This study examines the effect of charge on physical features of a gravastar model in the framework of Rastall gravity.A gravastar is an alternative model to a black hole consisting of three separate regions:the inner...This study examines the effect of charge on physical features of a gravastar model in the framework of Rastall gravity.A gravastar is an alternative model to a black hole consisting of three separate regions:the inner sector,the intermediate shell and the outer sector.Different values of the barotropic equation of state(EoS)parameter provide the mathematical basis for these regions.Field equations(FEs)are initially developed for a spherically symmetric spacetime coupled with charged matter distribution.We then use the temporal component of TolmanⅣspacetime to formulate the radial metric potential for both the inner region and intermediate shell.We also apply the matching criteria to ensure smooth matching of exterior and interior spacetimes so that the constants resulting from integrations can be determined.Afterwards,we explore various physical properties of the developed gravastar model such as the proper length,entropy,energy,and others to analyze how shell thickness and charge affect them.It is concluded that,in the background of Rastall theory,a gravastar model exists and serves as a viable alternative to the black hole.展开更多
This paper proposes an innovative form of group reduction or similarity transformation involving off-diagonal block matrices. The proposed method is applied to the Ablowitz-Kaup-Newell-Segur(AKNS) matrix spectral prob...This paper proposes an innovative form of group reduction or similarity transformation involving off-diagonal block matrices. The proposed method is applied to the Ablowitz-Kaup-Newell-Segur(AKNS) matrix spectral problem, leading to the generation of reduced matrix AKNS integrable hierarchies. As a result, a variety of reduced multiple-component integrable nonlinear Schr??dinger and modified Korteweg-de Vries models are derived from the analysis of the reduced AKNS matrix spectral problem.展开更多
Our analysis is particularly motivated by its relevance to understanding compact object instabilities,gravitational collapse thresholds,and the formation of dense structures under the influence of modified gravity the...Our analysis is particularly motivated by its relevance to understanding compact object instabilities,gravitational collapse thresholds,and the formation of dense structures under the influence of modified gravity theories.The interplay of anisotropic pressures,perturbative dynamics,and modified gravity contributions offers insight into both the stable configuration of dense fluids and the mechanisms leading to dynamical instability.Such considerations directly contribute to the aims of high energy density profiles,particularly in modeling physical systems where extreme pressure,curvature,and matter interactions co-exist.We consider an axially symmetric,dense structure with anisotropic matter content and employ a specific equation of state(EoS)to examine the interplay between static and dynamic quantities via the adiabatic index.To address the complex dynamics of the collapse process,a perturbative scheme is utilized under Newtonian and post-Newtonian approximations,enabling a detailed examination of the stability and structural evolution of the system under the influence of the considered minimally coupled gravity.Our results demonstrate that hydrostatic equilibrium is maintained when effective pressure,gravitational,and anti-gravitational forces are balanced,while deviations from this balance initiate dynamical instability.Graphical representations of stable and unstable regimes are presented,revealing how the choice of gravity functions significantly affects the outcome.This work provides insight into the behavior of dense,self-gravitating configurations under modified gravity,offering broader implications for the modeling of compact astrophysical objects and contributing to the understanding of gravitational collapse in energy density regimes.展开更多
In weak field limits,we compute the deflection angle of a gravitational decoupling extended black hole(BH)solution.We obtained the Gaussian optical curvature by examining the null geodesic equations with the help of G...In weak field limits,we compute the deflection angle of a gravitational decoupling extended black hole(BH)solution.We obtained the Gaussian optical curvature by examining the null geodesic equations with the help of Gauss-Bonnet theorem(GBT).We also looked into the deflection angle of light by a black hole in weak field limits with the use of the Gibbons-Werner method.We verify the graphical behavior of the black hole after determining the deflection angle of light.Additionally,in the presence of the plasma medium,we also determine the deflection angle of the light and examine its graphical behavior.Furthermore,we compute the Einstein ring via gravitational decoupling extended black hole solution.We also compute the quasi-periodic oscillations and discuss their graphical behavior.展开更多
