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F(R) gravity: From the Pioneer anomaly to cosmic acceleration
, Article Physical Review D - Particles, Fields, Gravitation and Cosmology ; Volume 77, Issue 10 , 2008 ; 15507998 (ISSN) ; Rahvar, S ; Sharif University of Technology
2008
Abstract
We use metric formalism in f(R) modified gravity to study the dynamics of various systems from the solar system to the cosmological scale. We assume an ansatz for the derivative of action as a function of distance and describe the Pioneer anomaly and the flat rotation curve of the spiral galaxies. Having the asymptotic behavior of action, we propose the action of f(R)=(R+Λ)(1+ln (R/Rc)/(R/R0+2/α)) where in galactic and solar system scales it can recover our desired form. The vacuum solution of this action also results in a positive late time acceleration for the Universe. We fix the parameters of this model, comparing with the Pioneer anomaly, rotation curve of spiral galaxies, and supernova...
Consistency of f(R)=R2-R02 gravity with cosmological observations in the palatini formalism
, Article Physical Review D - Particles, Fields, Gravitation and Cosmology ; Volume 76, Issue 4 , 2007 ; 15507998 (ISSN) ; Baghram, S ; Rahvar, S ; Sharif University of Technology
2007
Abstract
In this work we study the dynamics of the Universe in f(R)=R2-R02 modified gravity with the Palatini formalism. We use data from recent observations, such as the supernova type Ia Gold sample and Supernova Legacy Survey data, the size of the baryonic acoustic peak from the Sloan Digital Sky Survey, the position of the acoustic peak from the cosmic microwave background observations, and large-scale structure formation from the 2dFGRS survey, to put constraints on the parameters of the model. To check the consistency of this action, we compare the age of old cosmological objects with the age of the Universe. In the combined analysis with all the observations, we find the parameters of the...
Modified gravity with f(R)=R2-R02
, Article Physical Review D - Particles, Fields, Gravitation and Cosmology ; Volume 75, Issue 4 , 2007 ; 15507998 (ISSN) ; Farhang, M ; Rahvar, S ; Sharif University of Technology
2007
Abstract
In this work we propose a modified gravity with the action f(R)=R2-R02 instead of the Einstein-Hilbert action to describe the late time acceleration of the Universe. We obtain the equation of the modified gravity both in the metric and Palatini formalisms. An asymptotic solution of gravity equations corresponding to a constant Ricci scalar causes a late time acceleration of the Universe. We do a conformal transformation in the action of the modified gravity and obtain the equivalent minimally coupled scalar-tensor gravity. The equivalent Brans-Dicke gravity of this model is also studied. To examine this model with observation, the perihelion precession of the Mercury is compared with our...
Gravity on noncommutative D-branes
, Article International Journal of Modern Physics A ; Volume 18, Issue 7 , 2003 , Pages 1051-1066 ; 0217751X (ISSN) ; Arfaei, H ; Garousi, M. R ; Ghodsi, A ; Sharif University of Technology
2003
Abstract
The effective action for the low energy scattering of two gravitons with a D-brane in the presence of a constant antisymmetric B field in bosonic string theory is calculated and the modification to the standard D-brane action to first order in α′ is obtained
A near optimal midcourse guidance law based on spherical gravity
, Article Scientia Iranica ; Volume 10, Issue 4 , 2003 , Pages 436-442 ; 10263098 (ISSN) ; Massoumnia, M. A ; Sharif University of Technology
Sharif University of Technology
2003
Abstract
In [1], an optimal midcourse guidance law for close distances, where the difference of gravity for interceptor and ballistic missile is negligible, was introduced. There, a closed form solution, based on an optimization problem, was found, with very good performance for close distances but degraded performance in real problems with unequal gravity for missile and interceptor. In this paper, the difference of gravity is taken into account by considering a spherical gravity model. A new equation to express the relative motion between missile and interceptor is used to derive a "Near Optimal Guidance Law". The results found using the new derivation are similar to those in [1] but it provides a...
