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.Comment: 8 pages, no figure

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.Comment: 3 pages, no figure

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 $\sigma_8$ 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 $\Lambda$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 $\Lambda$CDM and the variations of $\sigma_{8}$ and $S_3$ between SCG and $\Lambda$CDM can not be discriminated.Comment: 10 pages, 6 figures, minor errors are correcte

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.Comment: 8 pages, no figures, some errors are correcte

New types of f(T)f(T) gravity

Recently $f(T)$ theories based on modifications of teleparallel gravity where torsion is the geometric object describing gravity instead of curvature have been proposed to explain the present cosmic accelerating expansion. The field equations are always second order, remarkably simpler than $f(R)$ theories. In analogy to the $f(R)$ theory, we consider here three types of $f(T)$ gravity, and find that all of them can give rise to cosmic acceleration with interesting features, respectively.Comment: 6 pages, 9 figures, some changes are mad

Dynamics of generalized tachyon field

We investigate the dynamics of generalized tachyon field in FRW spacetime. We obtain the autonomous dynamical system for the general case. Because the general autonomous dynamical system cannot be solved analytically, we discuss two cases in detail: $\beta=1$ and $\beta=2$. We find the critical points and study their stability. At these critical points, we also consider the stability of the generalized tachyon field, which is as important as the stability of critical points. The possible final states of the universe are discussed.Comment: 9 pages, 5 figures, published versio

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.Comment: 7 page

Horizon thermodynamics in f(R)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.Comment: 8 pages, no figur