Verlinde's emergent gravity(VEG)posits that gravity arises as an emergent phenomenon rooted in the entropic properties of spacetime,challenging the traditional view of gravity as a fundamental force.Building on th...Verlinde's emergent gravity(VEG)posits that gravity arises as an emergent phenomenon rooted in the entropic properties of spacetime,challenging the traditional view of gravity as a fundamental force.Building on this paradigm,recent developments have introduced a novel class of black holes within the VEG framework,revealing intriguing connections between apparent dark matter effects and the distribution of baryonic matter.In this study,we delve into the observational signatures of a Simpson–Visser(SV)Minkowski core regular black hole in VEG,focusing on its shadow images and intensity profiles.Our analysis highlights the profound influence of model parameters,including A(governing baryonic matter distribution),B(strength of interaction between apparent dark matter and baryonic matter),and n(characterizing diverse spacetime geometries),on the effective potential and observable properties.Notably,we find that the modifications introduced by these parameters lead to distinct changes in the black hole's shadow size and intensity distribution.Comparing our results to the Reissner–Nordström(RN)black hole,we uncover a striking reduction in the apparent shadow size and an enhancement in intensity for the SV solution in VEG.展开更多
The current sheath velocity in 0.25 Torr gas pressure of Filippov type plasma focus is studied experimentally. By using two tridimensional magnetic probes on top of the anode surface, the current sheath velocity is me...The current sheath velocity in 0.25 Torr gas pressure of Filippov type plasma focus is studied experimentally. By using two tridimensional magnetic probes on top of the anode surface, the current sheath velocity is measured for argon, oxygen and nitrogen. Additionally, the effect of charging voltage on the current sheath velocity is studied in both axial and radial phases. We found that, the maximum current sheath velocities at both radial and axial phases are respectively 4.33 ± 0.28 （cm/μs） and 3.92 ± 0.75 （cm/μs） with argon as the working gas at 17 kV. Also, the minimum values of current sheath velocity are 1.48 ± 0.15 （cm/μs） at the radial phase and 1.14 ± 0.09 （cm/μs） at the axial phase with oxygen at 12 kV. The current sheath velocity at the radial phase is higher than that at the axial phase for all gases and voltages. In this study, variation of the full width half maximum （FWHM） of magnetic probe signals with voltage is investigated for different gases at radial and axial phases.展开更多
We consider the recently developed black hole in massive Einstein-dilaton gravity including the coupling of the dilaton scalar field to massive graviton terms.This model has different horizon structures such as event ...We consider the recently developed black hole in massive Einstein-dilaton gravity including the coupling of the dilaton scalar field to massive graviton terms.This model has different horizon structures such as event horizons and inner horizons depending on the values of certain parameters.These variations influence how the black hole interacts with its surroundings.We utilize the well-known Novikov-Thorne model to investigate the thin accretion disks into this interesting model.Our research shows a crucial correlation between the dynamics of the accretion disk and the parameters of dilatonic black holes in dilaton-massive gravity.We observe that dilaton-massive gravity leads to significant contraction and outward expansion.We offer a detailed analysis of accretion by examining both direct and secondary images at various radial distances and observation angles.展开更多
In this paper,we investigate the thermodynamic properties and geometries of phantom(A)dS F(R)black holes in the presence of recently proposed generalized entropy.For this purpose,we evaluate the heat capacity,Helmholt...In this paper,we investigate the thermodynamic properties and geometries of phantom(A)dS F(R)black holes in the presence of recently proposed generalized entropy.For this purpose,we evaluate the heat capacity,Helmholtz free energy,Gibbs free energy,compressibility factor Z and the isothermal compressibility(κT)to analyze the local and global stability of this black hole.We also analyze the thermal geometries of this black hole,which provide useful insights.Furthermore,we present isotherms on the P-V diagram,illustrating phase behavior.It is interesting to mention here that generalized entropy provides a significant and useful impact on the results of thermal analysis of the aforementioned black holes.展开更多
