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
We investigate a class of gravity theories respecting only spatial covariance,termed spatially covariant gravity,in the presence of an auxiliary scalar field.We examine the conditions on the Lagrangian required to eli...We investigate a class of gravity theories respecting only spatial covariance,termed spatially covariant gravity,in the presence of an auxiliary scalar field.We examine the conditions on the Lagrangian required to eliminate scalar degrees of freedom,allowing only two tensorial degrees of freedom to propagate.Instead of strict constraint analysis,in this paper,we employ the perturbation method and focus on the necessary conditions to evade the scalar mode at the linear order in perturbations around a cosmological background.Beginning with a general action and solving the auxiliary perturbation variables in terms of a would-be dynamical scalar mode,we derive the condition to remove its kinetic term,thus ensuring that no scalar mode propagates.As an application of the general condition,we study a polynomial-type Lagrangian as a concrete example,in which all monomials are spatially covariant scalars containing two derivatives.We find that the auxiliary scalar field is essential,and new terms in the Lagrangian are allowed.Our analysis provides insights into constructing gravity theories with two degrees of freedom in the extended framework of spatially covariant gravity.展开更多
We investigate the impact of the modified gravity(MOG)field and the quintessence scalar field on horizon evolution,black hole(BH)shadow and the weak gravitational lensing around a static spherically symmetric BH.We fi...We investigate the impact of the modified gravity(MOG)field and the quintessence scalar field on horizon evolution,black hole(BH)shadow and the weak gravitational lensing around a static spherically symmetric BH.We first begin to write the BH metric associated with the MOG parameter and quintessence scalar field.We then determine the BH shadow and obtain numerical solutions for the photon sphere and shadow radius.We show that the MOG(α)and the quintessence(c)parameters have a significant impact on the BH shadow and photon sphere.Based on the analysis,we further show that the combined effects of the MOG parameter and quintessence field can increase the values of BH shadow and photon sphere radii.We also obtain constraints on the BH parameters by applying the observational data of Sgr A^(★)and M87^(★).Finally,we consider the weak deflection angle of BH within the context of the Gauss-Bonnet theorem(GBT)and show that the combined effects of the MOG and quintessence parameters do make the value of the deflection angle increase,and find this remarkable property is in good agreement with the physical meaning of both parameters that can maintain the strong gravitational field in the surrounding environment of a BH.展开更多
General relativity has been very successful since its proposal more than a century ago.However,various cosmological observations and theoretical consistency still motivate us to explore extended gravity theories.Hornd...General relativity has been very successful since its proposal more than a century ago.However,various cosmological observations and theoretical consistency still motivate us to explore extended gravity theories.Horndeski gravity stands out as one attractive theory by introducing only one scalar field.Here we formulate the post-Newtonian effective field theory of Horndeski gravity and investigate the conservative dynamics of inspiral compact binary systems.We calculate the leading effective Lagrangian for a compact binary and obtain the periastron advance per period.In particular,we apply our analytical calculation to two binary systems,PSR B 1534+12 and PSR J0737-3039,and constrain the relevant model parameters.This theoretical framework can also be systematically extended to higherorders.展开更多
The Weyl double copy builds the relation between gauge theory and gravity theory, in particular the correspondence between gauge solutions and gravity solutions. In this paper, we obtain the slowly rotating charge sol...The Weyl double copy builds the relation between gauge theory and gravity theory, in particular the correspondence between gauge solutions and gravity solutions. In this paper, we obtain the slowly rotating charge solutions from the Weyl double copy for the Kerr black hole with small Chern-Simons correction. Based on the Weyl double copy relation, for the Petrov type D solution in Chern-Simons modified gravity, we find the additional correction to the electromagnetic field strength tensor of the rotating charge. For the Petrov type I solution, we find that the additional electromagnetic field strength tensors have external sources, while the total sources vanish at the leading order.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
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
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
