In this study,we explore the Finch–Skea perfect fluid solution and extend its domain to three distinct anisotropic interior models within the framework of the f(R,T)theory,incorporating the influence of an electromag...In this study,we explore the Finch–Skea perfect fluid solution and extend its domain to three distinct anisotropic interior models within the framework of the f(R,T)theory,incorporating the influence of an electromagnetic field.We assume a static spherical spacetime initially coupled with an isotropic matter distribution.We then introduce a Lagrangian corresponding to an additional gravitating source,taking into account its role in inducing pressure anisotropy within the original fluid source.By deriving the field equations for the combined matter setup,we applied a radial component transformation,which yielded two distinct systems of equations.In addition,we consider a charged exterior spacetime to determine the three constants associated with the Finch–Skea solution at the boundary.Our findings suggest that under certain parametric choices,all three resulting models exhibited physical relevance within this modified theory.展开更多
Recently,corrections to the standard Einstein Hilbert action were proposed to explain the current cosmicacceleration in stead of introducing dark energy.In the Palatini formulation of those modified gravity models,the...Recently,corrections to the standard Einstein Hilbert action were proposed to explain the current cosmicacceleration in stead of introducing dark energy.In the Palatini formulation of those modified gravity models,thereis an important observation due to Arkani Hamed:matter loops will give rise to a correction to the modified gravityaction proportional to the Ricci scalar of the metric.In the presence of such a term,we show that the current formsof modified gravity models in Palatini formulation,specifically,the 1/R gravity and in R gravity,will have phantoms.Then we study the possible instabilities due to the presence of phantom fields.We show that the strong instability inthe metric formulation of 1/R gravity indicated by Dolgov and Kawasaki will not appear and the decay timescales forthe phantom fields may be long enough for the theories to make sense as effective field theory.On the other hand,if wechange the sign of the modification terms to eliminate the phantoms,some other inconsistencies will arise for the variousversions of the modified gravity models.Finally,we comment on the universal property of the Palatini formulation ofthe matter loops corrected modified gravity models and its implications.展开更多
We consider a Lagrangian to describe gravity using a nonlinear term depending on the Gauss-Bonnet topological invariant. We examine the conditions for a bouncing and the existence of an ulterior accelerated phase of t...We consider a Lagrangian to describe gravity using a nonlinear term depending on the Gauss-Bonnet topological invariant. We examine the conditions for a bouncing and the existence of an ulterior accelerated phase of the universe. The original concept of Einstein's first paper on cosmology was to modify the equations that control the gravitational metric. Based on this, several authors have proposed different formalisms to modify general relativity. Herein, a function of the topological invariant is used in a homogeneous and isotropic cosmological model. First, a case with the topological invariant as a constant is thoroughly examined, yielding specific constraints on the evolution of the Hubble parameter. Subsequently, we study a function of the topological invariant squared. In an empty space-time scenario, the term that modifies Einstein's equations functions as an effective geometrical density, and we map regions in the phase space in which the effective pressure is positive or negative. This results in non-trivial integral solutions such as cyclic stages of acceleration and deceleration of the scale factor in the model, among other behaviors. We add cosmic dust to the system, and for certain classes of solutions, we observe that the minimum positive value of the Hubble parameter is limited by the cosmic dust matter density. If this matter density reaches zero, the minimum value of the Hubble parameter also reaches zero.展开更多
In this study,we conduct an investigation on decoupling gravitational sources under the framework of f(R,T)gravity.Basically,the complete geometric deformation technique is employed,which facilitates finding the exact...In this study,we conduct an investigation on decoupling gravitational sources under the framework of f(R,T)gravity.Basically,the complete geometric deformation technique is employed,which facilitates finding the exact solutions to the anisotropic astrophysical system smoothly without imposing any particular ansatz for the deformation function.In addition,we used 5-dimensional Euclidean spacetime in order to describe the embedding Class Ⅰ spacetime in order to obtain a solvable spherical physical system.The resulting solutions are both physically interesting and viable with new possibilities for investigation.Notably,the present investigation demonstrates that the mixture of f(R,T)+CGD translates to a scenario beyond the pure GR realm and helps to enhance the features of the interior astrophysical aspects of compact stellar objects.To determine the physical acceptability and stability of the stellar system based on the obtained solutions,we conducted a series of physical tests that satisfied all stability criteria,including the nonsingular nature of density and pressure.展开更多