In this study,we explore the recently proposed f(R,A)gravity,a novel extension of modified gravity theories,where R represents the Ricci scalar and A denotes the anticurvature scalar.The addition of anticurvature term...In this study,we explore the recently proposed f(R,A)gravity,a novel extension of modified gravity theories,where R represents the Ricci scalar and A denotes the anticurvature scalar.The addition of anticurvature term might play a role in softening or avoiding singularities,offering smoother transitions in regions of high curvature,such as near black holes or during cosmic inflation(Amendol et al 2020 Phys.Lett.B 811135923).We also incorporate well-established cosmological bouncing scenarios,including the symmetric bounce,oscillatory bounce,matter bounce,little rip,and super bounce,to examine their implications within the framework of underlying gravity theory.In this context,we investigate some cosmic and thermodynamic aspects of flat FRW universe.We analyze the effective equation of state parameterω_(eff),which exhibits a transition from quintessence to phantom regimes across different bouncing cosmologies.The stability of the models is examined through the squared speed of sound parameterv_(s)^(2),confirming stability in certain bouncing scenarios.The validity of the generalized second law of thermodynamics is verified by ensuring the positivity of total entropy productionS_(tot).Additionally,theeff eff plane exhibits the thawing as well as the freezing regions.Our findings demonstrate that the chosen bouncing models provide viable cosmic and thermodynamic behavior,supporting their relevance in modified gravity theories.展开更多
As a dedicated solar radioheliograph,the MingantU SpEctral RadioHeliograph(MUSER)has a maximum baseline of more than 3000 m and a frequency range of 400 MHz–15 GHz.According to the classical radio interferometry theo...As a dedicated solar radioheliograph,the MingantU SpEctral RadioHeliograph(MUSER)has a maximum baseline of more than 3000 m and a frequency range of 400 MHz–15 GHz.According to the classical radio interferometry theory,the non-coplanar baseline effect(i.e.,w-term effect)would be considered and calibrated for such a radio instrument.However,little previous literature made the qualitative or quantitative analyses on w-term effects of solar radioheliograph in-depth.This study proposes a complete quantitative analysis of w-term effects for the MUSER.After a brief introduction of the MUSER,we systematically investigate the baseline variations over a year and analyze the corresponding variations of w-term.We further studied the effects of the w-term in the imaging for the specified extended source,i.e.,the Sun.We discussed the possible effects of the w-term,such as image distortion and so on.The simulated results show that the w-term is an essential and unavoidable issue for solar radio imaging with high spatial resolution.展开更多
This paper explores the dynamical feature of Hayward-Letelier black holes in AdS spacetime,emphasizing the effects of the Hayward parameter g,mass M,cosmological constant L,and modification parameterαon their geometr...This paper explores the dynamical feature of Hayward-Letelier black holes in AdS spacetime,emphasizing the effects of the Hayward parameter g,mass M,cosmological constant L,and modification parameterαon their geometry,thermodynamics,and observational features.By utilizing an effective potential method,we investigate the paths of particles,innermost stable circular orbit,and behavior of photon spheres,which connects them to the appearance of black hole shadows.Thermodynamic features such as Hawking temperature and entropy are studied for investigating the effect of L and thermal fluctuations on the stability of black holes.These discoveries connect theoretical ideas with observational astrophysics,which enhances our comprehension of ordinary black holes in AdS models.In this study,we analytically compute the greybody factor for a massless scalar field propagating in the vicinity of a black hole under the assumption of weak coupling to gravity.We investigate the behavior of the effective potential concerning the black hole's mass and charge,revealing that it reaches its maximum at lower values of the cloud of strings parameter.Our results indicate that the radial absorption rate of the scalar field exhibits significant fluctuations,which is influenced by the charge of the black hole and clouds of string,with implications for the dynamics of scalar fields in strong gravitational fields.展开更多
We investigate the optical properties of the spacetime surrounding a Sen black hole,focusing on the black hole shadow,weak gravitational lensing,and time delay effects.Our analysis reveals that the effective charge of...We investigate the optical properties of the spacetime surrounding a Sen black hole,focusing on the black hole shadow,weak gravitational lensing,and time delay effects.Our analysis reveals that the effective charge of the Sen black hole significantly influences these phenomena.Specifically,an increase in the effective charge leads to a decrease in the radius of the photon sphere and a corresponding decrease in the size of the black hole shadow.Additionally,the bending angle of light rays diminishes as the effective charge increases.Our study provides observational bounds on the effective charge based on these optical characteristics.We also examine the magnification of source brightness using the lens equation and analyze the time delay of light in the presence of a surrounding plasma medium.Our findings offer new insights into the impact of effective charge and plasma on the observational signatures of Sen black holes.展开更多