In this study,we examine the effects of weak gravitational lensing and determine the shadow radius around black holes within the Dyonic ModMax(DM)spacetime,also accounting for models with nonuniform plasma distributio...In this study,we examine the effects of weak gravitational lensing and determine the shadow radius around black holes within the Dyonic ModMax(DM)spacetime,also accounting for models with nonuniform plasma distributions.By analyzing various gravitational lens models,we compare corrections to vacuum lensing due to gravitational effects within plasma and plasma inhomogeneity,finding that these effects could be observed in hot gas within galaxy clusters.Starting with the orbits of photons around a black hole in DM,we investigate the shadow and weak gravitational lensing phenomena.Utilizing observational data from the Event Horizon Telescope for M87*and SgrA*,we constrain parameters within DM gravity.To connect our findings to observations,we examine the magnification and positioning of lensed images,along with the weak deflection angle and magnification for sources near different galaxies.展开更多
F(R)gravity is a modified gravity theory,and its applications for the compact star have attracted attention in the last decades.We review the basics of the F(R)gravity theory and the modified Tolman-Oppenheimer-Volkof...F(R)gravity is a modified gravity theory,and its applications for the compact star have attracted attention in the last decades.We review the basics of the F(R)gravity theory and the modified Tolman-Oppenheimer-Volkoff(TOV)equation.Recent studies show that the model dependence of equation of state(EOS)and modification of gravity degenerate to each other,which suggests the mass-radius(M-R)relation of the compact star alone cannot completely determine the EOS of the inner matter.Moreover,the effects of a new scalar field predicted in F(R)gravity on both the internal and external structure of the compact star are illustrated in the benchmark R^(2)model.Finally,We discuss the future directions for testing gravitational theories by observational measurements of the compact stars.展开更多
In this paper,we present several explicit reconstructions for the aether scalar tensor(AeST)theory derived from the background of the Friedmann-Lemaître-Robertson-Walker cosmological evolution.It is shown that th...In this paper,we present several explicit reconstructions for the aether scalar tensor(AeST)theory derived from the background of the Friedmann-Lemaître-Robertson-Walker cosmological evolution.It is shown that the Einstein-Hilbert Lagrangian with a positive cosmological constant is the only Lagrangian capable of accurately replicating the exact expansion history of theΛcold dark matter(ΛCDM)universe filled solely with dust-like matter.However,theΛCDM-era can be produced within the framework of the AeST theory for some other fluids,including a perfect fluid with p=−(1/3)ρ,multifluids,and nonisentropic perfect fluids.Moreover,we demonstrate that theΛCDM-era can be replicated with no real matter field for the AeST theory.The cosmic evolution resulting from both the power-law and de-Sitter solutions can also be obtained.展开更多
In this paper,we construct a bounce inflation cosmological scenario in the framework of the modified symmetric teleparallel gravity,namely f(Q)theory,and investigate the tensor perturbations therein.As is well-known,t...In this paper,we construct a bounce inflation cosmological scenario in the framework of the modified symmetric teleparallel gravity,namely f(Q)theory,and investigate the tensor perturbations therein.As is well-known,the tensor perturbations generated in the very early Universe(inflation and pre-inflation regions)can account for the primordial gravitational waves(PGWs)that are to be detected by the next generation of GW experiments.We discuss the stability condition of the tensor perturbations in the bounce inflation process and investigate in detail the evolution of the perturbation variable.The general form of the tensor power spectrum is obtained both for large as well as small scale modes.As a result,we show both kinds of modes(short or long wavelength modes),and the tensor spectrum may get a positive tilt in the parametric range where the tensor perturbation proves to be stable—this interestingly hints an enhancement of gravitational waves’amplitude in the background of the f(Q)bounce-inflation scenario.Moreover,we study the LQC-like scenario as a specific case of our model,in which,the primordial tensor power spectrum turns out to be nearly scale-invariant on both small and large scales.展开更多