We reconstruct the cosmological background evolution under the scenario of dynamical dark energy through the Gaussian process approach,using the latest Dark Energy Spectroscopic Instrument(DESI)baryon acoustic oscilla...We reconstruct the cosmological background evolution under the scenario of dynamical dark energy through the Gaussian process approach,using the latest Dark Energy Spectroscopic Instrument(DESI)baryon acoustic oscillations(BAO)combined with other observations.Our results reveal that the reconstructed dark-energy equation-of-state(EoS)parameter w(z)exhibits the so-called quintom-B behavior,crossing-1 from phantom to quintessence regime as the universe expands.We investigate under what situation this type of evolution could be achieved from the perspectives of field theories and modified gravity.In particular,we reconstruct the corresponding actions for f(R),f(T),and f(Q)gravity,respectively.We explicitly show that,certain modified gravity can exhibit the quintom dynamics and fit the recent DESI data efficiently,and for all cases the quadratic deviation from theΛCDM scenario is mildly favored.展开更多
There is a significant difference between the calculation based on the theory of general relativity and observation of rotation curves of spiral galaxies. To describe this discrepancy, two distinct theories have been ...There is a significant difference between the calculation based on the theory of general relativity and observation of rotation curves of spiral galaxies. To describe this discrepancy, two distinct theories have been proposed so far： existence of dark matter and modification of underlying gravitational theory. In the absence of dark matter, it is assumed that the theory of general relativity on galactic scales needs to be modified. This letter is devoted to explaining this difference in a modified teleparMIeI gravity. We show that modified teleparallel gravity favors flatness of rotation curves of spiral galaxies much in the same way as observation shows.展开更多
We explore the cosmological implications of five modified gravity(MG) models by using the recent cosmological observational data,including the recently released SNLS3 type Ia supernovae sample,the cosmic microwave bac...We explore the cosmological implications of five modified gravity(MG) models by using the recent cosmological observational data,including the recently released SNLS3 type Ia supernovae sample,the cosmic microwave background anisotropy data from the Wilkinson Microwave Anisotropy Probe 7-yr observations,the baryon acoustic oscillation results from the Sloan Digital Sky Survey data release 7,and the latest Hubble constant measurement utilizing the Wide Field Camera 3 on the Hubble Space Telescope.The MG models considered include the Dvali-Gabadadze-Porrati(DGP) model,two f(R) models,and two f(T) models.We find that compared with the ΛCDM model,MG models can not lead to an appreciable reduction of the χ 2 min.The analysis of AIC and BIC shows that the simplest cosmological constant model(ΛCDM) is still the most preferred by the current data,and the DGP model is strongly disfavored.In addition,from the observational constraints,we also reconstruct the evolutions of the growth factor in these models.We find that the current available growth factor data are not enough to distinguish these MG models from the ΛCDM model.展开更多
The aim of this work is to investigate anisotropic compact objects within the framework of f(G)modified theory of gravity.For our present work,we utilize Krori-Barua metrics,i.e.,λ(r)=Xr^(2)+Y andβ(r)=Zr^(2).We use ...The aim of this work is to investigate anisotropic compact objects within the framework of f(G)modified theory of gravity.For our present work,we utilize Krori-Barua metrics,i.e.,λ(r)=Xr^(2)+Y andβ(r)=Zr^(2).We use some matching conditions of spherically symmetric spacetime with Bardeen's model as an exterior geometry.Further,we establish some expressions of energy density and pressure components to analyze the stellar configuration of Bardeen compact stars by assuming viable f(G)models.We examine the energy conditions for different stellar structures to verify the viability of our considered models.Moreover,we also investigate some other physical features,such as equilibrium condition,equation of state parameters,adiabatic index,stability analysis,mass function,surface redshift,and compactness factor,respectively.It is worthwhile to mention here for the current study that our stellar structure in the background of Bardeen's model is more viable and stable.展开更多
The paper extends basic Einstein–Hilbert action by incorporating an invariant derived from a specific contraction between the Einstein tensor and energy momentum tensor.This represents a non–minimal coupling between...The paper extends basic Einstein–Hilbert action by incorporating an invariant derived from a specific contraction between the Einstein tensor and energy momentum tensor.This represents a non–minimal coupling between the space–time geometry and matter fields.The fundamental Einstein–Hilbert action is extended by considering a generic function f(R,GμνTμν).This function is subsequently dissected into its primary components:a geometric part reliant on the scalar curvature,and another segment encapsulating the interaction between geometry and matter fields.Distinct cosmological models are formulated at the backdrop of these dynamics,rooted in specific associations between the matter energy-momentum tensor and Einstein tensor.Upon deriving the consequent field equations,the cosmological model's physical implications are explored through a dynamical system analysis applied to diverse coupling functions.This exploration reveals that the current model aligns with various phases in the Universe's evolution,potentially elucidating several late-time historical epochs.展开更多
