324 research outputs found
Cosmological dynamics of dark energy in scalar-torsion gravity
It is investigated the cosmological dynamics of scalar-torsion
gravity as a dark energy model, where is the torsion scalar of teleparallel
gravity and is a canonical scalar field. In this context, we are
concerned with the phenomenology of the class of models with non-linear
coupling to gravity and exponential potential. We obtain the critical points of
the autonomous system, along with the stability conditions of each one of them
and their cosmological properties. Particularly, we show the existence of new
attractors with accelerated expansion, as well as, new scaling solutions in
which the energy density of dark energy scales as the background fluid density,
thus, defining the so-called scaling radiation and scaling matter epochs. The
scaling solutions are saddle points, and therefore, the system exits these
solutions to the current epoch of cosmic acceleration, towards an attractor
point describing the dark energy-dominated era.Comment: Accepted version for publication in EPJC. 17 pages, 3 tables and 10
figure
Scalar Perturbations of two-dimensional Horava-Lifshitz Black Holes
In this article, we study the stability of black hole solutions found in the
context of dilatonic Horava-Lifshitz gravity in dimensions by means of
the quasinormal modes approach. In order to find the corresponding quasinormal
modes, we consider the perturbations of massive and massless scalar fields
minimally coupled to gravity. In both cases, we found that the quasinormal
modes have a discrete spectrum and are completely imaginary, which leads to
damping modes. For a massive scalar field and a non-vanishing cosmological
constant, our results suggest unstable behaviour for large values of the scalar
field mass.Comment: 18 pages, 1 figure. Accepted version in EPJC. arXiv admin note: text
overlap with arXiv:gr-qc/070109
Odd-parity perturbations in the most general scalar-vector-tensor theory
In the context of the most general scalar-vector-tensor theory, we study the
stability of static spherically symmetric black holes under linear odd-parity
perturbations. We calculate the action to second order in the linear
perturbations to derive a master equation for these perturbations. For this
general class of models, we obtain the conditions of no-ghost and Laplacian
instability. Then, we study in detail the generalized Regge-Wheeler potential
of particular cases to find their stability conditions.Comment: 22 pages, Mathematica Noteboo
Revisiting the dynamics of interacting vector-like dark energy
We revise the dynamics of interacting vector-like dark energy, a theoretical
framework proposed to explain the accelerated expansion of the universe. By
investigating the interaction between vector-like dark energy and dark matter,
we analyze its effects on the cosmic expansion history and the thermodynamics
of the accelerating universe. Our results demonstrate that the presence of
interaction significantly influences the evolution of vector-like dark energy,
leading to distinct features in its equation of state and energy density. We
compare our findings with observational data and highlight the importance of
considering interactions in future cosmological studies.Comment: 15 pages, 19 figure
Phase-space analysis of torsion-coupled dilatonic ghost condensate
We studied the cosmological dynamics of a dilatonic ghost condensate field as
a source of dark energy, which is non-minimally coupled to gravity through
torsion. We performed a detailed phase-space analysis by finding all the
critical points and their stability conditions. Also, we compared our results
with the latest and Supernovae Ia observational data. In particular, we
found the conditions for the existence of scaling regimes during the dark
matter era. Furthermore, we obtained the conditions for a successful exit from
the scaling regime, such that, at late times, the universe tends towards an
attractor point describing the dark energy-dominated era. These intriguing
features can allow us to alleviate the energy scale problem of dark energy
since, during a scaling regime, the field energy density is not necessarily
negligible at early times.Comment: 11 pages, 14 figures, 3 Table
Growth of matter overdensities in non-minimal torsion-matter coupling theories
We study the evolution of cosmological perturbations around a homogeneous and
isotropic background in the framework of the non-minimal torsion-matter
coupling extension of gravity. We are concerned with the effects of the
non-minimal coupling term on the growth of matter overdensities. Under the
quasi-static approximation, we derive the equation which governs the evolution
of matter density perturbations, and it is shown that the effective
gravitational coupling 'constant' acquires an additional contribution due to
the non-minimal matter-torsion coupling term. In this way, this result
generalizes those previously obtained for the growth of matter overdensities in
the case of minimal gravity. In order to get a feeling of our results we
apply them to the important case of a power-law coupling function, which we
assume to be the responsible for the late-time accelerated expansion in the
dark energy regime. Thereby, analytic solutions for the matter density
perturbation equation in the regime of dark matter dominance and the dark
energy epoch are obtained, along with a complete numerical integration of this
equation. In particular, we show that this model predicts a growth index larger
than those obtained for CDM model, indicating therefore a smaller
growth rate. Concomitantly, we show that the model at hand is potentially
capable in alleviating the existing -tension, being that it can
provide us a prediction which is per cent below the
respective prediction of concordance model.Comment: 13 pages, 6 figure
Dynamics of dark energy in a scalar-vector-torsion theory
We study the cosmological dynamics of dark energy in a scalar-vector-torsion
theory. The vector field is described by the cosmic triad and the scalar field
is of the quintessence type with non-minimal coupling to gravity. The coupling
to gravity is introduced through the interaction between the scalar field and
torsion, where torsion is defined in the context of teleparallel gravity. We
derive the full set of field equations for the
Friedmann-Lema\^{i}tre-Robertson-Walker space-time background and obtain the
associated autonomous system. We obtain the critical points and their stability
conditions, along with the cosmological properties of them. Thus, we show that
the thermal history of the universe is successfully reproduced. Furthermore,
new scaling solutions in which the scalar and vector field densities scale in
the same way as the radiation and matter background fluids have been found.
Finally, we also show that there exist new attractor fixed points whose nature
is mainly vectorial, and which can explain the current accelerated expansion
and therefore the dark energy-domination.Comment: 21 pages, 21 figures, version published in EPJ Plu
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