This paper investigates the physical properties and predicted radii of compact stars generated by the Tolman-IV complexity-free model within the background of modified gravity theory,particularly the f(R,T)-gravity th...This paper investigates the physical properties and predicted radii of compact stars generated by the Tolman-IV complexity-free model within the background of modified gravity theory,particularly the f(R,T)-gravity theory,under complexity formalism for a spherically symmetric spacetime proposed by L.Herrera[Phys Rev D 97:044010,2018].By solving the resulting set of differential equations,we obtain the explicit forms of the energy-momentum(EM)tensor components,including the density,radial pressure,and tangential pressure.The influence of the parameterχon various physical properties of the star is thoroughly investigated.The model undergoes a series of rigorous tests to determine its physical relevance.The findings indicate that the model exhibits regularity,stability,and a surface with vanishing pressure.The boundary of this surface is determined by carefully selecting the parameter space.The complexity method employed in f(R,T)gravity offers an interesting approach for developing astrophysical models that are consistent with observable events as demonstrated by recent experiments.In this regard,we use observational data from the GW190814 event,detected by the LIGO and Virgo observatories,to investigate the validity of the Tolman-IV model in f(R,T)gravity.The analysis includes comparing the model's predictions with the observed characteristics of the compact object involved in the merger.In addition,data from two-millisecond pulsars,PSR J1614-2230 and PSR J0952-0607,are incorporated to further constrain the theoretical theories.However,we present a diagram depicting the relationship between the total mass and radius of the compact object candidates for different values ofχ.展开更多
We explore the impact of the Sandage-Loeb （SL） test on the precision of cosmological constraints for f（T） gravity theories. The SL test is an important supplement to current cosmological observations because it me...We explore the impact of the Sandage-Loeb （SL） test on the precision of cosmological constraints for f（T） gravity theories. The SL test is an important supplement to current cosmological observations because it measures the redshift drift in the Lyman-α forest in the spectra of distant quasars, covering the ＂redshift desert＂ of 2 ≤ z ≤ 5. To avoid data inconsistency, we use the best-fit models based on current combined observational data as fiducial models to simulate 30 mock SL test data. We quantify the impact of these SL test data on parameter estimation for f（T） gravity theories. Two typical f（T） models are considered, the power-law model f（T）pL and the exponential-form model f（T）Exp. The results show that the SL test can effectively break the existing strong degeneracy between the present-day matter density Ωm and the Hubble constant Ho in other cosmological observations. For the considered f（T） models, a 30-year observation of the SL test can improve the constraint precision of Ωm and H0 enormously but cannot effectively improve the constraint precision of the model parameters.展开更多
f(Ricci)gravity is a special kind of higher curvature gravity whose bulk Lagrangian density is the trace of a matrix-valued function of the Ricci tensor.It is shown that under some mild constraints,f(Ricci)gravity adm...f(Ricci)gravity is a special kind of higher curvature gravity whose bulk Lagrangian density is the trace of a matrix-valued function of the Ricci tensor.It is shown that under some mild constraints,f(Ricci)gravity admits Einstein manifolds as exact vacuum solutions,and can be ghost-free and tachyon-free around maximally symmetric Einstein vacua.It is also shown that the entropy for spherically symmetric black holes in f(Ricci)gravity calculated via the Wald method and the boundary Noether charge approach are in good agreement.展开更多
Assuming a Winterberg model for space where the vacuum consists of a very stiff two-component superfluid made up of positive and negative mass planckions, Q theory is the hypothesis, that Planck charge, <i>q<...Assuming a Winterberg model for space where the vacuum consists of a very stiff two-component superfluid made up of positive and negative mass planckions, Q theory is the hypothesis, that Planck charge, <i>q<sub>pl</sub></i>, was created at the same time as Planck mass. Moreover, the repulsive force that like-mass planckions experience is, in reality, due to the electrostatic force of repulsion between like charges. These forces also give rise to what appears to be a gravitational force of attraction between two like planckions, but this is an illusion. In reality, gravity is electrostatic in origin if our model is correct. We determine the spring constant associated with planckion masses, and find that, <img src="Edit_770c2a48-039c-4cc9-8f66-406c0cfc565c.png" width="90" height="15" alt="" />, where <i>ζ</i>(3) equals Apery’s constant, 1.202 …, and, <i>n</i><sub>+</sub>(0)=<i>n</i>_(0), is the relaxed, <i>i.e.</i>, <img src="Edit_813d5a6f-b79a-49ba-bdf7-5042541b58a0.png" width="25" height="12" alt="" />, number density of the positive and negative mass planckions. In the present epoch, we estimate that, <i>n</i><sub>+</sub>(0) equals, 7.848E54 m<sup>-3</sup>, and the relaxed distance of separation between nearest neighbor positive, or negative, planckion pairs is, <i>l</i><sub>+</sub>(0)=<i>l</i><sub>_</sub>(0)=5.032E-19 meters. These values were determined using box quantization for the positive and negative mass planckions, and considering transitions between energy states, much like as in the hydrogen atom. For the cosmos as a whole, given a net smeared macroscopic gravitational field of, <img src="Edit_efc8003d-5297-4345-adac-4ac95536934d.png" width="80" height="15" alt="" />, due to all the ordinary, and bound, matter contained within the observable universe, an average displacement from equilibrium for the planckion masses is a mere 7.566E-48 meters, within the vacuum made up of these particles. On the surface of the earth, where, <i>g</i>=9.81m/s<sup>2</sup>, the displacement amounts to, 7.824E-38 met展开更多