In this paper, we review modified <i>f(R)</i> theories of gravity in Palatini formalism. In this framework, we use the Raychaudhuri’s equation along with the requirement that the gravity is attractive, wh...In this paper, we review modified <i>f(R)</i> theories of gravity in Palatini formalism. In this framework, we use the Raychaudhuri’s equation along with the requirement that the gravity is attractive, which holds for any geometrical theory of gravity to discuss the energy conditions. Then, to derive these conditions, we obtain an expression for effective pressure and energy density by considering FLRW metric. To simply express the energy conditions, we write the Ricci scalar and its derivatives in terms of the deceleration (<i>q</i>), jerk (<i>j</i>) and snap (<i>s</i>) parameters. Energy conditions derived in Palatini version of <i>f(R)</i> Gravity differ from those derived in GR. We will see that the WEC (weak energy condition) derived in Palatini formalism has exactly the same expression in its metric approach.展开更多
In the present work, we reconstruct different f（T）-gravity models corre- sponding to the original and entropy-corrected versions of the holographic and new agegraphic dark energy models. We also obtain the equation ...In the present work, we reconstruct different f（T）-gravity models corre- sponding to the original and entropy-corrected versions of the holographic and new agegraphic dark energy models. We also obtain the equation of state parameters of the corresponding f（T）-gravity models. We conclude that the original holographic and new agegraphic f （T）-gravity models behave like the phantom or quintessence model, whereas in the entropy-corrected models, the equation of state parameter can justify the transition from the quintessence state to the phantom regime as indicated by the recent observations.展开更多
This study aims to discuss anisotropic solutions that are spherically symmetric in the quintessence field,which describe compact stellar objects in the modified Rastall teleparallel theory of gravity.To achieve this g...This study aims to discuss anisotropic solutions that are spherically symmetric in the quintessence field,which describe compact stellar objects in the modified Rastall teleparallel theory of gravity.To achieve this goal,the Krori and Barua arrangement for spherically symmetric components of the line element is incorporated.We explore the field equations by selecting appropriate off-diagonal tetrad fields.Born-Infeld function of torsion f(T)=β√λT+1-1 and power law form h(T)=δTn are used.The Born-Infeld gravity was the first modified teleparallel gravity to discuss inflation.We use the linear equation of state pr=ξρto separate the quintessence density.After obtaining the field equations,we investigate different physical parameters that demonstrate the stability and physical acceptability of the stellar models.We use observational data,such as the mass and radius of the compact star candidates PSRJ 1416-2230,Cen X-3,&4U 1820-30,to ensure the physical plausibility of our findings.展开更多
Inspired by the f（R） non-linear massive gravity, we propose a new kind of modified gravity model, namely f（T） non-linear massive gravity, by adding the dRGT mass term reformulated in the vierbein formalism, to th...Inspired by the f（R） non-linear massive gravity, we propose a new kind of modified gravity model, namely f（T） non-linear massive gravity, by adding the dRGT mass term reformulated in the vierbein formalism, to the f（T） theory. We then investigate the cosmological evolution of f（T） massive gravity, and constrain it by using the latest observational data. We find that it slightly favors a crossing of the phantom divide line from the quintessence-like phase （wae 〉 -1） to the phantom-like one （wae 〈 -1） as redshiff decreases.展开更多
Recently, a novel 4 D Einstein–Gauss–Bonnet gravity has been proposed by Glavan and Lin(2020 Phys. Rev. Lett. 124 081301) by rescaling the coupling α→α(D-4) and taking the limit D→ 4 at the level of equations of...Recently, a novel 4 D Einstein–Gauss–Bonnet gravity has been proposed by Glavan and Lin(2020 Phys. Rev. Lett. 124 081301) by rescaling the coupling α→α(D-4) and taking the limit D→ 4 at the level of equations of motion. This prescription, though was shown to bring non-trivial effects for some spacetimes with particular symmetries, remains mysterious and calls for scrutiny. Indeed, there is no continuous way to take the limit D→4 in the higher Ddimensional equations of motion because the tensor indices depend on the spacetime dimension and behave discretely. On the other hand, if one works with 4 D spacetime indices the contribution corresponding to the Gauss–Bonnet term vanishes identically in the equations of motion. A necessary condition(but may not be sufficient) for this procedure to work is that there is an embedding of the 4 D spacetime into the higher D-dimensional spacetime so that the equations in the latter can be properly interpreted after taking the limit. In this note, working with2 D Einstein gravity, we show several subtleties when applying the method used in(2020 Phys.Rev. Lett. 124 081301).展开更多
