Particle dynamics,black hole shadow and weak gravitational lensing in the f(Q) theory of gravity

We study the particle dynamics around a black hole(BH) in f(Q) gravity. First, we investigate the influence of the parameters of f(Q) gravity on the horizon structure of the BH, photon orbits and the radius of the innermost stable circular orbit(ISCO) of massive particles. We further study the effects of the parameters of f(Q) gravity on the shadow cast by the BH. Moreover, we consider weak gravitational lensing using the general method, where we also explore the deflection angle of light rays a...     

The Buchdahl stability bound in Eddington-inspired Born-Infeld gravity

We give the Buchdahl stability bound in Eddington-inspired Born-Infeld(EiBI)gravity.We show that this bound depends on an energy condition controlled by the model parameter K.From this bound,we can constrain κ≤10~8 m~2 if a neutron star with a mass around 3 M is observed in the future.In addition,to avoid the potential pathologies in EiBI,a Hagedorn-like equation of state associated with κ at the center of a compact star is inevitable,which is similar to the Hagedorn temperature in string theory.     

Thin accretion disk around a four-dimensional Einstein- Gauss-Bonnet black hole

Recently,a novel four-dimensional Einstein-Gauss-Bonnet(4 EGB) theory of gravity was proposed by Glavan and Lin [D.Glavan and C.Lin,Phys.Rev.Lett.124,081301(2020)],which includes a regularized GaussBonnet term using the re-scalaring of the Gauss-Bonnet coupling constant α→α/(D-4) in the limit D→4.This theory has also been reformulated to a specific class of the Homdeski theory with an additional scalar degree of freedom and to a spatial covariant version with a Lagrangian multiplier,which can eliminate the scalar mode.Here,we study the physical properties of the electromagnetic radiation emitted from a thin accretion disk around a static spherically symmetric black hole in 4 EGB gravity.For this purpose,we assume the disk is in a steady-state and in hydrodynamic and thermodynamic equilibrium,so that the emitted electromagnetic radiation is a black body spectrum.We study in detail the effects of the Gauss-Bonnet coupling constant α in 4 EGB gravity on the energy flux,temperature distribution,and electromagnetic spectrum of the disk.With an increase in the parameter α,the energy flux,temperature distribution,and electromagnetic spectrum of the accretion disk all increase.We also show that the accretion efficiency increases with the growth of the parameter α.Our results indicate that the thin accretion disk around a static spherically symmetric black hole in 4 EGB gravity is hotter,more luminous,and more efficient than that around a Schwarzschild black hole with the same mass for positive α,while it is cooler,less luminous,and less efficient for negative α.     

P-V criticality and Joule-Thomson expansion of charged AdS black holes in the Rastall gravity

We discuss the P-V criticality and the Joule-Thomson expansion of charged AdS black holes in the Rastall gravity. We find that although the equation-of-state of a charged AdS black hole in the Rastall gravity is related to the Rastall parameter λ, its reduced equation-of-state at the critical point is independent of the Rastall parameter λ, as is the case in the Einstein gravity where λ=0. This is the reason why the critical exponents are not related to the Rastall parameter λ. We also find that the inversion temperature T_i is related to the Rastall parameter λ,but that the minimum inversion temperature T_i~(min) and the ratio ε between the minimum inversion temperature and the critical temperature are both independent of the Rastall parameter λ. At the critical point, the thermodynamic evolution of a charged AdS black hole in the Rastall gravity behaves as in the van der Waals fluid and charged AdS black hole in the Einstein gravity. We show the inversion curves and isenthalpic curves in the T-P plane and analyze the effect of the Rastall constant λ on the inversion curves of a charged AdS black hole during the Joule-Thomson expansion.     

Iron Kα Emission in Seyfert（—Like） Active Galactic Nuclei：Revelation of a Rapidly Spinning Central Black Hole

Fe Kα lines are superimposed upon the x-ray continuum in most Seyfert(-like) active galactic nuclei(AGNs).By a data-fitting study,previous authors have claimed that the central black hole(BH)is either rotating or non-rotating according to the thin disc model.We develop the disc model to the torus model to determine the real spin of the BH.With formulations of the motion of both torus particles and photons near a BH in Kerr metric,we simulate iron emission lines from a thin luminous torus,It is found that only spinning BH galaxies can radiate observable profiles.The data-fitting to Fe lines of four AGNs observed by ASCA predicts that the cenrral BH is spinning rapidly with the dimensionless specific angular monentum approaching the maximal value of 1.     