The integrated spectrum of discrete X-ray sources (mainly the active galactic nuclei, AGN) is inconsistent with the observed spectrum of cosmic X-ray background (CXB), and it is so called CXB spectral paradox. The med...The integrated spectrum of discrete X-ray sources (mainly the active galactic nuclei, AGN) is inconsistent with the observed spectrum of cosmic X-ray background (CXB), and it is so called CXB spectral paradox. The medium X-ray spectra of 68 AGNs are adopted, the evolution function of X-ray spectral indices is analyzed statistically, the fraction of CXB is calculated due to AGNs X-ray emission, which shows that almost 100% CXB comes from AGNs X-ray emission. Especially, the integrated spectrum in 2-10 keV is consistent with the observed spectrum of CXB. The spectral paradox of CXB can be interpreted by this result.展开更多
We investigated a time-varying cosmological constant model using recent BAO measurements from DESI DR2,combined with Type Ia supernova samples(Pantheon^(+),DES-Dovekie,and Union3)and CMB shift parameters,to constrain ...We investigated a time-varying cosmological constant model using recent BAO measurements from DESI DR2,combined with Type Ia supernova samples(Pantheon^(+),DES-Dovekie,and Union3)and CMB shift parameters,to constrain the Λ(t)CDM model parameters via Markov Chain Monte Carlo analysis.We find that the interaction term Q(z)shows a sign change for all dataset combinations by crossing Q(z)=0,depending on the choice of the dataset:at low redshift,Q(z)<0,indicating vacuum energy decaying into dark matter,while at high redshift,Q(z)>O,corresponding to dark matter decaying into vacuum energy.The dynamical system analysis found three critical points,namely,P1,P2,and P3.The resulting critical points,determined by the underlying cosmological parameters,correspond to distinct epochs in cosmic evolution.Depending on the parameter combinations,these points characterize various cosmological phases,ranging from an accelerated stiff matter-dominated era to late-time accelerated expansion.The stability of each critical point is analyzed using linear stability theory,with the relevant physical constraints on the cosmological parameters duly incorporated throughout the analysis.For each dataset combinations,the Λ(t)CDM model predicts that ω_(0)>-1,showing a preference for dynamical dark energy over the cosmological constant scenario withω0=-1.Consequently,the model exhibits a transition phase in the range N≡loga(t)≈-0.51 to−0.48 and predicts q0 in the range−0.54 to−0.52,with the precise transition point depending on the choice of dataset.Finally,the Bayesian evidence shows strong support for the Λ(t)CDM model over ACDM.展开更多
Theories on the nucleosynthesis in the very early universe show that the baryonic matter is just a fraction of the cosmic matter. Moreover, the flatness of the space geometry of the universe shows that the cosmic dens...Theories on the nucleosynthesis in the very early universe show that the baryonic matter is just a fraction of the cosmic matter. Moreover, the flatness of the space geometry of the universe shows that the cosmic density factor Ω= 1, this means that most of the cosmic matter exists in the form of dark matter. Confirmed by experiments up to the present, neutrino展开更多
文摘This study examines the effect of charge on physical features of a gravastar model in the framework of Rastall gravity.A gravastar is an alternative model to a black hole consisting of three separate regions:the inner sector,the intermediate shell and the outer sector.Different values of the barotropic equation of state(EoS)parameter provide the mathematical basis for these regions.Field equations(FEs)are initially developed for a spherically symmetric spacetime coupled with charged matter distribution.We then use the temporal component of TolmanⅣspacetime to formulate the radial metric potential for both the inner region and intermediate shell.We also apply the matching criteria to ensure smooth matching of exterior and interior spacetimes so that the constants resulting from integrations can be determined.Afterwards,we explore various physical properties of the developed gravastar model such as the proper length,entropy,energy,and others to analyze how shell thickness and charge affect them.It is concluded that,in the background of Rastall theory,a gravastar model exists and serves as a viable alternative to the black hole.
基金supported in part by the Ministry of Science and Technology of China (G2021016032L and G2023016011L)the National Natural Science Foundation of China (12271488 and 11975145)。
文摘This paper proposes an innovative form of group reduction or similarity transformation involving off-diagonal block matrices. The proposed method is applied to the Ablowitz-Kaup-Newell-Segur(AKNS) matrix spectral problem, leading to the generation of reduced matrix AKNS integrable hierarchies. As a result, a variety of reduced multiple-component integrable nonlinear Schr??dinger and modified Korteweg-de Vries models are derived from the analysis of the reduced AKNS matrix spectral problem.
