34 research outputs found
On the Curvature Invariants of the Massive Banados-Teitelboim-Zanelli Black Holes and Their Holographic Pictures
In this paper, the curvature structure of a (2+1)-dimensional black hole in
the massive-charged-Born-Infeld gravity is investigated. The metric that we
consider is characterized by four degrees of freedom which are the mass and
electric charge of the black hole, the mass of the graviton field, and a
cosmological constant. For the charged and neutral cases separately, we present
various constraints among scalar polynomial curvature invariants which could
invariantly characterize our desired spacetimes. Specially, an appropriate
scalar polynomial curvature invariant and a Cartan curvature invariant which
together could detect the black hole horizon would be explicitly constructed.
Using algorithms related to the focusing properties of a bundle of light rays
on the horizon which are accounted by the Raychaudhuri equation, a procedure
for isolating the black hole parameters, as the algebraic combinations
involving the curvature invariants, would be presented. It will be shown that
this technique could specially be applied for black holes with zero electric
charge, contrary to the cases of solutions of lower-dimensional non-massive
gravity. In addition, for the case of massive (2+1)-dimensional black hole, the
irreducible mass, which quantifies the maximum amount of energy which could be
extracted from a black hole through the Penrose process would be derived.
Therefore, we show that the Hawking temperatures of these black holes could be
reduced to the pure curvature properties of the spacetimes. Finally, we comment
on the relationship between our analysis and the novel roles it could play in
numerical quark-gluon plasma simulations and other QCD models and also black
hole information paradox where the holographic correspondence could be
exploited.Comment: v3; 25 pages; 11 figures; 105 reference
Constraints on interacting dark energy models through cosmic chronometers and Gaussian process
Energy flows between dark energy and dark matter may alleviate the Hubble
tension and mitigate the coincidence problem. In this paper, after
reconstructing the redshift evolution of the Hubble function by adopting
Gaussian process techniques, we estimate the best-fit parameters for some flat
Friedmann cosmological models based on a Modified Chaplygin Gas interacting
with dark matter. In fact, the expansion history of the Universe will be
investigated because passively evolving early galaxies constitute cosmic
chronometers. An estimate for the present-day values of the deceleration
parameter, adiabatic speed of sound within the dark energy fluid, effective
dark energy, and dark matter equation of state parameters is provided. By this,
we mean that the interaction term between the two dark fluids, which breaks the
Bianchi symmetries, will be interpreted as an effective contribution to the
dark matter pressure similarly to the framework of the \lq\lq Generalized Dark
Matter". Fixing a certain value for the dark matter abundance at the present
day as initial condition will allow us to investigate whether the estimate of
the Hubble constant is sensitive to the dark matter - dark energy coupling. We
will also show that the cosmic chronometers data favor a hot dark matter, and
that our findings are in agreement with the Le Ch\^atelier-Braun principle
according to which dark energy should decay into dark matter (and not vice
versa).Comment: 14 pages, 2 figure
Friction forces in cosmological models
We investigate the dynamics of test particles undergoing friction forces in a
Friedmann-Robertson-Walker (FRW) spacetime. The interaction with the background
fluid is modeled by introducing a Poynting-Robertson-like friction force in the
equations of motion, leading to measurable (at least in principle) deviations
of the particle trajectories from geodesic motion. The effect on the peculiar
velocities of the particles is investigated for various equations of state of
the background fluid and different standard cosmological models. The friction
force is found to have major effects on particle motion in closed FRW
universes, where it turns the time-asymptotic value (approaching the
recollapse) of the peculiar particle velocity from ultra-relativistic (close to
light speed) to a co-moving one, i.e., zero peculiar speed. On the other hand,
for open or flat universes the effect of the friction is not so significant,
because the time-asymptotic peculiar particle speed is largely non-relativistic
also in the geodesic case.Comment: 8 pages, 2 figures; published versio
Revisiting the foundation and applicability of some dark energy fluid models in the Dirac-Born-Infeld framework
In this paper, we will deepen the understanding of some fluid models proposed
by other authors for the description of dark energy. Specifically, we will show
that the so-called (Modified) Berthelot fluid is the hydrodynamic realization
of the Dirac-Born-Infeld theory and that the Dieterici fluid admits a
non-relativistic -essence formulation; for the former model, the evolution
of the scalar field will be written in terms of some cosmographic parameters.
The latter model will also be tested using Machine Learning algorithms with
respect to cosmic chronometers data, and results about the dynamics at a
background level will be compared with those arising when other fluids
(Generalized Chaplygin Gas and Anton-Schmidt) are considered. Due to some
cosmic opacity effects, the background cosmology of universes filled by these
inequivalent fluids, as they arise in physically different theories, may not be
enough for discriminating among them. Thus, a perturbation analysis in the
long-wavelength limit is carried out revealing a rich variety of possible
behaviors.Comment: 19 pages, 8 figure
On the uniqueness of CDM-like evolution for homogeneous and isotropic cosmology in General Relativity
We address the question of the uniqueness of spatially flat CDM-like
evolution for FLRW cosmologies in General Relativity, i.e. whether any model
other than the spatially flat CDM can give rise to the same type of
scale factor evolution. Firstly, we elaborate on what we exactly imply by a
CDM-like evolution or kinematic/cosmographic degeneracy with the
CDM model, using the lessons from the statefinder diagnostic. Then, we
consider two models with interaction in the dark sector: coupled fluid-fluid
model and coupled quintessence model. We enforce the \emph{kinematic}
degeneracy with the spatially flat CDM model via the cosmographic
condition ( being the jerk parameter), which in turn fixes the
function of the interaction term that is a priori unspecified. We argue that in
General Relativity this cosmographic condition is consistent only with spatial
flatness. Employing a dynamical system approach, we show that the spatially
flat coupled fluid-fluid interacting models kinematically degenerate with
CDM must necessarily be based on a phantom fluid, whereas the set of
physically viable spatially flat coupled quintessence models with power law or
exponential potential kinematically degenerate to CDM is of measure
zero. Our analysis establishes that coupled fluid-fluid models with non-phantom
fluids or coupled quintessence models with power law and exponential potential
can never reproduce a cosmological evolution similar to that of the
CDM. The astrophysical consequences of our findings are qualitatively
discussed in light of observational cosmological tensions.Comment: 15 pages. 3 figures. Accepted for publication in PL
Curvature Invariants and Lower Dimensional Black Hole Horizons
It is known that the event horizon of a black hole can often be identified
from the zeroes of some curvature invariants. The situation in lower dimensions
has not been thoroughly clarified. In this work we investigate both (2+1)- and
(1+1)-dimensional black hole horizons of static, stationary and dynamical black
holes, identified with the zeroes of scalar polynomial and Cartan curvature
invariants, with the purpose of discriminating the different roles played by
the Weyl and Riemann curvature tensors. The situations and applicability of the
methods are found to be quite different from that in 4-dimensional spacetime.
The suitable Cartan invariants employed for detecting the horizon can be
interpreted as a local extremum of the tidal force suggesting that the horizon
of a black hole is a genuine special hypersurface within the full manifold,
contrary to the usual claim that there is nothing special at the horizon, which
is said to be a consequence of the equivalence principle.Comment: Matches published versio