Hawking Radiation of Grumiller Black Hole

In this paper, we consider the relativistic Harnilton-Jacobi （HJ） equation and study Hawking radiation （HR） of scalar particles from uncharged Grumiller black hole （GBH） which is affordable for testing in astrophysics. It is a/so known as Rindler modified Schwarzschild BH. Our aim is not only to investigate the effect of the Rindler parameter a on the Hawking temperature （TH ）, but to examine whether there is any discrepancy between the computed horizon temperature and the standard TH as well. For this purpose, in addition to its naive coordinate system, we study on the three regular coordinate systems, which are Painlevd--Gullstrand （PG）, ingoing Edding~on-Finkelstein （IEF）, and Kruskal-Szekeres （KS） coordinates. In o21 coordinate systems, we calculate the tunneling probabilities of incoming and outgoing scalar particles from the event horizon by using the HJ equation. It has been shown in detail that the considered HJ method is concluded with the conventional T~ in all these coordinate systems without giving rise to the famous factor-2 problem. Filrthermore, in the PG coordinates Parikh-Wilczek＇s tunneling （PWT） method is employed in order to show how one can integrate the quantum gravity （QG） corrections to the semiclassical tunneling rate by including the effects of self-gravitation and back reaction. We then show how this yields a modification in the TH.     

Optical appearance of the Schwarzschild black hole in the string cloud context

The image of a black hole(BH) consists of direct and secondary images that depend on the observer position. We investigate the optical appearance of a Schwarzschild BH in the context of a string cloud to reveal how the BH’s observable characteristics are influenced by the inclination angle, string cloud parameter, and impact parameter. Following Luminet’s work [Astron. Astrophys. 75, 228(1979)], we adopt a semi-analytic method to calculate the total bending angle of the light ray and derive the ...     

Effect of gravity gradient in weak equivalence principle test

A high accuracy test of the weak equivalence principle(WEP) is of great scientific significance no matter whether its result is positive. We analyze the gravity gradient effect which is a main systematic error source in the test of WEP.The result shows that the uncompensated gravity gradient effect from the coupling term of the dominated gravity gradient multipole moment component q_(21) and the relative multipole field component Q_(21) contributes to an uncertainty of 1×10~(-11) on the E¨otv¨os parameter. We make a Q_(21) compensation to reduce the effect by about 20 times, and the limit of the test precision due to this coupling is improved to a level of a part in 10~(13).     

Parameter Space for Evolution of Black Hole Systems and Gamma—Ray Bursts

An analytical model of black hole(BH) evolution is proposed by considering the Blandford-Znajek process with a tansient magnetized accretion disc,which is used as a central engine for powering gamma ray bursts(GRBs),The state of BH evolution can be described by the representative points in the corresponding sub-regions of a parameter space.It is shown that the BH spin might be spun up rather than slowed down.provided that the ratio of the angular velocity of the magnetic field lines to that of the BH horizon is greater than a critical value.In addition,the effects of some initial values of the BH system on the evolution terminals and the output energy for GRBs are discussed in the parameter space.     

Simulation of the relationship between porosity and tortuosity in porous media with cubic particles

Tortuosity is an important parameter used in areas such as vascular medicine,neurobiology,and the field of soil permeability and diffusion to express the mass transport in porous media.It is a function of the porosity and the shape and distribution of particles.In this paper,the tortuosity of cubic particles is calculated.With the assumption that the porous medium is homogeneous,the problem is converted to the micro-level over a unit cell,and geometry models of flow paths are proposed.In three-dimensional(3D) cells,the flow paths are too complicated to define.Hence,the 3D models are converted to two-dimensional(2D) models to simplify the calculation process.It is noticed that the path in the 2D model is shorter than that in the 3D model.As a result,triangular particles and the interaction are also taken into consideration to account for the longer distance respectively.We have proposed quadrate particle and interaction(QI) and quadrate and triangular particle(QT) models with cubic particles.Both models have shown good agreement with the experimental data.It is also found that they can predict the toruosities of some kinds of porous media,like freshwater sediment and Negev chalk.     