文摘Our analysis is particularly motivated by its relevance to understanding compact object instabilities,gravitational collapse thresholds,and the formation of dense structures under the influence of modified gravity theories.The interplay of anisotropic pressures,perturbative dynamics,and modified gravity contributions offers insight into both the stable configuration of dense fluids and the mechanisms leading to dynamical instability.Such considerations directly contribute to the aims of high energy density profiles,particularly in modeling physical systems where extreme pressure,curvature,and matter interactions co-exist.We consider an axially symmetric,dense structure with anisotropic matter content and employ a specific equation of state(EoS)to examine the interplay between static and dynamic quantities via the adiabatic index.To address the complex dynamics of the collapse process,a perturbative scheme is utilized under Newtonian and post-Newtonian approximations,enabling a detailed examination of the stability and structural evolution of the system under the influence of the considered minimally coupled gravity.Our results demonstrate that hydrostatic equilibrium is maintained when effective pressure,gravitational,and anti-gravitational forces are balanced,while deviations from this balance initiate dynamical instability.Graphical representations of stable and unstable regimes are presented,revealing how the choice of gravity functions significantly affects the outcome.This work provides insight into the behavior of dense,self-gravitating configurations under modified gravity,offering broader implications for the modeling of compact astrophysical objects and contributing to the understanding of gravitational collapse in energy density regimes.
基金funded by the National Natural Science Foundation of China under Grant No.11975145。
文摘In weak field limits,we compute the deflection angle of a gravitational decoupling extended black hole(BH)solution.We obtained the Gaussian optical curvature by examining the null geodesic equations with the help of Gauss-Bonnet theorem(GBT).We also looked into the deflection angle of light by a black hole in weak field limits with the use of the Gibbons-Werner method.We verify the graphical behavior of the black hole after determining the deflection angle of light.Additionally,in the presence of the plasma medium,we also determine the deflection angle of the light and examine its graphical behavior.Furthermore,we compute the Einstein ring via gravitational decoupling extended black hole solution.We also compute the quasi-periodic oscillations and discuss their graphical behavior.
基金the Deanship of Research and Graduate Studies at King Khalid University for funding this work through Large Research Project under grant number RGP2/333/46。
文摘Verlinde's emergent gravity(VEG)posits that gravity arises as an emergent phenomenon rooted in the entropic properties of spacetime,challenging the traditional view of gravity as a fundamental force.Building on this paradigm,recent developments have introduced a novel class of black holes within the VEG framework,revealing intriguing connections between apparent dark matter effects and the distribution of baryonic matter.In this study,we delve into the observational signatures of a Simpson–Visser(SV)Minkowski core regular black hole in VEG,focusing on its shadow images and intensity profiles.Our analysis highlights the profound influence of model parameters,including A(governing baryonic matter distribution),B(strength of interaction between apparent dark matter and baryonic matter),and n(characterizing diverse spacetime geometries),on the effective potential and observable properties.Notably,we find that the modifications introduced by these parameters lead to distinct changes in the black hole's shadow size and intensity distribution.Comparing our results to the Reissner–Nordström(RN)black hole,we uncover a striking reduction in the apparent shadow size and an enhancement in intensity for the SV solution in VEG.
文摘The current sheath velocity in 0.25 Torr gas pressure of Filippov type plasma focus is studied experimentally. By using two tridimensional magnetic probes on top of the anode surface, the current sheath velocity is measured for argon, oxygen and nitrogen. Additionally, the effect of charging voltage on the current sheath velocity is studied in both axial and radial phases. We found that, the maximum current sheath velocities at both radial and axial phases are respectively 4.33 ± 0.28 （cm/μs） and 3.92 ± 0.75 （cm/μs） with argon as the working gas at 17 kV. Also, the minimum values of current sheath velocity are 1.48 ± 0.15 （cm/μs） at the radial phase and 1.14 ± 0.09 （cm/μs） at the axial phase with oxygen at 12 kV. The current sheath velocity at the radial phase is higher than that at the axial phase for all gases and voltages. In this study, variation of the full width half maximum （FWHM） of magnetic probe signals with voltage is investigated for different gases at radial and axial phases.
