664 research outputs found

### Curvature Quintessence

The issues of quintessence and cosmic acceleration can be discussed in the
framework of higher order theories of gravity. We can define effective pressure
and energy density directly connected to the Ricci scalar of curvature of a
generic fourth order theory and then ask for the conditions to get an
accelerated expansion. Exact accelerated expanding solutions can be achieved
for several fourth order theories so that we get an alternative scheme to the
standard quintessence scalar field, minimally coupled to gravity, usually
adopted. We discuss also conformal transformations in order to see the links of
quintessence between the Jordan and Einstein frames.Comment: 10 pages, LATEX files, to appear on IJMP

### Hojman Symmetry Approach for Scalar-Tensor Cosmology

Scalar-tensor Cosmologies can be dealt under the standard of the Hojman
conservation theorem that allows to fix the form of the coupling $F(\phi)$, of
the potential $V(\phi)$ and to find out exact solutions for related
cosmological models. Specifically, the existence of a symmetry transformation
vector for the equations of motion gives rise to a Hojman conserved quantity on
the corresponding minisuperpace and exact solutions for the cosmic scale factor
$a$ and the scalar field $\phi$ can be achieved. In particular, we take
advantage of the fact that minimally coupled solutions, previously obtained in
the Einstein frame, can be conformally transformed in non-minimally coupled
solutions in the Jordan frame. Some physically relevant examples are worked
out.Comment: 6 pages, 4 figures, to appear in Phys. Lett.

### The fate of Schwarzschild-de Sitter Black Holes in $f(R)$ gravity

The semiclassical effects of antievaporating black holes can be discussed in
the framework of $f(R)$ gravity. In particular, the
Bousso-Hawking-Nojiri-Odinstov antievaporation instability of degenerate
Schwarzschild-de Sitter black holes (the so called Nariai space-time) leads to
a dynamical increasing of black hole horizon in $f(R)$ gravity. This phenomenon
causes the following transition: emitting marginally trapped surfaces become
space-like surfaces before the effective Bekenstein-Hawking emission time. As a
consequence, Bousso-Hawking thermal radiation cannot be emitted in an
antievaporating Nariai black hole. Possible implications in cosmology and black
hole physics are also discussed.Comment: 9 pages, to appear in Mod. Phys. Lett.

### Noether Symmetry Approach in $f(T,B)$ teleparallel cosmology

We consider the cosmology derived from $f(T,B)$ gravity where $T$ is the
torsion scalar and $B=\frac{2}{e}\partial_{\mu}(e T^{\mu})$ a boundary term. In
particular we discuss how it is possible to recover, under the same standard,
the teleparallel $f(T)$ gravity, the curvature $f(R)$ gravity and the
teleparallel-curvature $f(R,T)$ gravity, which are particular cases of
$f(T,B)$. We adopt the Noether Symmetry Approach to study the related dynamical
systems and to find out cosmological solutions.Comment: 21 page

### External stability for Spherically Symmetric Solutions in Lorentz Breaking Massive Gravity

We discuss spherically symmetric solutions for point-like sources in
Lorentz-breaking massive gravity theories. This analysis is valid for
St\"uckelberg's effective field theory formulation, for Lorentz Breaking
Massive Bigravity and general extensions of gravity leading to an extra term
$-Sr^{\gamma}$ added to the Newtonian potential. The approach consists in
analyzing the stability of the geodesic equations, at the first order
(deviation equation). The main result is a strong constrain in the space of
parameters of the theories. This motivates higher order analysis of geodesic
perturbations in order to understand if a class of spherically symmetric
Lorentz-breaking massive gravity solutions, for self-gravitating systems,
exists. Stable and phenomenologically acceptable solutions are discussed in the
no-trivial case $S\neq 0$.Comment: 10 pg, 1 figure, to appear in Int. Jou. Theor. Phy

### Gravitational waves in modified teleparallel theories of gravity

Teleparallel theory of gravity and its modifications have been studied
extensively in literature. However, gravitational waves has not been studied
enough in the framework of teleparallelism. In the present study, we discuss
gravitational waves in general theories of teleparallel gravity containing the
torsion scalar $T$, the boundary term $B$ and a scalar field $\phi$. The goal
is to classify possible new polarizations generalizing results presented in
Ref.[15]. We show that, if the boundary term is minimally coupled to the
torsion scalar and the scalar field, gravitational waves have the same
polarization modes of General Relativity.Comment: 9 pages, to be published in Eur.Phys.J.

### The emission of Gamma Ray Bursts as a test-bed for modified gravity

The extreme physical conditions of Gamma Ray Bursts can constitute a useful
observational laboratory to test theories of gravity where very high curvature
regimes are involved. Here we propose a sort of curvature engine capable, in
principle, of explaining the huge energy emission of Gamma Ray Bursts.
Specifically, we investigate the emission of radiation by charged particles
non-minimally coupled to the gravitational background where higher order
curvature invariants are present. The coupling gives rise to an additional
force inducing a non-geodesics motion of particles. This fact allows a strong
emission of radiation by gravitationally accelerated particles. As we will show
with some specific model, the energy emission is of the same order of magnitude
of that characterizing the Gamma Ray Burst physics. Alternatively, strong
curvature regimes can be considered as a natural mechanism for the generation
of highly energetic astrophysical events. Possible applications to cosmology
are discussed.Comment: 4 pages, 1 figure, accepted for publication in Phys. Lett.

### Exact cosmological solutions from Hojman conservation quantities

We present a new approach to find exact solutions for cosmological models. By
requiring the existence of a symmetry transformation vector for the equations
of motion of the given cosmological model (without using either Lagrangian or
Hamiltonian), one can find corresponding Hojman conserved quantities. With the
help of these conserved quantities, the analysis of the cosmological model can
be simplified. In the case of quintessence scalar-tensor models, we show that
the Hojman conserved quantities exist for a wide range of V(\phi)-potentials
and allow to find exact solutions for the cosmic scale factor and the scalar
field. Finally, we investigate the general cosmological behavior of solutions
by adopting a phase-space view.Comment: 11 pages, 13 figures, accepted for publication in Phys. Lett.

### Information entropy and dark energy evolution

The information entropy is here investigated in the context of early and late
cosmology under the hypothesis that distinct phases of universe evolution are
entangled between them. The approach is based on the \emph{entangled state
ansatz}, representing a coarse-grained definition of primordial \emph{dark
temperature} associated to an \emph{effective entangled energy density}. The
dark temperature definition comes from assuming either Von Neumann or linear
entropy as sources of cosmological thermodynamics. We interpret the involved
information entropies by means of probabilities of forming structures during
cosmic evolution. Following this recipe, we propose that quantum entropy is
simply associated to the thermodynamical entropy and we investigate the
consequences of our approach using the adiabatic sound speed. As byproducts, we
analyze two phases of universe evolution: the late and early stages. To do so,
we first recover that dark energy reduces to a pure cosmological constant, as
zero-order entanglement contribution, and second that inflation is
well-described by means of an effective potential. In both cases, we infer
numerical limits which are compatible with current observations.Comment: 12 pages, 1 figur

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