The reason for the present accelerated expansion of the Universe stands as one of the most profound questions in the realm of science,with deep connections to both cosmology and fundamental physics.From a cosmological...The reason for the present accelerated expansion of the Universe stands as one of the most profound questions in the realm of science,with deep connections to both cosmology and fundamental physics.From a cosmological point of view,physical models aimed at elucidating the observed expansion can be categorized into two major classes:dark energy and modified gravity.We review various major approaches that employ a single scalar field to account for the accelerating phase of our present Universe.Dynamic system analysis was employed in several important models to find cosmological solutions that exhibit an accelerating phase as an attractor.For scalar field models of dark energy,we consistently focused on addressing challenges related to the fine-tuning and coincidence problems in cosmology,as well as exploring potential solutions to them.For scalar–tensor theories and their generalizations,we emphasize the importance of constraints on theoretical parameters to ensure overall consistency with experimental tests.Models or theories that could potentially explain the Hubble tension are also emphasized throughout this review.展开更多
We study gravitational baryogenesis in the context of f(R, T) gravity where the gravitational Lagrangian is given by a generic function of the Ricci scalar R and the trace of the stress-energy tensor T. We explore how...We study gravitational baryogenesis in the context of f(R, T) gravity where the gravitational Lagrangian is given by a generic function of the Ricci scalar R and the trace of the stress-energy tensor T. We explore how this type of modified gravity is capable to shed light on the issue of baryon asymmetry in a successful manner. We consider various forms of baryogenesis interaction and discuss the effect of these interaction terms on the baryon to entropy ratio in this setup. We show that baryon asymmetry during the radiation era of the expanding universe can be non-zero in this framework. Then, we calculate the baryon to entropy ratio for some specific f(R, T) models and by using the observational data, we give some constraints on the parameter spaces of these models.展开更多
In this paper, a general FRW cosmological model has been constructed in f（R, T） gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizin...In this paper, a general FRW cosmological model has been constructed in f（R, T） gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman＇s law of constant deceleration parameter q =m - 1. The possible decelerating and accelerating solutions have been investigated. For （q 〉 0） we get a stable fiat decelerating radiation-dominated universe at q = 1. For （q 〈 0） we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed.展开更多
We propose a new exponential f（R） gravity model with f（R） = （R - λc） e^λ（c/R）n and n 〉 3, λ ≥ 1, c 〉 0 to explain late-time acceleration of the universe. At the high curvature region, the model behaves l...We propose a new exponential f（R） gravity model with f（R） = （R - λc） e^λ（c/R）n and n 〉 3, λ ≥ 1, c 〉 0 to explain late-time acceleration of the universe. At the high curvature region, the model behaves like the A CDM model. In the asymptotic future, it reaches a stable de-Sitter spaeetime. It is a cosmologically viable model and can evade the local gravity constraints easily. This model shares many features with other f（R） dark energy models like Hu-Sawicki model and ExponentiM gravity model. In it the dark energy equation of state is of an oscillating form and can cross phantom divide line ωde = -1. In particular, in the parameter range 3 〈 n ≤ 4, λ ～ 1, the model is most distinguishable from other models. For instance, when n = 4, λ = 1, the dark energy equation of state will cross -1 in the earlier future and has a stronger oscillating form than the other models, the dark energy density in asymptotical future is smaller than the one in the high curvature region. This new model can evade the local gravity tests easily when n 〉 3 and λ 〉 1.展开更多
This investigation assesses the feasibility of a traversable wormhole by examining the energy densities associated with charged Casimir phenomena.We focus on the influence of the electromagnetic field created by an el...This investigation assesses the feasibility of a traversable wormhole by examining the energy densities associated with charged Casimir phenomena.We focus on the influence of the electromagnetic field created by an electric charge as well as the negative energy density arising from the Casimir source.We have developed different shape functions by defining energy densities from this combination.This paper explores various configurations of Casimir energy densities,specifically those occurring between parallel plates,cylinders and spheres positioned at specified distances from each other.Furthermore,the impact of the generalized uncertainty principle correction is also examined.The behavior of wormhole conditions is evaluated based on the Gauss–Bonnet coupled parameter(μ)and electric charge(Q)through the electromagnetic energy density constraint.This is attributed to the fact that the electromagnetic field satisfies the characteristicρ=-pr.Subsequently,we examine the active gravitational mass of the generated wormhole geometries and explore the behavior ofμand Q concerning active mass.The embedding representations for all formulated shape functions are examined.Investigations of the complexity factor of the charged Casimir wormhole have demonstrated that the values of the complexity factor consistently fall within a particular range in all scenarios.Finally,using the generalized Tolman–Oppenheimer–Volkoff equation,we examine the stability of the resulting charged Casimir wormhole solutions.展开更多
