The modified cosmology like quintessence model with kination phase predicted the Hubble expansion rate of the universe before Big Bang Nucleosynthesis is different from the standard cosmological scenario. The modified...The modified cosmology like quintessence model with kination phase predicted the Hubble expansion rate of the universe before Big Bang Nucleosynthesis is different from the standard cosmological scenario. The modified expansion rate leaves its imprint on the relic density of asymmetric dark matter. In this work, we review the calculation of relic density of asymmetric WIMP dark matter in the standard cosmological scenario and quintessence model with kination phase. Then we use the Planck data to find constraints on the annihilation cross section and the mass of the asymmetric dark matter in those models.展开更多
To obtain analytically relativistic quintessence anisotropic spherical solutions in the f(T)paradigm is the primary objective of this paper.To do this,the pressure anisotropy condition is imposed,and we employ a metri...To obtain analytically relativistic quintessence anisotropic spherical solutions in the f(T)paradigm is the primary objective of this paper.To do this,the pressure anisotropy condition is imposed,and we employ a metric potential of the Tolman–Kuchowicz(TK)type.We also suppose that our current model incorporates a quintessence field characterized by a parameterωq,in addition to the anisotropic matter distribution.In the presence of the parameterα,the field equations are modified by the choice of the f(T)function.The f(T)gravity parameterαadds new components to the basic physical characteristics,such as density,pressure,subliminal sound velocity,surface redshift,etc,of the present model.By selecting the compact star Her X-1 and varyingαfrom 0.5 to 2.5,we examined all the physical characteristics of the model parameter of the configuration.The graphical process demonstrates that a more compact item is produced with greater values ofα.The hydrostatic equilibrium condition of the model is discussed,as well as the mass-radius relationship for our current model is obtained.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
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
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
A model is presented where the quintessence parameter, w, is related to a time-varying gravitational constant. Assuming a present value of w = -0.98 , we predict a current variation of ?/G = -0.06H0, a value within cu...A model is presented where the quintessence parameter, w, is related to a time-varying gravitational constant. Assuming a present value of w = -0.98 , we predict a current variation of ?/G = -0.06H0, a value within current observational bounds. H0 is Hubble’s parameter, G is Newton’s constant and ? is the derivative of G with respect to time. Thus, G has a cosmic origin, is decreasing with respect to cosmological time, and is proportional to H0, as originally proposed by the Dirac-Jordan hypothesis, albeit at a much slower rate. Within our model, we can explain the cosmological constant fine-tuning problem, the discrepancy between the present very weak value of the cosmological constant, and the much greater vacuum energy found in earlier epochs (we assume a connection exists). To formalize and solidify our model, we give two distinct parametrizations of G with respect to “a”, the cosmic scale parameter. We treat G-1 as an order parameter, which vanishes at high energies;at low temperatures, it reaches a saturation value, a value we are close to today. Our first parametrization for G-1 is motivated by a charging capacitor;the second treats G-1(a) by analogy to a magnetic response, i.e., as a Langevin function. Both parametrizations, even though very distinct, give a remarkably similar tracking behavior for w(a) , but not of the conventional form, w(a) = w0 + wa(1-a) , which can be thought of as only holding over a limited range in “a”. Interestingly, both parametrizations indicate the onset of G formation at a temperature of approximately 7×1021 degrees Kelvin, in contrast to the ΛCDM model where G is taken as a constant all the way back to the Planck temperature, 1.42×1032 degrees Kelvin. At the temperature of formation, we find that G has increased to roughly 4×1020 times its current value. For most of cosmic evolution, however, our variable G model gives results similar to the predictions of the ΛCDM model, except in the very early universe, as we shall demonstrate. In fact, in the limit where展开更多