In this study,the gravitational decoupling approach via extended geometric deformation is utilized to generate analytical black hole solutions owing to its simplicity and effectiveness.Considering the external fields ...In this study,the gravitational decoupling approach via extended geometric deformation is utilized to generate analytical black hole solutions owing to its simplicity and effectiveness.Considering the external fields surrounding Schwarzschild AdS black holes,we derive hairy black hole solutions in asymptotic AdS spacetime,satisfying the strong and dominant energy conditions.Moreover,we find that if the black hole spacetime is a fluid system,the fluid under each of these conditions is anisotropic.展开更多
The major goal of this work is to find solutions of Einstein–Maxwell field equations for anisotropic,expansion-free,non-static,spherically distributed matter content.The analytical models that highlight the major ben...The major goal of this work is to find solutions of Einstein–Maxwell field equations for anisotropic,expansion-free,non-static,spherically distributed matter content.The analytical models that highlight the major benefit of simplicity are shown and this makes it possible to use them as a toy model to illustrate how cavities evolve.Furthermore,the transport equations,quasi-homologous constraints and the junction conditions are also evaluated along with their useful implications.Eventually,the consequences of electric force on this system are summed up in the last section.展开更多
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 theNatural Science Foundation of China(11975020)。
文摘We investigate a class of gravity theories respecting only spatial covariance,termed spatially covariant gravity,in the presence of an auxiliary scalar field.We examine the conditions on the Lagrangian required to eliminate scalar degrees of freedom,allowing only two tensorial degrees of freedom to propagate.Instead of strict constraint analysis,in this paper,we employ the perturbation method and focus on the necessary conditions to evade the scalar mode at the linear order in perturbations around a cosmological background.Beginning with a general action and solving the auxiliary perturbation variables in terms of a would-be dynamical scalar mode,we derive the condition to remove its kinetic term,thus ensuring that no scalar mode propagates.As an application of the general condition,we study a polynomial-type Lagrangian as a concrete example,in which all monomials are spatially covariant scalars containing two derivatives.We find that the auxiliary scalar field is essential,and new terms in the Lagrangian are allowed.Our analysis provides insights into constructing gravity theories with two degrees of freedom in the extended framework of spatially covariant gravity.
基金supported by the National Natural Science Foundation of China under Grant No. 11675143the National Key Research and Development Program of China under Grant No. 2020YFC2201503the support from Research Grant F-FA-2021-432 of the Ministry of Higher Education, Science and Innovations of the Republic of Uzbekistan。
文摘We investigate the impact of the modified gravity(MOG)field and the quintessence scalar field on horizon evolution,black hole(BH)shadow and the weak gravitational lensing around a static spherically symmetric BH.We first begin to write the BH metric associated with the MOG parameter and quintessence scalar field.We then determine the BH shadow and obtain numerical solutions for the photon sphere and shadow radius.We show that the MOG(α)and the quintessence(c)parameters have a significant impact on the BH shadow and photon sphere.Based on the analysis,we further show that the combined effects of the MOG parameter and quintessence field can increase the values of BH shadow and photon sphere radii.We also obtain constraints on the BH parameters by applying the observational data of Sgr A^(★)and M87^(★).Finally,we consider the weak deflection angle of BH within the context of the Gauss-Bonnet theorem(GBT)and show that the combined effects of the MOG and quintessence parameters do make the value of the deflection angle increase,and find this remarkable property is in good agreement with the physical meaning of both parameters that can maintain the strong gravitational field in the surrounding environment of a BH.