In this assignment we will present a reconstruction scheme between f(R) gravity with ordinary and entropy corrected(m, n)-type holographic dark energy. The correspondence is established and expressions for the reconst...In this assignment we will present a reconstruction scheme between f(R) gravity with ordinary and entropy corrected(m, n)-type holographic dark energy. The correspondence is established and expressions for the reconstructed f(R) models are determined. To study the evolution of the reconstructed models plots are generated. The stability of the calculated models are also investigated using the squared speed of sound in the background of the reconstructed gravities.展开更多
文摘In this study,we explore the Finch–Skea perfect fluid solution and extend its domain to three distinct anisotropic interior models within the framework of the f(R,T)theory,incorporating the influence of an electromagnetic field.We assume a static spherical spacetime initially coupled with an isotropic matter distribution.We then introduce a Lagrangian corresponding to an additional gravitating source,taking into account its role in inducing pressure anisotropy within the original fluid source.By deriving the field equations for the combined matter setup,we applied a radial component transformation,which yielded two distinct systems of equations.In addition,we consider a charged exterior spacetime to determine the three constants associated with the Finch–Skea solution at the boundary.Our findings suggest that under certain parametric choices,all three resulting models exhibited physical relevance within this modified theory.
基金National Natural Science Foundation of China under Grant No.10675062the Second Stage Brain Korea 21 Program in Hanyang University
文摘Recently,corrections to the standard Einstein Hilbert action were proposed to explain the current cosmicacceleration in stead of introducing dark energy.In the Palatini formulation of those modified gravity models,thereis an important observation due to Arkani Hamed:matter loops will give rise to a correction to the modified gravityaction proportional to the Ricci scalar of the metric.In the presence of such a term,we show that the current formsof modified gravity models in Palatini formulation,specifically,the 1/R gravity and in R gravity,will have phantoms.Then we study the possible instabilities due to the presence of phantom fields.We show that the strong instability inthe metric formulation of 1/R gravity indicated by Dolgov and Kawasaki will not appear and the decay timescales forthe phantom fields may be long enough for the theories to make sense as effective field theory.On the other hand,if wechange the sign of the modification terms to eliminate the phantoms,some other inconsistencies will arise for the variousversions of the modified gravity models.Finally,we comment on the universal property of the Palatini formulation ofthe matter loops corrected modified gravity models and its implications.
文摘We consider a Lagrangian to describe gravity using a nonlinear term depending on the Gauss-Bonnet topological invariant. We examine the conditions for a bouncing and the existence of an ulterior accelerated phase of the universe. The original concept of Einstein's first paper on cosmology was to modify the equations that control the gravitational metric. Based on this, several authors have proposed different formalisms to modify general relativity. Herein, a function of the topological invariant is used in a homogeneous and isotropic cosmological model. First, a case with the topological invariant as a constant is thoroughly examined, yielding specific constraints on the evolution of the Hubble parameter. Subsequently, we study a function of the topological invariant squared. In an empty space-time scenario, the term that modifies Einstein's equations functions as an effective geometrical density, and we map regions in the phase space in which the effective pressure is positive or negative. This results in non-trivial integral solutions such as cyclic stages of acceleration and deceleration of the scale factor in the model, among other behaviors. We add cosmic dust to the system, and for certain classes of solutions, we observe that the minimum positive value of the Hubble parameter is limited by the cosmic dust matter density. If this matter density reaches zero, the minimum value of the Hubble parameter also reaches zero.
基金TRC Project(Grant No.BFP/RGP/CBS-/19/099),the Sultanate of Omancontinuous support and encouragement from the administration of University of Nizwa。
文摘In this study,we conduct an investigation on decoupling gravitational sources under the framework of f(R,T)gravity.Basically,the complete geometric deformation technique is employed,which facilitates finding the exact solutions to the anisotropic astrophysical system smoothly without imposing any particular ansatz for the deformation function.In addition,we used 5-dimensional Euclidean spacetime in order to describe the embedding Class Ⅰ spacetime in order to obtain a solvable spherical physical system.The resulting solutions are both physically interesting and viable with new possibilities for investigation.Notably,the present investigation demonstrates that the mixture of f(R,T)+CGD translates to a scenario beyond the pure GR realm and helps to enhance the features of the interior astrophysical aspects of compact stellar objects.To determine the physical acceptability and stability of the stellar system based on the obtained solutions,we conducted a series of physical tests that satisfied all stability criteria,including the nonsingular nature of density and pressure.
