18 research outputs found
Coupling parameters and the form of the potential via Noether symmetry
We explore the conditions for the existence of Noether symmetries in the
dynamics of FRW metric, non minimally coupled with a scalar field, in the most
general situation, and with nonzero spatial curvature. When such symmetries are
present we find general exact solution for the Einstein equations. We also show
that non Noether symmetries can be found.
Finally,we present an extension of the procedure to the Kantowski- Sachs
metric which is particularly interesting in the case of degenerate Lagrangian.Comment: 13 pages, no figure
Cosmological models in scalar tensor theories of gravity and observations: a class of general solutions
We consider cosmological models in scalar tensor theories of gravity that
describe an accelerating universe, and we study a family of inverse power law
potentials, for which exact solutions of the Einstein equations are known. We
also compare theoretical predictions of our models with observations. For this
we use the following data: the publicly available catalogs of type Ia
supernovae and high redshift Gamma Ray Bursts, the parameters of large scale
structure determined by the 2-degree Field Galaxy Redshift Survey (2dFGRS), and
measurements of cosmological distances based on the Sunyaev-Zel'dovich effect,
among others.Comment: 26 pages,23 figures, accepted for publication in A&
Dynamics of tachyon field in spatially curved FRW universe
The dynamics of a tachyon field plus a barotropic fluid is investigated in
spatially curved FRW universe. We perform a phase-plane analysis and obtain
scaling solutions accompanying with a discussion on their stability.
Furthermore, we construct the form of scalar potential which may give rise to
stable solutions for spatially open and closed universe separately.Comment: 16 pages, 2 figures, version to be published in PL
A unified approach to scaling solutions in a general cosmological background
Our ignorance about the source of cosmic acceleration has stimulated study of
a wide range of models and modifications to gravity. Cosmological scaling
solutions in any of these theories are privileged because they represent
natural backgrounds relevant to dark energy. We study scaling solutions in a
generalized background in the presence of a scalar field
\vp and a barotropic perfect fluid, where is a Hubble rate and
is a total energy density. The condition for the existence of scaling solutions
restricts the form of Lagrangian to be p=X^{1/n}g(Xe^{n\lambda \vp}), where
X=-g^{\mu\nu} \partial_\mu \vp \partial_\nu \vp /2 and is an arbitrary
function. This is very useful to find out scaling solutions and corresponding
scalar-field potentials in a broad class of dark energy models including
(coupled)-quintessence, ghost-type scalar field, tachyon and k-essence. We
analytically derive the scalar-field equation of state w_\vp and the
fractional density \Omega_\vp and apply it to a number of dark energy models.Comment: 7 pages, no figures, references updated; final version to appear in
PL
WMAP constraints on low redshift evolution of dark energy
The conceptual difficulties associated with a cosmological constant have led
to the investigation of alternative models in which the equation of state
parameter, , of the dark energy evolves with time. We show that
combining the supernova type Ia observations {\it with the constraints from
WMAP observations} restricts large variation of at low redshifts. The
combination of these two observational constraints is stronger than either one.
The results are completely consistent with the cosmological constant as the
source of dark energy.Comment: Final version to appear in MNRAS (Letters); discussion enlarged and
clarifications and references added; 6 pages; 3 figure
Understanding the origin of CMB constraints on Dark Energy
We study the observational constraints of CMB temperature and polarization
anisotropies on models of dark energy, with special focus on models with
variation in properties of dark energy with time. We demonstrate that the key
constraint from CMB observations arises from the location of acoustic peaks. An
additional constraint arises from the limits on Omega_nr from the relative
amplitudes of acoustic peaks. Further, we show that the distance to the last
scattering surface is not how the CMB observations constrain the combination of
parameters for models of dark energy. We also use constraints from Supernova
observations and show that unlike the Gold and Silver samples, the SNLS sample
prefers a region of parameter space that has a significant overlap with the
region preferred by the CMB observations. This is a verification of a
conjecture made by us in an earlier work. We discuss combined constraints from
WMAP5 and SNLS observations. We find that models with w ~ -1 are preferred for
models with a constant equation of state parameters. In case of models with a
time varying dark energy, we show that constraints on evolution of dark energy
density are almost independent of the type of variation assumed for the
equation of state parameter. This makes it easy to get approximate constraints
from CMB observations on arbitrary models of dark energy. Constraints on models
with a time varying dark energy are predominantly due to CMB observations, with
Supernova constraints playing only a marginal role.Comment: 12 pages, 10 figures, accepted for publication in MNRA
Extended Theories of Gravity and their Cosmological and Astrophysical Applications
We review Extended Theories of Gravity in metric and Palatini formalism
pointing out their cosmological and astrophysical application. The aim is to
propose an alternative approach to solve the puzzles connected to dark
components.Comment: 44 pages, 11 figure
Extended Theories of Gravity
Extended Theories of Gravity can be considered a new paradigm to cure
shortcomings of General Relativity at infrared and ultraviolet scales. They are
an approach that, by preserving the undoubtedly positive results of Einstein's
Theory, is aimed to address conceptual and experimental problems recently
emerged in Astrophysics, Cosmology and High Energy Physics. In particular, the
goal is to encompass, in a self-consistent scheme, problems like Inflation,
Dark Energy, Dark Matter, Large Scale Structure and, first of all, to give at
least an effective description of Quantum Gravity. We review the basic
principles that any gravitational theory has to follow. The geometrical
interpretation is discussed in a broad perspective in order to highlight the
basic assumptions of General Relativity and its possible extensions in the
general framework of gauge theories. Principles of such modifications are
presented, focusing on specific classes of theories like f (R)-gravity and
scalar-tensor gravity in the metric and Palatini approaches. The special role
of torsion is also discussed. The conceptual features of these theories are
fully explored and attention is payed to the issues of dynamical and conformal
equivalence between them considering also the initial value problem. A number
of viability criteria are presented considering the post-Newtonian and the
post-Minkowskian limits. In particular, we discuss the problems of neutrino
oscillations and gravitational waves in Extended Gravity. Finally, future
perspectives of Extended Gravity are considered with possibility to go beyond a
trial and error approach.Comment: 184 pages, 3 figures, survey to appear in Physics Report