基金Supported by National Key Research and Development Program of China (2021YFC2201901)and National Natural Science Foundation of China (12147103,11851302)。
文摘General relativity has been very successful since its proposal more than a century ago.However,various cosmological observations and theoretical consistency still motivate us to explore extended gravity theories.Horndeski gravity stands out as one attractive theory by introducing only one scalar field.Here we formulate the post-Newtonian effective field theory of Horndeski gravity and investigate the conservative dynamics of inspiral compact binary systems.We calculate the leading effective Lagrangian for a compact binary and obtain the periastron advance per period.In particular,we apply our analytical calculation to two binary systems,PSR B 1534+12 and PSR J0737-3039,and constrain the relevant model parameters.This theoretical framework can also be systematically extended to higherorders.
基金supported by the National Key Research and Development Program of China (No. 2023YFC2206200)the National Natural Science Foundation of China (No.12375059)the Fundamental Research Funds for the Central Universities。
文摘The Weyl double copy builds the relation between gauge theory and gravity theory, in particular the correspondence between gauge solutions and gravity solutions. In this paper, we obtain the slowly rotating charge solutions from the Weyl double copy for the Kerr black hole with small Chern-Simons correction. Based on the Weyl double copy relation, for the Petrov type D solution in Chern-Simons modified gravity, we find the additional correction to the electromagnetic field strength tensor of the rotating charge. For the Petrov type I solution, we find that the additional electromagnetic field strength tensors have external sources, while the total sources vanish at the leading order.
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.
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 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.
基金partly supported by Research Grants FZ-20200929344 and F-FA-2021-510 of the Uzbekistan Ministry for Innovative DevelopmentTUBITAKSCOAP3 for their support。
文摘In this study,we examine the effects of weak gravitational lensing and determine the shadow radius around black holes within the Dyonic ModMax(DM)spacetime,also accounting for models with nonuniform plasma distributions.By analyzing various gravitational lens models,we compare corrections to vacuum lensing due to gravitational effects within plasma and plasma inhomogeneity,finding that these effects could be observed in hot gas within galaxy clusters.Starting with the orbits of photons around a black hole in DM,we investigate the shadow and weak gravitational lensing phenomena.Utilizing observational data from the Event Horizon Telescope for M87*and SgrA*,we constrain parameters within DM gravity.To connect our findings to observations,we examine the magnification and positioning of lensed images,along with the weak deflection angle and magnification for sources near different galaxies.
文摘F(R)gravity is a modified gravity theory,and its applications for the compact star have attracted attention in the last decades.We review the basics of the F(R)gravity theory and the modified Tolman-Oppenheimer-Volkoff(TOV)equation.Recent studies show that the model dependence of equation of state(EOS)and modification of gravity degenerate to each other,which suggests the mass-radius(M-R)relation of the compact star alone cannot completely determine the EOS of the inner matter.Moreover,the effects of a new scalar field predicted in F(R)gravity on both the internal and external structure of the compact star are illustrated in the benchmark R^(2)model.Finally,We discuss the future directions for testing gravitational theories by observational measurements of the compact stars.
基金the National Natural Science Foundation of China(11975072,11875102,11835009)National SKA Program of China(2022SKA0110200,2022SKA0110203)+1 种基金Shaanxi Provincial Education Department(21JK0556)Shaanxi Provincial Department of Science and Technology(2023-JC-QN-0077)。
文摘In this paper,we present several explicit reconstructions for the aether scalar tensor(AeST)theory derived from the background of the Friedmann-Lemaître-Robertson-Walker cosmological evolution.It is shown that the Einstein-Hilbert Lagrangian with a positive cosmological constant is the only Lagrangian capable of accurately replicating the exact expansion history of theΛcold dark matter(ΛCDM)universe filled solely with dust-like matter.However,theΛCDM-era can be produced within the framework of the AeST theory for some other fluids,including a perfect fluid with p=−(1/3)ρ,multifluids,and nonisentropic perfect fluids.Moreover,we demonstrate that theΛCDM-era can be replicated with no real matter field for the AeST theory.The cosmic evolution resulting from both the power-law and de-Sitter solutions can also be obtained.
