arXiv subjects

Rong-Jia Yang

Publications and source records attributed to Rong-Jia Yang.

At least 55 records · Page 3Linked to original sources

arXiv2020

Accretion on High Derivative Asymptotically Safe Black Holes

Asymptotically safe gravity is one effective approach to quantum gravity. It is important to differentiate the modified gravity inspired by asymptotically safe gravity. In this paper, we examine the matter particles dynamics near the improved version of Schwarzschild black hole. We assume that in the context of asymptotically safe gravity scenario the ambient matter surrounding the black hole is of isothermal in nature and investigate the spherical accretion of matter by deriving solutions at critical points. The analysis for the various values of the state parameter for isothermal test fluids, viz., $k=1,~1/2,~1/3,~1/4$ show the possibility of accretion onto asymptotically safe black hole. We formulate the accretion problem as Hamiltonian dynamical system and explain its phase flow in detail which reveals interesting results in asymptotically safe gravity theory.

gr-qc↗

arXiv2020

Entropy and Energy of Static Spherically Symmetric Black Hole in $f(R)$ theory

We consider the new horizon first law in $f(R)$ theory with general spherically symmetric black hole. We derive the general formulas to computed the entropy and energy of the black hole. For applications, some nontrivial black hole solutions in some popular $f(R)$ theories are investigated, the entropies and the energies of black holes in these models are first calculated.

gr-qc↗

arXiv2019

Effects of Lorentz breaking on the accretion onto a Schwarzschild-like black hole

We formulate and solve the problem of spherically symmetric, steady state, adiabatic accretion onto a Schwarzschild-like black hole obtained recently. We derive the general analytic expressions for the critical points, the critical velocity, the critical speed of sound, and subsequently the mass accretion rate. The case for polytropic gas is discussed in detail. We find the parameter characterizing the breaking of Lorentz symmetry will slow down the mass accretion rate, while has no effect on the gas compression and the temperature profile below the critical radius and at the event horizon.

gr-qc↗

arXiv2019

Constraints from accretion onto a Tangherlini-Reissner-Nordstrom black hole

We investigate spherically symmetric, steady state, adiabatic accretion onto a Tangherlini-Reissner-Nordstrom black hole in arbitrary dimensions by using $D$-dimensional general relativity. We obtain basic equations for accretion and determine analytically the critical points, the critical fluid velocity, and the critical sound speed. We lay emphasis on the condition under which the accretion is possible. This condition constrains the ratio of mass to charge in a narrow limit, which is independent of dimension for large dimension. This condition may challenge the validity of the cosmic censorship conjecture since a naked singularity is eventually produced as the magnitude of charge increases compared to the mass of black hole.

gr-qc↗

arXiv2018

Horizon thermodynamics in $f(R)$ theory

We investigate whether the new horizon first law proposed recently still work in $f(R)$ theory. We identify the entropy and the energy of black hole as quantities proportional to the corresponding value of integration, supported by the fact that the new horizon first law holds true as a consequence of equations of motion in $f(R)$ theories. The formulas for the entropy and energy of black hole found here are in agreement with the results obtained in literatures. For applications, some nontrivial black hole solutions in $f(R)$ theories have been considered, the entropies and the energies of black holes in these models are firstly computed, which may be useful for future researches.

gr-qc↗

arXiv2018

Gravitational waves in conformal gravity

We consider the gravitational radiation in conformal gravity theory. We perturb the metric from flat Mikowski space and obtain the wave equation after introducing the appropriate transformation for perturbation. We derive the effective energy-momentum tensor for the gravitational radiation, which can be used to determine the energy carried by gravitational waves.

gr-qc↗

arXiv2017

Accretion onto a Moving Reissner-Nordström Black Hole

We obtain an analytic solution for accretion of a gaseous medium with a adiabatic equation of state ($P=ρ$) onto a Reissner-Nordström black hole which moves at a constant velocity through the medium. We obtain the specific expression for each component of the velocity and present the mass accretion rate which depends on the mass and the electric charge. The result we obtained may be helpful to understand the physical mechanism of accretion onto a moving black hole.

gr-qc↗

arXiv2017

Accretion onto a Kiselev black hole

We consider accretion onto a Kiselev black hole. We obtain the fundamental equations for accretion without the back-reaction. We determine the general analytic expressions for the critical points and the mass accretion rate and find the physical conditions the critical points should fulfill. The case of polytropic gas are discussed in detail. It turns out that the quintessence parameter plays an important role in the accretion process.

gr-qc↗

arXiv2016

Transient acceleration in $f(T)$ gravity

Recently a $f(T)$ gravity based on the modification of the teleparallel gravity was proposed to explain the accelerated expansion of the universe without the need of dark energy. We use observational data from Type Ia Supernovae, Baryon Acoustic Oscillations, and Cosmic Microwave Background to constrain this $f(T)$ theory and reconstruct the effective equation of state and the deceleration parameter. We obtain the best-fit values of parameters and find an interesting result that the $f(T)$ theory considered here allows for the accelerated Hubble expansion to be a transient effect.

