperature T. At strong 't Hooft coupling and at small k level, we have considered one-loop correction to the M-theory effective action. At large k level, we have considered the alpha' correction to the type IIA effective action. In both cases the correction to the ratio of shear viscosity to the entropy density is positive.

hep-th↗

arXiv2008

On thermodynamics of N=6 superconformal Chern-Simons theories at strong coupling

Recently it has been conjectured that N=6, U(N)_{k} \times U(N)_{-k} Chern-Simons theory is dual to M-theory on AdS_4\times S^7/Z_{k}. By studying one-loop correction to the M-theory effective action, we calculate the correction to the entropy of thermal field theory at strong coupling. For large k level, we have also found the alpha' correction to the entropy from the string correction of the type IIA effective action. The structure of these two corrections at strong t'Hooft coupling are different.

hep-th↗

arXiv2007

Entropy Function for Non-extremal D1D5 and D2D6NS5-branes

We apply the entropy function formalism to non-extremal D1D5 and D2D6NS5-branes whose throat approximation is given by the Schwarzschild black hole in AdS_3\times S^3\times T^4 and AdS_3\times S^2\times S^1\times T^4, respectively. We find the Bekenstein-Hawking entropy and the (alpha')^3R^4 corrections from the value of the entropy function at its saddle point. While the higher derivative terms have no effect on the temperature, they decrease the value of the entropy.

hep-th↗

arXiv2007

On Attractor Mechanism and Entropy Function for Non-extremal Black Holes/Branes

We examine in details the entropy function formalism for non-extremal D3, M2, and M5-branes that their throat approximation is given by Schwarzschild black hole in AdS_{p+2} \times S^{D-(p+2)}. We show that even though there is no attractor mechanism in the non-extremal black holes/branes, the entropy function formalism does work and the entropy is given by the entropy function at its saddle point.

hep-th↗

arXiv2006

R^4 Corrections to D1D5p Black Hole Entropy from Entropy Function Formalism

We show that in IIB string theory and for D1D5p black holes in ten dimensions the method of entropy function works. Despite the more complicated Wald formula for the entropy of D1D5p black holes in ten dimensions, their entropy is given by entropy function at its extremum point. We use this method for computing the entropy of the system both at the level of supergravity and for its higher order alpha'^3R^4 corrections.

hep-th↗

arXiv2005

D-brane Dynamics in RR Deformation of NS5-branes Background and Tachyon Cosmology

We study D-brane dynamics in the gravity dual background of ODp theories using the effective action on the worldvolume of the brane. We explore the similarities of the system with the rolling tachyon, including the exponential decrease of pressure at late times. We also consider the case where the worldvolume theory on a D3-brane is coupled to gravity and construct a cosmological model. By considering the time reversal symmetric solutions, we find that we can have both closed and open universes depending on the initial value of the radial mode on the brane. We compare the models with tachyon driven cosmologies and find limits where slow roll inflation is realized.

hep-th↗

arXiv2004

Braneworld inflation

We discuss various realizations of the four dimensional braneworld inflation in warped geometries of string theory. In all models the inflaton field is represented by a Dp probe brane scalar specifying its position in the warped throat of the compactification manifold. We study existing inflationary throat local geometries, and construct a new example. The inflationary brane is either a D3- or a D5-brane of type IIB string theory. In the latter case the inflationary brane is wrapping a two-cycle of the compactification manifold. We discuss some phenomenological aspects of the model where slow-roll conditions are under computational control.

hep-th↗

arXiv2004

On Isolated Conformal Fixed Points and Noncritical String Theory

We search for the gravity description of unidentified field theories at their conformal fixed points by studying the low energy effective action of six dimensional noncritical string theory. We find constant dilaton solutions by solving both the equations of motion and BPS equations. Our solutions include a free parameter provided by a stack of uncharged space filling branes. We find several AdS_p\times S^q solutions with constant radii for AdS_p and S^q. The curvature of the solutions are of the order of the string scale.

hep-th↗

arXiv2003

One-Loop Perturbative Corrections to non(anti)commutativity Parameter of N=1/2 Supersymmetric U(N) Gauge Theory

Perturbative corrections to N=1/2 supersymmetric U(N) gauge theory at one-loop order are studied. It is shown that whereas the quantum corrections to N=1 sector of the theory are not affected by the C-deformation, the non(anti)commutativity parameter C receives one-loop perturbative corrections. These perturbative corrections are computed by performing an explicit one-loop calculation of the three and four-point functions of the theory. The running of the non(anti)commutativity parameter C is also studied using an appropriate Callan-Symanzik equation.

hep-th↗