基金supported by the National Key Research and Development Program of China(Grant No.2021YFC2203100)the National Natrual Science Foundation of China(Grant No.11875141)。
文摘In this paper,we construct a bounce inflation cosmological scenario in the framework of the modified symmetric teleparallel gravity,namely f(Q)theory,and investigate the tensor perturbations therein.As is well-known,the tensor perturbations generated in the very early Universe(inflation and pre-inflation regions)can account for the primordial gravitational waves(PGWs)that are to be detected by the next generation of GW experiments.We discuss the stability condition of the tensor perturbations in the bounce inflation process and investigate in detail the evolution of the perturbation variable.The general form of the tensor power spectrum is obtained both for large as well as small scale modes.As a result,we show both kinds of modes(short or long wavelength modes),and the tensor spectrum may get a positive tilt in the parametric range where the tensor perturbation proves to be stable—this interestingly hints an enhancement of gravitational waves’amplitude in the background of the f(Q)bounce-inflation scenario.Moreover,we study the LQC-like scenario as a specific case of our model,in which,the primordial tensor power spectrum turns out to be nearly scale-invariant on both small and large scales.
基金supported via funding from Prince Sattam bin Abdulaziz University project number(PSAU/2024/R/1445).
文摘This paper investigates the physical properties and predicted radii of compact stars generated by the Tolman-IV complexity-free model within the background of modified gravity theory,particularly the f(R,T)-gravity theory,under complexity formalism for a spherically symmetric spacetime proposed by L.Herrera[Phys Rev D 97:044010,2018].By solving the resulting set of differential equations,we obtain the explicit forms of the energy-momentum(EM)tensor components,including the density,radial pressure,and tangential pressure.The influence of the parameterχon various physical properties of the star is thoroughly investigated.The model undergoes a series of rigorous tests to determine its physical relevance.The findings indicate that the model exhibits regularity,stability,and a surface with vanishing pressure.The boundary of this surface is determined by carefully selecting the parameter space.The complexity method employed in f(R,T)gravity offers an interesting approach for developing astrophysical models that are consistent with observable events as demonstrated by recent experiments.In this regard,we use observational data from the GW190814 event,detected by the LIGO and Virgo observatories,to investigate the validity of the Tolman-IV model in f(R,T)gravity.The analysis includes comparing the model's predictions with the observed characteristics of the compact object involved in the merger.In addition,data from two-millisecond pulsars,PSR J1614-2230 and PSR J0952-0607,are incorporated to further constrain the theoretical theories.However,we present a diagram depicting the relationship between the total mass and radius of the compact object candidates for different values ofχ.
基金This work was supported by the National Natural Science Foundation of China under Grant Nos. 11175042 and 11522540, the Provincial Department of Education of Liaon- ing under Grant No. L2012087, and the Fundamental Research Funds for the Central Universities under Grant Nos. N140505002, N140506002, and N140504007.
文摘We explore the impact of the Sandage-Loeb （SL） test on the precision of cosmological constraints for f（T） gravity theories. The SL test is an important supplement to current cosmological observations because it measures the redshift drift in the Lyman-α forest in the spectra of distant quasars, covering the ＂redshift desert＂ of 2 ≤ z ≤ 5. To avoid data inconsistency, we use the best-fit models based on current combined observational data as fiducial models to simulate 30 mock SL test data. We quantify the impact of these SL test data on parameter estimation for f（T） gravity theories. Two typical f（T） models are considered, the power-law model f（T）pL and the exponential-form model f（T）Exp. The results show that the SL test can effectively break the existing strong degeneracy between the present-day matter density Ωm and the Hubble constant Ho in other cosmological observations. For the considered f（T） models, a 30-year observation of the SL test can improve the constraint precision of Ωm and H0 enormously but cannot effectively improve the constraint precision of the model parameters.