On the validity of microscopic calculations of double-quantum-dot spin qubits based on Fock-Darwin states

We consider two typical approximations that are used in the microscopic calculations of double-quantum dot spin qubits, namely,the Heitler-London(HL) and the Hund-Mulliken(HM) approximations, which use linear combinations of Fock-Darwin states to approximate the two-electron states under the double-well confinement potential. We compared these results to a case in which the solution to a one-dimensional Schr ¨odinger equation was exactly known and found that typical microscopic calculations based on Fock-Darwin states substantially underestimate the value of the exchange interaction, which is the key parameter that controls the quantum dot spin qubits. This underestimation originates from the lack of tunneling of Fock-Darwin states, which is accurate only in the case with a single potential well. Our results suggest that the accuracies of the current two-dimensional molecularorbit-theoretical calculations based on Fock-Darwin states should be revisited since underestimation could only deteriorate in dimensions that are higher than one.     

Standing Rankine-Hugoniot Shocks in Black Hole Accretion Discs

We study the problem of standing shocks in viscous disc-like accretion flows around black holes. For the first time we parametrize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K. By providing the global dependence of shock formation in the j- K parameter space, we show that a significant parameter region can ensure solutions with Rankine-Hugoniot shocks; and that the possibilities of shock formation are the largest for inviscid flows, decreasing with increasing viscosity, and ceasing to exist for a strong enough viscosity. Our results support the view that the standing shock is an essential ingredient in black hole accretion discs and is a general phenomenon in astrophysics, and that there should be a continuous change from the properties of inviscid flows to those of viscous ones.     

Flocking and clustering in mixtures of self-propelled particles with or without active reorientation

We study phase behaviors of mixtures comprising active particles with and without active reorientation by varying mixing ratios. We observe that the order parameter characterizing flocking in the steady state exhibits a linear decrease with an increase in mixing ratio. While the order parameter characterizing clustering in the steady state presents a sharp leap as the mixing ratio increases. Particularly, we obtain phase diagrams of flocking under different mixing ratios and observe that the dom...     

Post-Newtonian parameter γ in generalized non-local gravity

We investigate the post-Newtonian parameter γ and derive its formalism in generalized non-local(GNL) gravity, which is the modified theory of general relativity(GR) obtained by adding a term m~(2n-2)R□(-n)R to the Einstein-Hilbert action. Concretely,based on parametrizing the generalized non-local action in which gravity is described by a series of dynamical scalar fields φ~i in addition to the metric tensor g_(μν), the post-Newtonian limit is computed, and the effective gravitational constant as well as the post-Newtonian parameters are directly obtained from the generalized non-local gravity. Moreover, by discussing the values of the parametrized post-Newtonian parameters γ, we can compare our expressions and results with those in Hohmann and Jrv et al.(2016), as well as current observational constraints on the values of γ in Will(2006). Hence, we draw restrictions on the nonminimal coupling terms F around their background values.     

Magnetic Field of a Compact Spherical Star under f(R,T) Gravity

We present the interior solutions of distributions of magnetized fluid inside a sphere in f(R,T) gravity. The magnetized sphere is embedded in an exterior Reissner–Nordstr?m metric. We assume that all physical quantities are in static equilibrium. The perfect fluid matter is studied under a particular form of the Lagrangian density f(R,T). The magnetic field profile in modified gravity is calculated. Observational data of neutron stars are used to plot suitable models of magnetized compact objects. We reveal the effect of f(R,T) gravity on the magnetic field profile, with application to neutron stars, especially highly magnetized neutron stars found in x-ray pulsar systems. Finally, the effective potential V_(eff) and innermost stable circular orbits, arising out of the motion of a test particle of negligible mass influenced by attraction or repulsion from the massive center, are discussed.     