Holographic entanglement entropy for 4D conformal gravity
, Article Journal of High Energy Physics ; Vol. 2014, issue. 2 , 2014 ; ISSN: 11266708 ; Faraji Astaneh, A ; Mohammadi Mozaffar, M. R ; Sharif University of Technology
Abstract
Using the proposal for holographic entanglement entropy in higher derivative gravities, we compute holographic entanglement entropy for the conformal gravity in four dimensions which turns out to be finite. However, if one subtracts the contribution of the four dimensional Gauss-Bonnet term, the corresponding entanglement entropy has a divergent term and indeed restricted to an Einstein solution of the conformal gravity, the resultant entanglement entropy is exactly the same as that in the Einstein gravity. We will also make a comment on the first law of the entanglement thermodynamics for the conformal gravity in four dimensions
Consistency condition of spherically symmetric solutions in f(R) gravity
, Article Modern Physics Letters A ; Volume 24, Issue 4 , 2009 , Pages 305-309 ; 02177323 (ISSN) ; Rahvar, S ; Sharif University of Technology
2009
Abstract
In this work we study the spherical symmetric solutions of f(R) gravity in the metric formalism. We show that for a generic f(R) gravity, the spherical symmetric solution is consistent with the modified gravity equations except in the case of imposing an extra condition for the metric. © 2009 World Scientific Publishing Company
An inverse f(R) gravitation for cosmic speed up, and dark energy equivalent
, Article Modern Physics Letters A ; Volume 23, Issue 23 , 2008 , Pages 1929-1937 ; 02177323 (ISSN) ; Sobouti, Y ; Sharif University of Technology
2008
Abstract
To explain the cosmic speed up, brought to light by the recent SNIa and CMB observations, we propose the following: (a) In a spacetime endowed with a FRW metric, we choose an empirical scale factor that best explains the observations. (b) We assume a modified gravity, generated by an unspecified field Lagrangian, f(R). (c) We use the adopted empirical scale factor to work back retroactively to obtain f(R), hence the term "Inverse f(R)". (d) Next we consider the classic GR and a conventional FRW universe that, in addition to its known baryonic content, possesses a hypothetical "Dark Energy" component. We compare the two scenarios and find the density, the pressure, and the equation of the...
Holographic two-point functions for 4d log-gravity
, Article Journal of High Energy Physics ; Volume 2012, Issue 9 , 2012 ; 11266708 (ISSN) ; Naseh, A ; Zojer, T ; Sharif University of Technology
Springer
2012
Abstract
We compute holographic one- and two-point functions of critical highercurvature gravity in four dimensions. The two most important operators are the stress tensor and its logarithmic partner, sourced by ordinary massless and by logarithmic nonnormalisable gravitons, respectively. In addition, the logarithmic gravitons source two ordinary operators, one with spin-one and one with spin-zero. The one-point function of the stress tensor vanishes for all Einstein solutions, but has a non-zero contribution from logarithmic gravitons. The two-point functions of all operators match the expectations from a three-dimensional logarithmic conformal field theory
Moving quark in a viscous fluid
, Article Journal of High Energy Physics ; Volume 2012, Issue 6 , 2012 ; 11266708 (ISSN) ; Davody, A ; Sharif University of Technology
Springer
2012
Abstract
To study the rate of energy and momentum loss of a heavy quark in QGP, specifically in the hydrodynamic regime, we use fluid/gravity duality and construct a perturbative procedure to find the string solution in gravity side. We show that by this construction, the position of the world-sheet horizon and thereby the drag force exerted on the quark can be computed perturbatively, order by order in a boundary derivative expansion, for a wide range of quark velocities. At ideal order, our result is just the localized expression of the drag force exerted on a moving quark in thermal plasma, while for a quark whose velocity does not belong to the mentioned range, we predict a nonlocal drag force....
On holography of Julia-Zee dyon
, Article Journal of High Energy Physics ; Volume 2011, Issue 9 , September , 2011 ; 11266708 (ISSN) ; Sharif University of Technology
2011
Abstract
The holographic dual of self gravitating Julia-Zee dyon is discussed. It is shown that the dual field theory is generally a field theory with a vortex condensate. The vacuum expectation values of the dual operators, as functions of the sources in the field theory, are studied in a class of bulk solutions. In these solutions the sign of the vacuum expectation values of the dual operators change, by changing the sources in the model
Quantum local quench, AdS/BCFT and Yo-Yo string
, Article Journal of High Energy Physics ; Volume 2015, Issue 5 , May , 2015 ; 11266708 (ISSN) ; Mosaffa, A. E ; Sharif University of Technology
Springer Verlag
2015
Abstract
Abstract: We propose a holographic model for local quench in 1 + 1 dimensional Conformal Field Theory (CFT). The local quench is produced by joining two identical CFT’s on semi-infinite lines. When these theories have a zero boundary entropy, we use the AdS/Boundary CFT proposal to describe this process in terms of bulk physics. Boundaries of the original CFT’s are extended in AdS as dynamical surfaces. In our holographic picture these surfaces detach from the boundary and form a closed folded string which can propagate in the bulk. The dynamics of this string is governed by the tensionless Yo-Yo string solution and its subsequent evolution determines the time dependence after quench. We use...
Holographic phase transition to topological dyons
, Article Journal of High Energy Physics ; Volume 2010, Issue 10 , 2010 ; 11266708 (ISSN) ; Ardalan, F ; Sharif University of Technology
2010
Abstract
The dynamical stability of a Julia-Zee solution in the AdS background in a four dimensional Einstein-Yang-Mills-Higgs theory is studied. We find that the model with a vanishing scalar field develops a non-zero value for the field at a certain critical temperature which corresponds to a topological dyon in the bulk and a topological phase transition at the boundary
Quantum jump from singularity to outside of black hole
, Article Journal of High Energy Physics ; Volume 2016, Issue 2 , 2016 , Pages 1-20 ; 11266708 (ISSN) ; Hajian, K ; Sharif University of Technology
Springer Verlag
Abstract
Considering the role of black hole singularity in quantum evolution, a resolution to the firewall paradox is presented. It is emphasized that if an observer has the singularity as a part of his spacetime, then the semi-classical evolution would be non-unitary as viewed by him. Specifically, a free-falling observer inside the black hole would have a Hilbert space with non-unitary evolution; a quantum jump for particles encountering the singularity to outside of the horizon as late Hawking radiations. The non-unitarity in the jump resembles the one in collapse of wave function, but preserves entanglements. Accordingly, we elaborate the first postulate of black hole complementarity: freely...