We were interested, along this work, in the phenomena of the quintessence and the inflation due to the F-harmonic maps, in other words, in the functions of the scalar field such as the exponential and trigo-harmonic m...We were interested, along this work, in the phenomena of the quintessence and the inflation due to the F-harmonic maps, in other words, in the functions of the scalar field such as the exponential and trigo-harmonic maps. We showed that some F-harmonic map such as the trigonometric functions instead of the scalar field in the lagrangian, allow, in the absence of term of potential, reproduce the inflation. However, there are other F-harmonic maps such as exponential maps which can’t produce the inflation;the pressure and the density of this exponential harmonic field being both of the same sign. On the other hand, these exponential harmonic fields redraw well the phenomenon of the quintessence when the variation of these fields remains weak. The problem of coincidence, however remains.展开更多
Dark energy and dark matter are only indirectly measured though their gravitational effects.It is possibie that there is some direct,non-gravitational interaction between DE and DM,which can be used to solve(or,at lea...Dark energy and dark matter are only indirectly measured though their gravitational effects.It is possibie that there is some direct,non-gravitational interaction between DE and DM,which can be used to solve(or,at least alleviate)several important theoretical problems.In the present work,by analysing the cosmological dynamical system with a dark-sector interaction which changes its sign during the cosmological evolution,we find a scaling attractor to help to alleviate the cosmic-coincidence problem.This result shows that this interaction can bring new features to the cosmology.展开更多
In this study,we observe that,in the presence of the string cloud parameter a and the quintessence parameterγ,with the equation of state parameter,ω_(q) = -2/3the radius of the shadow of the Schwarzschild black hole...In this study,we observe that,in the presence of the string cloud parameter a and the quintessence parameterγ,with the equation of state parameter,ω_(q) = -2/3the radius of the shadow of the Schwarzschild black hole increases as compared with that in the pure Schwarzschild black hole case.The existence of both quintessential dark energy and the cloud of strings increases the shadow size;hence,the strength of the gravitational field around the Schwarzschild black hole increases.Using the data collected by the Event Horizon Telescope(EHT)collaboration for M87*and Sgr A*,we obtain upper bounds on the values of a andγ.Further,we see the effects of a andγon the rate of emission energy for the Schwarzschild black hole.We notice that the rate of emission energy is higher in the presence of clouds of strings and quintessence.Moreover,we study the weak deflection angle using the Gauss-Bonnet theorem.We show the influence of a andγon the weak deflection angle.We notice that both a andγincrease the deflection angleα.展开更多
文摘The modified cosmology like quintessence model with kination phase predicted the Hubble expansion rate of the universe before Big Bang Nucleosynthesis is different from the standard cosmological scenario. The modified expansion rate leaves its imprint on the relic density of asymmetric dark matter. In this work, we review the calculation of relic density of asymmetric WIMP dark matter in the standard cosmological scenario and quintessence model with kination phase. Then we use the Planck data to find constraints on the annihilation cross section and the mass of the asymmetric dark matter in those models.
基金National Research Foundation(NRF)of South Africa for the postdoctoral fellowship award。
文摘To obtain analytically relativistic quintessence anisotropic spherical solutions in the f(T)paradigm is the primary objective of this paper.To do this,the pressure anisotropy condition is imposed,and we employ a metric potential of the Tolman–Kuchowicz(TK)type.We also suppose that our current model incorporates a quintessence field characterized by a parameterωq,in addition to the anisotropic matter distribution.In the presence of the parameterα,the field equations are modified by the choice of the f(T)function.The f(T)gravity parameterαadds new components to the basic physical characteristics,such as density,pressure,subliminal sound velocity,surface redshift,etc,of the present model.By selecting the compact star Her X-1 and varyingαfrom 0.5 to 2.5,we examined all the physical characteristics of the model parameter of the configuration.The graphical process demonstrates that a more compact item is produced with greater values ofα.The hydrostatic equilibrium condition of the model is discussed,as well as the mass-radius relationship for our current model is obtained.