基金The authors extend their appreciation to the Deanship of Scientific Research at Northern Border University,Arar,KSA for funding this research work through the project number NBU-FFR-2025-1102-06.
文摘We consider the recently developed black hole in massive Einstein-dilaton gravity including the coupling of the dilaton scalar field to massive graviton terms.This model has different horizon structures such as event horizons and inner horizons depending on the values of certain parameters.These variations influence how the black hole interacts with its surroundings.We utilize the well-known Novikov-Thorne model to investigate the thin accretion disks into this interesting model.Our research shows a crucial correlation between the dynamics of the accretion disk and the parameters of dilatonic black holes in dilaton-massive gravity.We observe that dilaton-massive gravity leads to significant contraction and outward expansion.We offer a detailed analysis of accretion by examining both direct and secondary images at various radial distances and observation angles.
文摘In this paper,we investigate the thermodynamic properties and geometries of phantom(A)dS F(R)black holes in the presence of recently proposed generalized entropy.For this purpose,we evaluate the heat capacity,Helmholtz free energy,Gibbs free energy,compressibility factor Z and the isothermal compressibility(κT)to analyze the local and global stability of this black hole.We also analyze the thermal geometries of this black hole,which provide useful insights.Furthermore,we present isotherms on the P-V diagram,illustrating phase behavior.It is interesting to mention here that generalized entropy provides a significant and useful impact on the results of thermal analysis of the aforementioned black holes.
基金funded by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan(Grant No.AP23483654).
文摘In this study,we explore the recently proposed f(R,A)gravity,a novel extension of modified gravity theories,where R represents the Ricci scalar and A denotes the anticurvature scalar.The addition of anticurvature term might play a role in softening or avoiding singularities,offering smoother transitions in regions of high curvature,such as near black holes or during cosmic inflation(Amendol et al 2020 Phys.Lett.B 811135923).We also incorporate well-established cosmological bouncing scenarios,including the symmetric bounce,oscillatory bounce,matter bounce,little rip,and super bounce,to examine their implications within the framework of underlying gravity theory.In this context,we investigate some cosmic and thermodynamic aspects of flat FRW universe.We analyze the effective equation of state parameterω_(eff),which exhibits a transition from quintessence to phantom regimes across different bouncing cosmologies.The stability of the models is examined through the squared speed of sound parameterv_(s)^(2),confirming stability in certain bouncing scenarios.The validity of the generalized second law of thermodynamics is verified by ensuring the positivity of total entropy productionS_(tot).Additionally,theeff eff plane exhibits the thawing as well as the freezing regions.Our findings demonstrate that the chosen bouncing models provide viable cosmic and thermodynamic behavior,supporting their relevance in modified gravity theories.
基金supported by the National SKA Program of China(2020SKA0110300)the Joint Research Fund in Astronomy(U1831204 and U1931141)+3 种基金the National Natural Science Foundation of China(NSFC)the Chinese Academy of Sciences(CAS)the Funds for International Cooperation and Exchange of the NSFC(11961141001)the NSFC(Grant No.11903009)。
文摘As a dedicated solar radioheliograph,the MingantU SpEctral RadioHeliograph(MUSER)has a maximum baseline of more than 3000 m and a frequency range of 400 MHz–15 GHz.According to the classical radio interferometry theory,the non-coplanar baseline effect(i.e.,w-term effect)would be considered and calibrated for such a radio instrument.However,little previous literature made the qualitative or quantitative analyses on w-term effects of solar radioheliograph in-depth.This study proposes a complete quantitative analysis of w-term effects for the MUSER.After a brief introduction of the MUSER,we systematically investigate the baseline variations over a year and analyze the corresponding variations of w-term.We further studied the effects of the w-term in the imaging for the specified extended source,i.e.,the Sun.We discussed the possible effects of the w-term,such as image distortion and so on.The simulated results show that the w-term is an essential and unavoidable issue for solar radio imaging with high spatial resolution.
基金Supported by the Ongoing Research Funding Program,(ORF-2025-650),King Saud University,Riyadh,Saudi Arabia.