Observational constraints with recent data on the DGP modified gravity
, Article International Journal of Theoretical Physics ; Volume 48, Issue 4 , 2009 , Pages 1203-1230 ; 00207748 (ISSN) ; Farhang, M ; Rahvar, S ; Sharif University of Technology
2009
Abstract
We study one of the simplest covariant modified-gravity models based on the Dvali-Gabadadze-Porrati (DGP) brane cosmology, a self-accelerating universe. In this model gravitational leakage into extra dimensions is responsible of late-time acceleration. We mainly focus on the effects of the model parameters on the geometry and the age of universe. Also we investigate the evolution of matter density perturbations in the modified gravity model, and obtain an analytical expression for the growth index, f. We show that increasing Ωrcleads to less growth of the density contrast δ, and also decreases the growth index. We give a fitting formula for the growth index at the present time and indicate...
2D gravity on AdS 2 with chern-simons corrections
, Article Journal of High Energy Physics ; Volume 2009, Issue 1 , 2009 ; 11266708 (ISSN) ; Fareghbal, R ; Esmaeil Mosaffa, A ; Sharif University of Technology
2009
Abstract
We study 2D Maxwell-dilaton gravity with higher order corrections given by the Chern-Simons term. The model admits three distinctive AdS 2 vacuum solutions. By making use of the entropy function formalism we find the entropy of the solutions which is corrected due to the presence of the Chern-Simons term. We observe that the form of the correction depends not only on the coefficient of the Chern-Simons term, but also on the sign of the electric charge; pointing toward the chiral nature of the dual CFT. Using the asymptotic symmetry of the theory as well as requiring a consistent picture we can find the central charge and the level of U(1) current. Upon uplifting the solutions to three...
Quantum chaos, pole-skipping and hydrodynamics in a holographic system with chiral anomaly
, Article Journal of High Energy Physics ; Volume 2020, Issue 3 , 2020 ; Tabatabaei, J ; Sharif University of Technology
Springer
2020
Abstract
It is well-known that chiral anomaly can be macroscopically detected through the energy and charge transport, due to the chiral magnetic effect. On the other hand, in a holographic many body system, the chaotic modes might be only associated with the energy conservation. This suggests that, perhaps, one can detect microscopic anomalies through the diagnosis of quantum chaos in such systems. To investigate this idea, we consider a magnetized brane in AdS space time with a Chern-Simons coupling in the bulk. By studying the shock wave geometry in this background, we first compute the corresponding butterfly velocities, in the presence of an external magnetic field B, in μ « T and B « T2 limit....
Fermat's principle in quantum gravitational optics
, Article Journal of Cosmology and Astroparticle Physics ; Volume 2008, Issue 5 , 2008 ; 14757516 (ISSN) ; Khoeini Moghaddam, S ; Nouri Zonoz, M ; Sharif University of Technology
2008
Abstract
Interactions incorporating the vacuum polarization effects in curved backgrounds modify the null cone structure in such a way that the photon trajectories would not be the space-time geodesics any longer. The gravitational birefringence introduced as a direct consequence of these effects will allow shifts in the photon velocities leading to polarization dependent superluminal propagation. Taking these effects into account, we study Fermat's principle in the context of the 1+3 (threading) formulation of the space-time decomposition. We find an expression for the modified space-time refractive index and show that it is proportional to the light cone correction to first order. Consequences of...
Island in the presence of higher derivative terms
, Article Journal of High Energy Physics ; Volume 2021, Issue 2 , 2021 ; 10298479 (ISSN) ; Astaneh, A. F ; Naseh, A ; Sharif University of Technology
Springer Science and Business Media Deutschland GmbH
2021
Abstract
Using extended island formula we compute entanglement entropy of Hawking radiation for black hole solutions of certain gravitational models containing higher derivative terms. To be concrete we consider two different four dimensional models to compute entropy for both asymptotically flat and AdS black holes. One observes that the resultant entropy follows the Page curve, thanks to the contribution of the island, despite the fact that the corresponding gravitational models might be non-unitary. © 2021, The Author(s)
A Review on Structure Formation Via f(R) Gravity Models
, M.Sc. Thesis Sharif University of Technology ; Rahvar, Sohrab (Supervisor)
Abstract
It has been about a decade that cosmological datas have shown the universe is accelerating. To describe this acceleration, cosmologosts use cosmological constant as the easiest way to describe,or the modified gravity models or smooth dark energy models as other ways. In this thesis we solved a specific structure formation problem, density contrast, for linear structures, in two different methods. the first method was LCDM model and the second method was a specific f (R) gravity model. We shpwed that these two solutions are about the same.