gr-qc↗

arXiv2016

Effects of quantum fluctuations of metric on the universe

We consider a model of modified gravity from the nonperturbative quantization of a metric. We obtain the modified gravitational field equations and the modified conservational equations. We apply it to the FLRW spacetime and find that due to the quantum fluctuations a bounce universe can be obtained and a decelerated expansion can also possibly be obtained in a dark energy dominated epoch. We also discuss the effects of quantum fluctuations on inflation parameters (such as slow-roll parameters, spectral index, and the spectrum of the primordial curvature perturbation) and find values of parameters in the comparing the predictions of inflation can also work to drive the current epoch of acceleration. We obtain the constraints on the parameter of the theory from the observation of the big bang nucleosynthesis.

gr-qc↗

arXiv2016

Quantum hoop conjecture and a natural cutoff for vacuum energy of a scalar field

We propose here a quantum hoop conjecture which states: the de Broglie wavelength of a quantum system cannot be arbitrarily small, it must be larger than the characterized Schwarzschild radius of the quantum system. Based on this conjecture, we find an upper bound for the wave number (or the momentum) of a particle, which offers a natural cutoff for the vacuum energy of a scalar field.

gr-qc↗

arXiv2015

Quantum gravity corrections to accretion onto a Schwarzschild black hole

Quantum gravity corrections to accretion onto a Schwarzschild black hole are considered in the context of asymptotically safe scenario. The possible positions of the critical points are discussed and the general conditions for critical points are obtained. The explicit expressions for matter density compression and temperature profile both below the critical radius and at the event horizon are derived. For polytropic matter, we determine the corrected temperature and the integrated flux resulting from quantum gravity effects at the event horizon, which might be as a test of asymptotically safe scenario.

gr-qc↗

arXiv2015

The evolution of the power law k-essence cosmology

We investigate the evolution of the power law k-essence field in FRWL spacetime. The autonomous dynamical system and critical points are obtained. The corresponding cosmological parameters, such as $Ω_{ϕ}$ and $w_{ϕ}$, are calculated at these critical points. We find it is possible to achieve an equation of state crossing through $-1$ for k-essence field. The results we obtained indicate that the power law k-essence dark energy model can be compatible with observations.

gr-qc↗

arXiv2014

Discriminating dark energy models by using the statefinder hierarchy and the growth rate of matter perturbations

We apply the Statefinder hierarchy and the growth rate of matter perturbations to discriminate modified Chaplygin gas (MCG), generalized Chaplygin gas (GCG), superfluid Chaplygin gas (SCG), purely kinetic k-essence (PKK), and $Λ$CDM model. We plot the evolutional trajectories of these models in the statefinder plane and in the composite diagnostic plane. We find that GCG, MCG, SCG, PKK, and $Λ$CDM can be distinguished well from each other at the present epoch by using the composite diagnostic $\{ε(z), S^{(1)}_{5}\}$. Using other combinations, such as $\{S^{(1)}_{3}, S^{(1)}_4\}$, $\{S^{(1)}_{3}, S_{5}\}$, $\{ε(z), S^{(1)}_{3}\}$, and $\{ε(z), S_4 \}$, some of these five dark energy models cannot be distinguished.

gr-qc↗

arXiv2014

Is gravity entropic force?

If we assume that the source of thermodynamic system, $ρ$ and $p$, are also the source of gravity, thermal quantities, such as entropy, temperature, and chemical potential, can induce effects of gravity, or gravity can induce thermal effects. We find only for systems with constant temperature and zero chemical potential, gravity can be seen as an entropic force. The case for Newtonian approximation is discussed.

gr-qc↗

arXiv2014

Spatial Ricci scalar dark energy model

Inspired by holographic principle, we suggest that the density of dark energy is proportional to the spatial Ricci scalar curvature (SRDE). Such model is phenomenologically viable. The best fit values of its parameters at 68% confidence level are found to be: $Ω_{\rm m0}=0.259\pm0.016$ and $α=0.261\pm0.0122$, constrained from the Union+CFA3 sample of 397 SNIa and the BAO measurement. We find the equation of state of SRDE crosses -1 at $z\simeq-0.14$. The present values of the deceleration parameter $q(z)$ for SRDE is found to be $q_{z=0}\sim -0.85$. The phase transition from deceleration to acceleration of the Universe for SRDE occurs at the redshift $z_{q=0}\sim 0.4$. After studying on the perturbation of each component of the Universe, we show that the matter power spectra and cosmic microwave background temperature anisotropy is slightly affected by SRDE, compared with $Λ$CDM.

gr-qc↗

arXiv2014

Large-scale structure in superfluid Chaplygin gas cosmology

We investigate the growth of large-scale structure in the superfluid Chaplygin gas (SCG) model. Both linear and non-linear growth, such as $σ_8$ and the skewness $S_3$, are discussed. We find the growth factor of SCG reduces to the EdS case at early times while differs from the $Λ$CDM case in the large $a$ limit. We also find there will be much stricture growth on large scales in the SCG scenario than in $Λ$CDM and the variations of $σ_{8}$ and $S_3$ between SCG and $Λ$CDM can not be discriminated.

gr-qc↗

arXiv2013

Test of conformal gravity with astrophysical observations

Since it can describe the rotation curves of galaxies without dark matter and can give rise to accelerated expansion, conformal gravity attracts much attention recently. As a theory of modified gravity, it is important to test conformal gravity with astrophysical observations. Here we constrain conformal gravity with SNIa and Hubble parameter data and investigate whether it suffers from an age problem with the age of APM~08279+5255. We find conformal gravity can accommodate the age of APM~08279+5255 at 3 $σ$ deviation, unlike most of dark energy models which suffer from an age problem.

gr-qc↗