基金Supported by the National Natural Science Foundation of China(11575088)
文摘f(Ricci)gravity is a special kind of higher curvature gravity whose bulk Lagrangian density is the trace of a matrix-valued function of the Ricci tensor.It is shown that under some mild constraints,f(Ricci)gravity admits Einstein manifolds as exact vacuum solutions,and can be ghost-free and tachyon-free around maximally symmetric Einstein vacua.It is also shown that the entropy for spherically symmetric black holes in f(Ricci)gravity calculated via the Wald method and the boundary Noether charge approach are in good agreement.
文摘Assuming a Winterberg model for space where the vacuum consists of a very stiff two-component superfluid made up of positive and negative mass planckions, Q theory is the hypothesis, that Planck charge, <i>q<sub>pl</sub></i>, was created at the same time as Planck mass. Moreover, the repulsive force that like-mass planckions experience is, in reality, due to the electrostatic force of repulsion between like charges. These forces also give rise to what appears to be a gravitational force of attraction between two like planckions, but this is an illusion. In reality, gravity is electrostatic in origin if our model is correct. We determine the spring constant associated with planckion masses, and find that, <img src="Edit_770c2a48-039c-4cc9-8f66-406c0cfc565c.png" width="90" height="15" alt="" />, where <i>ζ</i>(3) equals Apery’s constant, 1.202 …, and, <i>n</i><sub>+</sub>(0)=<i>n</i>_(0), is the relaxed, <i>i.e.</i>, <img src="Edit_813d5a6f-b79a-49ba-bdf7-5042541b58a0.png" width="25" height="12" alt="" />, number density of the positive and negative mass planckions. In the present epoch, we estimate that, <i>n</i><sub>+</sub>(0) equals, 7.848E54 m<sup>-3</sup>, and the relaxed distance of separation between nearest neighbor positive, or negative, planckion pairs is, <i>l</i><sub>+</sub>(0)=<i>l</i><sub>_</sub>(0)=5.032E-19 meters. These values were determined using box quantization for the positive and negative mass planckions, and considering transitions between energy states, much like as in the hydrogen atom. For the cosmos as a whole, given a net smeared macroscopic gravitational field of, <img src="Edit_efc8003d-5297-4345-adac-4ac95536934d.png" width="80" height="15" alt="" />, due to all the ordinary, and bound, matter contained within the observable universe, an average displacement from equilibrium for the planckion masses is a mere 7.566E-48 meters, within the vacuum made up of these particles. On the surface of the earth, where, <i>g</i>=9.81m/s<sup>2</sup>, the displacement amounts to, 7.824E-38 met
基金financial support from the Outstanding Young Teacher programme of Yangzhou University(137050368)。
文摘In this study,the gravitational decoupling approach via extended geometric deformation is utilized to generate analytical black hole solutions owing to its simplicity and effectiveness.Considering the external fields surrounding Schwarzschild AdS black holes,we derive hairy black hole solutions in asymptotic AdS spacetime,satisfying the strong and dominant energy conditions.Moreover,we find that if the black hole spacetime is a fluid system,the fluid under each of these conditions is anisotropic.
基金Researchers Supporting Project number:RSPD2023R650,King Saud University,Riyadh,Saudi Arabia。
文摘The major goal of this work is to find solutions of Einstein–Maxwell field equations for anisotropic,expansion-free,non-static,spherically distributed matter content.The analytical models that highlight the major benefit of simplicity are shown and this makes it possible to use them as a toy model to illustrate how cavities evolve.Furthermore,the transport equations,quasi-homologous constraints and the junction conditions are also evaluated along with their useful implications.Eventually,the consequences of electric force on this system are summed up in the last section.
基金Supported by the National Natural Science Foundation of China under Grant Nos. 10535060/A050207, 10975172, and 10821504Ministry of Science and Technology 973 Program under Grant No. 2007CB815401Fellowships from FQI and IPP
文摘We review the problem of dark energy,including a survey of theoretical models and some aspects ofnumerical studies.
