130 research outputs found
Scalar field phase dynamics in preheating
We study the model of a massive inflaton field coupled to another
scalar filed with interaction term for the first stage
of preheating. We obtain the the behavior of the phase in terms of the
iteration of a simple family of circle maps. When expansion of the universe is
taken into account the qualitative behavior of the phase and growth number
evolution is reminiscent of the behavior found in the case without expansion.Comment: 4 pages, 4 figures, LaTeX; submitted to the Proceedings of Eleventh
Marcel Grossmann Meetin
Dynamical study of the empty Bianchi type I model in generalised scalar-tensor theory
A dynamical study of the generalised scalar-tensor theory in the empty
Bianchi type I model is made. We use a method from which we derive the sign of
the first and second derivatives of the metric functions and examine three
different theories that can all tend towards relativistic behaviours at late
time. We determine conditions so that the dynamic be in expansion and
decelerated at late time.Comment: 18 pages, 3 figures, to appear in General Relativity and Gravitatio
An anti-Schwarzshild solution: wormholes and scalar-tensor solutions
We investigate a static solution with an hyperbolic nature, characterised by
a pseudo-spherical foliation of space. This space-time metric can be perceived
as an anti-Schwarzschild solution, and exhibits repulsive features. It belongs
to the class of static vacuum solutions termed "a degenerate static solution of
class A". In the present work we review its fundamental features, discuss the
existence of generalised wormholes, and derive its extension to scalar-tensor
gravity theories in general.Comment: 3 pages, contribution to the proceedings of the Spanish Relativity
Meeting-ERE200
Self-interacting scalar field cosmologies: unified exact solutions and symmetries
We investigate a mechanism that generates the exact solutions of scalar field
cosmologies in a unified way. The procedure investigated here permits to
recover allmost all known solutions, and allows one to derive new solutions as
well. In particular, we derive and discuss one novel solution defined in terms
of the Lambert function. The solutions are organized in a classification which
depends on the choice of a generating function which we have denoted by
that reflects the underlying thermodynamics of the model. We also
analyse and discuss the existence of form invariances and dualities between
solutions. A general way of defining the latter in an appropriate fashion for
scalar fields is put forward.Comment: 6 figures, 30 page
What if Newton's Gravitational Constant was negative?
In this work, we seek a cosmological mechanism that may define the sign of
the effective gravitational coupling constant, {\em G}. To this end, we
consider general scalar-tensor gravity theories as they provide the field
theory natural framework for the variation of the gravitational coupling. We
find that models with a quadratic potential naturally stabilize the value of
{\em G} into the positive branch of the evolution and further, that de Sitter
inflation and a relaxation to General Relativity is easily attained.Comment: 16 pages, 16 figure
Brane Isotropisation in Extra-Dimensional Tolman-Bondi Universe
We consider the dynamics of a 3-brane embedded in an extra-dimensional
Tolman-Bondi Universe where the origin of space plays a special role. The
embedding is chosen such that the induced matter distribution on the brane
respects the spherical symmetry of matter in the extra dimensional space. The
mirage cosmology on the probe brane is studied, resulting in an inhomogeneous
and anisotropic four dimensional cosmology where the origin of space is also
special. We then focus on the spatial geometry around the origin and show that
the induced geometry, which is initially inhomogeneous and anisotropic,
converges to an isotropic and homogeneous Friedmann-Lemaitre 4d space-time. For
instance, when a 3-brane is embedded in a 5d matter dominated model, the 4d
dynamics around the origin converge to a Friedmann-Lemaitre Universe in a
radiation dominated epoch. We analyse this isotropisation process and show that
it is a late time attractor.Comment: 16 pages, 8 figures, one reference adde
Reheating induced by competing decay modes
We address the problem of studying the decay of the inflaton field to
another scalar field through parametric resonance in the case of a
coupling that involves several decay modes. This amounts to the presence of
extra harmonic terms in the perturbation of the field dynamics. For the
case of two frequencies we compute the geometry of the resonance regions, which
is significantly altered due to the presence of non-cuspidal resonance regions
associated to higher harmonics and to the emergence of instability `pockets'.
We discuss the effect of this change in the efficiency of the energy transfer
process for the simplest case of a coupling given by a combination of the two
interaction terms of homogeneous degree usually considered in the literature.
We find that the presence of higher harmonics has limited cosmological
implications.Comment: 14 pages, 4 figures Added references. Corrected typo
Testing -Free f(Q) Cosmology
We study a model of Symmetric Teleparallel gravity that is able to account
for the current accelerated expansion of the universe without the need for dark
energy component. We investigate this model by making use of dynamical system
analysis techniques to identify the regions of the parameter space with viable
cosmologies and constrain it using type Ia supernova (SnIa), cosmic microwave
background (CMB) data and make forecasts using standard siren (SS) events. We
conclude that this model is disfavored with respect to CDM and
forthcoming standard siren events can be decisive in testing the viability of
the model.Comment: 9 pages, 3 figure
Plane-symmetric inhomogeneous Brans-Dicke cosmology with an equation of state
We present a new exact solution in Brans-Dicke theory. The solution describes
inhomogeneous plane-symmetric perfect fluid cosmological model with an equation
of state . Some main properties of the solution are discussed.Comment: 6 pages, Late
Scaling solutions from interacting fluids
We examine the dynamical implications of an interaction between some of the
fluid components of the universe. We consider the combination of three matter
components, one of which is a perfect fluid and the other two are interacting.
The interaction term generalizes the cases found in scalar field cosmologies
with an exponential potential. We find that attracting scaling solutions are
obtained in several regions of parameter space, that oscillating behaviour is
possible, and that new curvature scaling solutions exist. We also discuss the
inflationary behaviour of the solutions and present some of the constraints on
the strength of the coupling, namely those arising from nucleosynthesis.Comment: RevTeX, 21 pages, 8 figure
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