3,902 research outputs found
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
-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
and the skewness , are discussed. We find the growth factor of
SCG reduces to the EdS case at early times while differs from the CDM
case in the large limit. We also find there will be much stricture growth
on large scales in the SCG scenario than in CDM and the variations of
and between SCG and 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 gravity
Recently 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 theories. In
analogy to the theory, we consider here three types of 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: and . 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 theory
We investigate whether the new horizon first law proposed recently still work
in 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 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
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
- …