基金National Key Research and Development Program of China(2021YFC2203100)National Natural Science Foundation of China(12261131497 and 12003029)+2 种基金CAS young interdisciplinary innovation team(JCTD2022-20)111 Project(B23042)USTC Fellowship for International Cooperation,and USTC Research Funds of the Double First-Class Initiative。
文摘We reconstruct the cosmological background evolution under the scenario of dynamical dark energy through the Gaussian process approach,using the latest Dark Energy Spectroscopic Instrument(DESI)baryon acoustic oscillations(BAO)combined with other observations.Our results reveal that the reconstructed dark-energy equation-of-state(EoS)parameter w(z)exhibits the so-called quintom-B behavior,crossing-1 from phantom to quintessence regime as the universe expands.We investigate under what situation this type of evolution could be achieved from the perspectives of field theories and modified gravity.In particular,we reconstruct the corresponding actions for f(R),f(T),and f(Q)gravity,respectively.We explicitly show that,certain modified gravity can exhibit the quintom dynamics and fit the recent DESI data efficiently,and for all cases the quadratic deviation from theΛCDM scenario is mildly favored.
文摘There is a significant difference between the calculation based on the theory of general relativity and observation of rotation curves of spiral galaxies. To describe this discrepancy, two distinct theories have been proposed so far： existence of dark matter and modification of underlying gravitational theory. In the absence of dark matter, it is assumed that the theory of general relativity on galactic scales needs to be modified. This letter is devoted to explaining this difference in a modified teleparMIeI gravity. We show that modified teleparallel gravity favors flatness of rotation curves of spiral galaxies much in the same way as observation shows.
基金supported by the National Natural Science Foundation of China (Grant Nos. 10535060/A050207,10821504 and 10975167)the Project of Knowledge Innovation Program of Chinese Academy of Science
文摘We explore the cosmological implications of five modified gravity(MG) models by using the recent cosmological observational data,including the recently released SNLS3 type Ia supernovae sample,the cosmic microwave background anisotropy data from the Wilkinson Microwave Anisotropy Probe 7-yr observations,the baryon acoustic oscillation results from the Sloan Digital Sky Survey data release 7,and the latest Hubble constant measurement utilizing the Wide Field Camera 3 on the Hubble Space Telescope.The MG models considered include the Dvali-Gabadadze-Porrati(DGP) model,two f(R) models,and two f(T) models.We find that compared with the ΛCDM model,MG models can not lead to an appreciable reduction of the χ 2 min.The analysis of AIC and BIC shows that the simplest cosmological constant model(ΛCDM) is still the most preferred by the current data,and the DGP model is strongly disfavored.In addition,from the observational constraints,we also reconstruct the evolutions of the growth factor in these models.We find that the current available growth factor data are not enough to distinguish these MG models from the ΛCDM model.
基金Adnan Malik acknowledges the Grant No.YS304023912 to support his Postdoctoral Fellowship at Zhejiang Normal University,China.
文摘The aim of this work is to investigate anisotropic compact objects within the framework of f(G)modified theory of gravity.For our present work,we utilize Krori-Barua metrics,i.e.,λ(r)=Xr^(2)+Y andβ(r)=Zr^(2).We use some matching conditions of spherically symmetric spacetime with Bardeen's model as an exterior geometry.Further,we establish some expressions of energy density and pressure components to analyze the stellar configuration of Bardeen compact stars by assuming viable f(G)models.We examine the energy conditions for different stellar structures to verify the viability of our considered models.Moreover,we also investigate some other physical features,such as equilibrium condition,equation of state parameters,adiabatic index,stability analysis,mass function,surface redshift,and compactness factor,respectively.It is worthwhile to mention here for the current study that our stellar structure in the background of Bardeen's model is more viable and stable.
文摘The paper extends basic Einstein–Hilbert action by incorporating an invariant derived from a specific contraction between the Einstein tensor and energy momentum tensor.This represents a non–minimal coupling between the space–time geometry and matter fields.The fundamental Einstein–Hilbert action is extended by considering a generic function f(R,GμνTμν).This function is subsequently dissected into its primary components:a geometric part reliant on the scalar curvature,and another segment encapsulating the interaction between geometry and matter fields.Distinct cosmological models are formulated at the backdrop of these dynamics,rooted in specific associations between the matter energy-momentum tensor and Einstein tensor.Upon deriving the consequent field equations,the cosmological model's physical implications are explored through a dynamical system analysis applied to diverse coupling functions.This exploration reveals that the current model aligns with various phases in the Universe's evolution,potentially elucidating several late-time historical epochs.