文摘This paper explores the dynamical feature of Hayward-Letelier black holes in AdS spacetime,emphasizing the effects of the Hayward parameter g,mass M,cosmological constant L,and modification parameterαon their geometry,thermodynamics,and observational features.By utilizing an effective potential method,we investigate the paths of particles,innermost stable circular orbit,and behavior of photon spheres,which connects them to the appearance of black hole shadows.Thermodynamic features such as Hawking temperature and entropy are studied for investigating the effect of L and thermal fluctuations on the stability of black holes.These discoveries connect theoretical ideas with observational astrophysics,which enhances our comprehension of ordinary black holes in AdS models.In this study,we analytically compute the greybody factor for a massless scalar field propagating in the vicinity of a black hole under the assumption of weak coupling to gravity.We investigate the behavior of the effective potential concerning the black hole's mass and charge,revealing that it reaches its maximum at lower values of the cloud of strings parameter.Our results indicate that the radial absorption rate of the scalar field exhibits significant fluctuations,which is influenced by the charge of the black hole and clouds of string,with implications for the dynamics of scalar fields in strong gravitational fields.
基金partly supported by Research(FZ-20200929344,F-FA-2021-510)of the Uzbekistan Ministry for Innovative DevelopmentSERB-DST for project(CRG/2021/005771)。
文摘We investigate the optical properties of the spacetime surrounding a Sen black hole,focusing on the black hole shadow,weak gravitational lensing,and time delay effects.Our analysis reveals that the effective charge of the Sen black hole significantly influences these phenomena.Specifically,an increase in the effective charge leads to a decrease in the radius of the photon sphere and a corresponding decrease in the size of the black hole shadow.Additionally,the bending angle of light rays diminishes as the effective charge increases.Our study provides observational bounds on the effective charge based on these optical characteristics.We also examine the magnification of source brightness using the lens equation and analyze the time delay of light in the presence of a surrounding plasma medium.Our findings offer new insights into the impact of effective charge and plasma on the observational signatures of Sen black holes.
文摘The integrated spectrum of discrete X-ray sources (mainly the active galactic nuclei, AGN) is inconsistent with the observed spectrum of cosmic X-ray background (CXB), and it is so called CXB spectral paradox. The medium X-ray spectra of 68 AGNs are adopted, the evolution function of X-ray spectral indices is analyzed statistically, the fraction of CXB is calculated due to AGNs X-ray emission, which shows that almost 100% CXB comes from AGNs X-ray emission. Especially, the integrated spectrum in 2-10 keV is consistent with the observed spectrum of CXB. The spectral paradox of CXB can be interpreted by this result.
基金Supported by UGC,Govt.of India,for providing Senior Research Fellowship(NTA Ref.No.211610083890)。
文摘We investigated a time-varying cosmological constant model using recent BAO measurements from DESI DR2,combined with Type Ia supernova samples(Pantheon^(+),DES-Dovekie,and Union3)and CMB shift parameters,to constrain the Λ(t)CDM model parameters via Markov Chain Monte Carlo analysis.We find that the interaction term Q(z)shows a sign change for all dataset combinations by crossing Q(z)=0,depending on the choice of the dataset:at low redshift,Q(z)<0,indicating vacuum energy decaying into dark matter,while at high redshift,Q(z)>O,corresponding to dark matter decaying into vacuum energy.The dynamical system analysis found three critical points,namely,P1,P2,and P3.The resulting critical points,determined by the underlying cosmological parameters,correspond to distinct epochs in cosmic evolution.Depending on the parameter combinations,these points characterize various cosmological phases,ranging from an accelerated stiff matter-dominated era to late-time accelerated expansion.The stability of each critical point is analyzed using linear stability theory,with the relevant physical constraints on the cosmological parameters duly incorporated throughout the analysis.For each dataset combinations,the Λ(t)CDM model predicts that ω_(0)>-1,showing a preference for dynamical dark energy over the cosmological constant scenario withω0=-1.Consequently,the model exhibits a transition phase in the range N≡loga(t)≈-0.51 to−0.48 and predicts q0 in the range−0.54 to−0.52,with the precise transition point depending on the choice of dataset.Finally,the Bayesian evidence shows strong support for the Λ(t)CDM model over ACDM.
基金Project supported by the National Natural Science Foundation of China.
文摘Theories on the nucleosynthesis in the very early universe show that the baryonic matter is just a fraction of the cosmic matter. Moreover, the flatness of the space geometry of the universe shows that the cosmic density factor Ω= 1, this means that most of the cosmic matter exists in the form of dark matter. Confirmed by experiments up to the present, neutrino