A New Solution in Understanding Massive White Dwarfs

The observed high over-luminous type-Ia supernovae imply the existence of super-Chandrasekhar limit white dwarfs,which raises a challenge to the classical white dwarf theories.By employing the Eddington-inspired Born-Infeld(EiBI) gravity,we reinvestigate the structures and properties of white dwarfs,and find out that the EiBI gravity provides a new way to understand the observations.It is shown that by choosing an appropriate positive Eddington parameter k,a massive white dwarf with mass up to 2.8M_☉ can be supported by the equation of state of free electron gas.Unlike the classical white dwarf theory,the maximum mass of the white dwarf sequence in the EiBI gravity is not decided by the mass-radius relations,but is decided by the central density,ρ_C = 4.3 × 10~(14) kg/m~3,above which neutronization cannot be avoided and the white dwarf will transform into a neutron star.On the other hand,if the gravity in the massive white dwarf really behaves as the EiBI gravity predicts,then one can obtain a constraint on the Eddington parameter in the EiBI gravity,that is,8πρ0KG/c~2 ≥80(where ρ_0 = 10~(18) kg/m~3) to support a massive white dwarf with mass up to 2.8M_☉.Moreover,we find out that the fast Keplarian frequency of the massive white dwarf raises a degeneration between the two kinds of compact stars,that is,one cannot distinguish whether the observed massive pulsar is a massive neutron star or a massive white dwarf only through the observed pulse frequency and mass.     

Testing noncommutative spacetimes and violations of the Pauli Exclusion Principle through underground experiments

We propose to deploy limits that arise from different tests of the Pauli Exclusion Principle: i) to provide theories of quantum gravity with experimental guidance; ii) to distinguish, among the plethora of possible models, the ones that are already ruled out by current data; iii) to direct future attempts to be in accordance with experimental constraints. We first review experimental bounds on nuclear processes forbidden by the Pauli Exclusion Principle,which have been derived by several experimental collaborations making use of various detector materials. Distinct features of the experimental devices entail sensitivities on the constraints hitherto achieved that may differ from one another by several orders of magnitude. We show that with choices of these limits, well-known examples of flat noncommutative space-time instantiations of quantum gravity can be heavily constrained, and eventually ruled out.We devote particular attention to the analysis of the κ-Minkowski and θ-Minkowski noncommutative spacetimes.These are deeply connected to some scenarios in string theory, loop quantum gravity, and noncommutative geometry.We emphasize that the severe constraints on these quantum spacetimes, although they cannot rule out theories of top-down quantum gravity to which they are connected in various ways, provide a powerful limitation for those models. Focus on this will be necessary in the future.     

Dark-matter pair production at the ILC in the littlest Higgs model with T-parity

In the littlest Higgs model with T-parity, the heavy photon (A_H) is supposed to be a possible dark matter (DM) candidate. The direct proof of the validity of this model is to produce the heavy photon at an accelerator. In this paper, we study the production rate of e~+ e~- → AH AH at the international e~+ e~- linear collider (ILC) in the littlest Higgs model with T-parity, and show the distributions of the transverse momenta of A H . The numerical results indicate that the heavy photon production rate could reach the 10~-1 fb level at some parameter space, so this could be a good chance to observe the heavy photon via the pair production process with high luminosity at the ILC (500 fb~-1). We know that DM is composed of weakly interacting massive particles, so the interactions with standard model particles are weak. How to detect heavy photons at a collider and distinguish them from other DM candidates are discussed in the final section of the paper.     

Oscillating Quintom Model with Time Periodic Varying Deceleration Parameter

We propose a new law for the deceleration parameter that varies periodically with time. According to the law we give a model of the oscillating universe with quintom matter in the framework of a 4-dimensional Friedmann Ftobertson Walker background. We find that, in the model, the Hubble parameter oscillates and keeps positive The universe undergoes decelerating expansion and accelerating expansion alternately without singularity.     

Inconsistencies in four-dimensional Einstein-Gauss-Bonnet gravity

We attempt to clarify several aspects concemi ng the recently presented four-dimensional Ein stein-Gauss-Bonnet gravity.We argue that the limiting procedure outlined in[Phys.Rev.Lett.124,081301(2020)]generally involves ill-defined terms in the four dimensional field equations.Potential ways to circumvent this issue are discussed,alongside remarks regarding specific solutions of the theory.We prove that,although linear perturbations are well behaved around maximally symmetric backgrounds,the equations for second-order perturbations are illdefined even around a Minkowskia n background.Additi on ally,we perform a detailed analysis of the spherically symmetric solutions and find that the central curvature singularity can be reached within a finite proper time.