文摘In this paper, we review modified <i>f(R)</i> theories of gravity in Palatini formalism. In this framework, we use the Raychaudhuri’s equation along with the requirement that the gravity is attractive, which holds for any geometrical theory of gravity to discuss the energy conditions. Then, to derive these conditions, we obtain an expression for effective pressure and energy density by considering FLRW metric. To simply express the energy conditions, we write the Ricci scalar and its derivatives in terms of the deceleration (<i>q</i>), jerk (<i>j</i>) and snap (<i>s</i>) parameters. Energy conditions derived in Palatini version of <i>f(R)</i> Gravity differ from those derived in GR. We will see that the WEC (weak energy condition) derived in Palatini formalism has exactly the same expression in its metric approach.
基金supported financially by the Research Institute for Astronomy and Astrophysics of Maragha (RIAAM) under research project No.1/1999
文摘In the present work, we reconstruct different f（T）-gravity models corre- sponding to the original and entropy-corrected versions of the holographic and new agegraphic dark energy models. We also obtain the equation of state parameters of the corresponding f（T）-gravity models. We conclude that the original holographic and new agegraphic f （T）-gravity models behave like the phantom or quintessence model, whereas in the entropy-corrected models, the equation of state parameter can justify the transition from the quintessence state to the phantom regime as indicated by the recent observations.
基金funded by the National Natural Science Foundation of China (Grant No. 11975145)
文摘This study aims to discuss anisotropic solutions that are spherically symmetric in the quintessence field,which describe compact stellar objects in the modified Rastall teleparallel theory of gravity.To achieve this goal,the Krori and Barua arrangement for spherically symmetric components of the line element is incorporated.We explore the field equations by selecting appropriate off-diagonal tetrad fields.Born-Infeld function of torsion f(T)=β√λT+1-1 and power law form h(T)=δTn are used.The Born-Infeld gravity was the first modified teleparallel gravity to discuss inflation.We use the linear equation of state pr=ξρto separate the quintessence density.After obtaining the field equations,we investigate different physical parameters that demonstrate the stability and physical acceptability of the stellar models.We use observational data,such as the mass and radius of the compact star candidates PSRJ 1416-2230,Cen X-3,&4U 1820-30,to ensure the physical plausibility of our findings.
基金Supported by National Natural Science Foundation of China under Grant Nos.11175016 and 10905005New Century Excellent Talents in University under Grant No.NCET-11-0790
文摘Inspired by the f（R） non-linear massive gravity, we propose a new kind of modified gravity model, namely f（T） non-linear massive gravity, by adding the dRGT mass term reformulated in the vierbein formalism, to the f（T） theory. We then investigate the cosmological evolution of f（T） massive gravity, and constrain it by using the latest observational data. We find that it slightly favors a crossing of the phantom divide line from the quintessence-like phase （wae 〉 -1） to the phantom-like one （wae 〈 -1） as redshiff decreases.
文摘Recently, a novel 4 D Einstein–Gauss–Bonnet gravity has been proposed by Glavan and Lin(2020 Phys. Rev. Lett. 124 081301) by rescaling the coupling α→α(D-4) and taking the limit D→ 4 at the level of equations of motion. This prescription, though was shown to bring non-trivial effects for some spacetimes with particular symmetries, remains mysterious and calls for scrutiny. Indeed, there is no continuous way to take the limit D→4 in the higher Ddimensional equations of motion because the tensor indices depend on the spacetime dimension and behave discretely. On the other hand, if one works with 4 D spacetime indices the contribution corresponding to the Gauss–Bonnet term vanishes identically in the equations of motion. A necessary condition(but may not be sufficient) for this procedure to work is that there is an embedding of the 4 D spacetime into the higher D-dimensional spacetime so that the equations in the latter can be properly interpreted after taking the limit. In this note, working with2 D Einstein gravity, we show several subtleties when applying the method used in(2020 Phys.Rev. Lett. 124 081301).
基金supported by the National Natural Science Foundation of China(11991053)Beijing Natural Science Foundation(1242018)+2 种基金National SKA Program of China(2020SKA0120300)Max Planck Partner Group program funded by the Max Planck SocietyHigh-Performance Computing Platform of Peking University。
文摘The reason for the present accelerated expansion of the Universe stands as one of the most profound questions in the realm of science,with deep connections to both cosmology and fundamental physics.From a cosmological point of view,physical models aimed at elucidating the observed expansion can be categorized into two major classes:dark energy and modified gravity.We review various major approaches that employ a single scalar field to account for the accelerating phase of our present Universe.Dynamic system analysis was employed in several important models to find cosmological solutions that exhibit an accelerating phase as an attractor.For scalar field models of dark energy,we consistently focused on addressing challenges related to the fine-tuning and coincidence problems in cosmology,as well as exploring potential solutions to them.For scalar–tensor theories and their generalizations,we emphasize the importance of constraints on theoretical parameters to ensure overall consistency with experimental tests.Models or theories that could potentially explain the Hubble tension are also emphasized throughout this review.