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 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A model is presented where the quintessence parameter, w, is related to a time-varying gravitational constant. Assuming a present value of w = -0.98 , we predict a current variation of ?/G = -0.06H0, a value within current observational bounds. H0 is Hubble’s parameter, G is Newton’s constant and ? is the derivative of G with respect to time. Thus, G has a cosmic origin, is decreasing with respect to cosmological time, and is proportional to H0, as originally proposed by the Dirac-Jordan hypothesis, albeit at a much slower rate. Within our model, we can explain the cosmological constant fine-tuning problem, the discrepancy between the present very weak value of the cosmological constant, and the much greater vacuum energy found in earlier epochs (we assume a connection exists). To formalize and solidify our model, we give two distinct parametrizations of G with respect to “a”, the cosmic scale parameter. We treat G-1 as an order parameter, which vanishes at high energies;at low temperatures, it reaches a saturation value, a value we are close to today. Our first parametrization for G-1 is motivated by a charging capacitor;the second treats G-1(a) by analogy to a magnetic response, i.e., as a Langevin function. Both parametrizations, even though very distinct, give a remarkably similar tracking behavior for w(a) , but not of the conventional form, w(a) = w0 + wa(1-a) , which can be thought of as only holding over a limited range in “a”. Interestingly, both parametrizations indicate the onset of G formation at a temperature of approximately 7×1021 degrees Kelvin, in contrast to the ΛCDM model where G is taken as a constant all the way back to the Planck temperature, 1.42×1032 degrees Kelvin. At the temperature of formation, we find that G has increased to roughly 4×1020 times its current value. For most of cosmic evolution, however, our variable G model gives results similar to the predictions of the ΛCDM model, except in the very early universe, as we shall demonstrate. In fact, in the limit where
文摘We were interested, along this work, in the phenomena of the quintessence and the inflation due to the F-harmonic maps, in other words, in the functions of the scalar field such as the exponential and trigo-harmonic maps. We showed that some F-harmonic map such as the trigonometric functions instead of the scalar field in the lagrangian, allow, in the absence of term of potential, reproduce the inflation. However, there are other F-harmonic maps such as exponential maps which can’t produce the inflation;the pressure and the density of this exponential harmonic field being both of the same sign. On the other hand, these exponential harmonic fields redraw well the phenomenon of the quintessence when the variation of these fields remains weak. The problem of coincidence, however remains.
基金supported by the National Natural Science Foundation of China(Grant Nos.11147017 and 11347605)the"Applied Nonlinear Science and Technology of Zhejiang Province(Grant No.zx2012000070)the Ministry of Education Doctoral Program Funds(Grant No.20126101110004)
文摘Dark energy and dark matter are only indirectly measured though their gravitational effects.It is possibie that there is some direct,non-gravitational interaction between DE and DM,which can be used to solve(or,at least alleviate)several important theoretical problems.In the present work,by analysing the cosmological dynamical system with a dark-sector interaction which changes its sign during the cosmological evolution,we find a scaling attractor to help to alleviate the cosmic-coincidence problem.This result shows that this interaction can bring new features to the cosmology.
基金the Grant No.ZC304022919 to support his Postdoctoral Fellowship at Zhejiang Normal Universitythe support of Inha University in Tashkent and research work has been supported by the Visitor Research Fellowship at Zhejiang Normal Universitysupported by Research Grant FZ-20200929344,F-FA-2021-510 and F-FA-2021-432 of the Uzbekistan Ministry for Innovative Development。
文摘In this study,we observe that,in the presence of the string cloud parameter a and the quintessence parameterγ,with the equation of state parameter,ω_(q) = -2/3the radius of the shadow of the Schwarzschild black hole increases as compared with that in the pure Schwarzschild black hole case.The existence of both quintessential dark energy and the cloud of strings increases the shadow size;hence,the strength of the gravitational field around the Schwarzschild black hole increases.Using the data collected by the Event Horizon Telescope(EHT)collaboration for M87*and Sgr A*,we obtain upper bounds on the values of a andγ.Further,we see the effects of a andγon the rate of emission energy for the Schwarzschild black hole.We notice that the rate of emission energy is higher in the presence of clouds of strings and quintessence.Moreover,we study the weak deflection angle using the Gauss-Bonnet theorem.We show the influence of a andγon the weak deflection angle.We notice that both a andγincrease the deflection angleα.
