3,084 research outputs found
Phantom Energy Accretion by Stringy Charged Black Hole
We investigate the dynamical behavior of phantom energy near stringy
magnetically charged black hole. For this purpose, we derive equations of
motion for steady-state spherically symmetric flow of phantom energy onto the
stringy magnetically charged black hole. It is found that phantom energy
accreting onto black hole decreases its mass. Further, the location of critical
points of accretion is explored, which yields mass to charge ratio. This ratio
implies that accretion process cannot transform a black hole into an extremal
black hole or a naked singularity, hence cosmic censorship hypothesis remains
valid here.Comment: 7 pages, no figur
Darboux class of cosmological fluids with time-dependent adiabatic indices
A one-parameter family of time dependent adiabatic indices is introduced for
any given type of cosmological fluid of constant adiabatic index by a
mathematical method belonging to the class of Darboux transformations. The
procedure works for zero cosmological constant at the price of introducing a
new constant parameter related to the time dependence of the adiabatic index.
These fluids can be the real cosmological fluids that are encountered at
cosmological scales and they could be used as a simple and efficient
explanation for the recent experimental findings regarding the present day
accelerating universe. In addition, new types of cosmological scale factors,
corresponding to these fluids, are presentedComment: document with the following three latex files: 1) quhm.tex: 17 pages,
10 figs, 16 numbered refs, Honorable Mention GRF 2000, 2) errad.tex: Errata
and Addenda (EaA) of 5 pages with 2 figs enclosed, 3) analogy.tex: Negative
friction of Darboux cosmological fluids of 4 page
Cosmological constant influence on cosmic string spacetime
We investigate the line element of spacetime around a linear cosmic string in
the presence of a cosmological constant. We obtain the metric and argue that it
should be discarded because of asymptotic considerations. Then a time dependent
and consistent form of the metric is obtained and its properties are discussed.Comment: 3 page
Phantom Accretion onto the Schwarzschild de-Sitter Black Hole
We deal with phantom energy accretion onto the Schwarzschild de-Sitter black
hole. The energy flux conservation, relativistic Bernoulli equation and mass
flux conservation equation are formulated to discuss the phantom accretion. We
discuss the conditions for critical accretion. It is found that mass of the
black hole decreases due to phantom accretion. There exist two critical points
which lie in the exterior of horizons (black hole and cosmological horizons).
The results for the phantom energy accretion onto the Schwarzschild black hole
can be recovered by taking .Comment: 9 pages, no figur
Natural extension of the Generalised Uncertainty Principle
We discuss a gedanken experiment for the simultaneous measurement of the
position and momentum of a particle in de Sitter spacetime. We propose an
extension of the so-called generalized uncertainty principle (GUP) which
implies the existence of a minimum observable momentum. The new GUP is directly
connected to the nonzero cosmological constant, which becomes a necessary
ingredient for a more complete picture of the quantum spacetime.Comment: 4 pages, 1 figure, v2 with added references, revised and extended as
published in CQ
F(T) gravity and k-essence
Modified teleparallel gravity theory with the torsion scalar have recently
gained a lot of attention as a possible explanation of dark energy. We perform
a thorough reconstruction analysis on the so-called models, where
is some general function of the torsion term, and derive conditions for the
equivalence between of models with purely kinetic k-essence. We present
a new class models of -gravity and k-essence. We also proposed some new
models of generalized gases and knot universes as well as some generalizations
of gravity.Comment: 25 page
The Newtonian Limit of F(R) gravity
A general analytic procedure is developed to deal with the Newtonian limit of
gravity. A discussion comparing the Newtonian and the post-Newtonian
limit of these models is proposed in order to point out the differences between
the two approaches. We calculate the post-Newtonian parameters of such theories
without any redefinition of the degrees of freedom, in particular, without
adopting some scalar fields and without any change from Jordan to Einstein
frame. Considering the Taylor expansion of a generic theory, it is
possible to obtain general solutions in term of the metric coefficients up to
the third order of approximation. In particular, the solution relative to the
component gives a gravitational potential always corrected with
respect to the Newtonian one of the linear theory . Furthermore, we
show that the Birkhoff theorem is not a general result for -gravity since
time-dependent evolution for spherically symmetric solutions can be achieved
depending on the order of perturbations. Finally, we discuss the
post-Minkowskian limit and the emergence of massive gravitational wave
solutions.Comment: 16 page
Is the Lambda CDM Model Consistent with Observations of Large-Scale Structure?
The claim that large-scale structure data independently prefers the Lambda
Cold Dark Matter model is a myth. However, an updated compilation of
large-scale structure observations cannot rule out Lambda CDM at 95%
confidence. We explore the possibility of improving the model by adding Hot
Dark Matter but the fit becomes worse; this allows us to set limits on the
neutrino mass.Comment: To appear in Proceedings of "Sources and Detection of Dark
Matter/Energy in the Universe", ed. D. B. Cline. 6 pages, including 2 color
figure
CPT symmetry and antimatter gravity in general relativity
The gravitational behavior of antimatter is still unknown. While we may be
confident that antimatter is self-attractive, the interaction between matter
and antimatter might be either attractive or repulsive. We investigate this
issue on theoretical grounds. Starting from the CPT invariance of physical
laws, we transform matter into antimatter in the equations of both
electrodynamics and gravitation. In the former case, the result is the
well-known change of sign of the electric charge. In the latter, we find that
the gravitational interaction between matter and antimatter is a mutual
repulsion, i.e. antigravity appears as a prediction of general relativity when
CPT is applied. This result supports cosmological models attempting to explain
the Universe accelerated expansion in terms of a matter-antimatter repulsive
interaction.Comment: 6 pages, to be published in EPL (http://epljournal.edpsciences.org/
K-essential Phantom Energy: Doomsday around the Corner? Revisited
We generalize some of those results reported by Gonz\'{a}lez-D\'{i}az by
further tuning the parameter () which is closely related to the
canonical kinetic term in -essence formalism. The scale factor could
be negative and decreasing within a specific range of (, : the equation-of-state parameter) during the initial
evolutional period.Comment: 1 Figure, 6 page
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