基金Supported financially by Research Institute for Astronomy and Astrophysics of Maragha(RIAAM)under Research Project Number1/5750/16
文摘We study gravitational baryogenesis in the context of f(R, T) gravity where the gravitational Lagrangian is given by a generic function of the Ricci scalar R and the trace of the stress-energy tensor T. We explore how this type of modified gravity is capable to shed light on the issue of baryon asymmetry in a successful manner. We consider various forms of baryogenesis interaction and discuss the effect of these interaction terms on the baryon to entropy ratio in this setup. We show that baryon asymmetry during the radiation era of the expanding universe can be non-zero in this framework. Then, we calculate the baryon to entropy ratio for some specific f(R, T) models and by using the observational data, we give some constraints on the parameter spaces of these models.
文摘In this paper, a general FRW cosmological model has been constructed in f（R, T） gravity reconstruction with variable cosmological constant. A number of solutions to the field equations has been generated by utilizing a form for the Hubble parameter that leads to Berman＇s law of constant deceleration parameter q =m - 1. The possible decelerating and accelerating solutions have been investigated. For （q 〉 0） we get a stable fiat decelerating radiation-dominated universe at q = 1. For （q 〈 0） we get a stable accelerating solution describing a flat universe with positive energy density and negative cosmological constant. Nonconventional mechanisms that are expected to address the late-time acceleration with negative cosmological constant have been discussed.
基金Supported by National Natural Science Foundation of China under Grant Nos.10975005 and 11335012
文摘We propose a new exponential f（R） gravity model with f（R） = （R - λc） e^λ（c/R）n and n 〉 3, λ ≥ 1, c 〉 0 to explain late-time acceleration of the universe. At the high curvature region, the model behaves like the A CDM model. In the asymptotic future, it reaches a stable de-Sitter spaeetime. It is a cosmologically viable model and can evade the local gravity constraints easily. This model shares many features with other f（R） dark energy models like Hu-Sawicki model and ExponentiM gravity model. In it the dark energy equation of state is of an oscillating form and can cross phantom divide line ωde = -1. In particular, in the parameter range 3 〈 n ≤ 4, λ ～ 1, the model is most distinguishable from other models. For instance, when n = 4, λ = 1, the dark energy equation of state will cross -1 in the earlier future and has a stronger oscillating form than the other models, the dark energy density in asymptotical future is smaller than the one in the high curvature region. This new model can evade the local gravity tests easily when n 〉 3 and λ 〉 1.
基金the Deanship of Research and Graduate Studies at King Khalid University for funding this work through a Large Research Project under Grant No.RGP2/453/45partially supported by the National Natural Science Foundation of China under Grant No.11988101。
文摘This investigation assesses the feasibility of a traversable wormhole by examining the energy densities associated with charged Casimir phenomena.We focus on the influence of the electromagnetic field created by an electric charge as well as the negative energy density arising from the Casimir source.We have developed different shape functions by defining energy densities from this combination.This paper explores various configurations of Casimir energy densities,specifically those occurring between parallel plates,cylinders and spheres positioned at specified distances from each other.Furthermore,the impact of the generalized uncertainty principle correction is also examined.The behavior of wormhole conditions is evaluated based on the Gauss–Bonnet coupled parameter(μ)and electric charge(Q)through the electromagnetic energy density constraint.This is attributed to the fact that the electromagnetic field satisfies the characteristicρ=-pr.Subsequently,we examine the active gravitational mass of the generated wormhole geometries and explore the behavior ofμand Q concerning active mass.The embedding representations for all formulated shape functions are examined.Investigations of the complexity factor of the charged Casimir wormhole have demonstrated that the values of the complexity factor consistently fall within a particular range in all scenarios.Finally,using the generalized Tolman–Oppenheimer–Volkoff equation,we examine the stability of the resulting charged Casimir wormhole solutions.
文摘In this assignment we will present a reconstruction scheme between f(R) gravity with ordinary and entropy corrected(m, n)-type holographic dark energy. The correspondence is established and expressions for the reconstructed f(R) models are determined. To study the evolution of the reconstructed models plots are generated. The stability of the calculated models are also investigated using the squared speed of sound in the background of the reconstructed gravities.
