21 research outputs found
Conditions for the cosmological viability of f(R) dark energy models
We clarify the conditions under which dark energy models whose Lagrangian densities f are written in terms of the Ricci scalar R are cosmologically viable. The existence of a viable matter dominated epoch prior to a late-time acceleration requires that the variable m=Rf_{,RR}/f_{,R} (where f_{,R}=df/dR) satisfies the conditions m(r) approx +0 and dm/dr>-1 at r approx -1 where r=-Rf_{,R}/f. For the existence of a viable late-time acceleration we require instead either (i) m=-r-1, (sqrt{3}-1)/2 0 and n<-1 and are thus cosmologically unacceptable. Similar conclusions can be reached for many other examples discussed in the text. In most cases the standard matter era is replaced by a cosmic expansion with scale factor a=t^{1/2}. We show that the cosmological behavior of f(R) models can be understood by a geometrical approach consisting in studying the m(r) curve on the (r,m) plane. This allows us to classify the f(R) models into four general classes, depending on the existence of a standard matter epoch and on the final accelerated stage. Among several other results, we find that f(R) models can have a strongly phantom attractor but in this case there is no acceptable matter era
Spherically symmetric solutions in f(R)-gravity via Noether Symmetry Approach
We search for spherically symmetric solutions of f(R) theories of gravity via
the Noether Symmetry Approach. A general formalism in the metric framework is
developed considering a point-like f(R)-Lagrangian where spherical symmetry is
required. Examples of exact solutions are given.Comment: 17 pages, to appear in Class. Quant. Gra
Expansion history and f(R) modified gravity
We attempt to fit cosmological data using modified Lagrangians
containing inverse powers of the Ricci scalar varied with respect to the
metric. While we can fit the supernova data well, we confirm the behaviour at medium to high redshifts reported elsewhere and argue
that the easiest way to show that this class of models are inconsistent with
the data is by considering the thickness of the last scattering surface. For
the best fit parameters to the supernova data, the simplest 1/R model gives
rise to a last scattering surface of thickness , inconsistent
with observations.Comment: accepted in JCAP, presentation clarified, results and conclusions
unchange
f(R) actions, cosmic acceleration and local tests of gravity
We study spherically symmetric solutions in f(R) theories and its
compatibility with local tests of gravity. We start by clarifying the range of
validity of the weak field expansion and show that for many models proposed to
address the Dark Energy problem this expansion breaks down in realistic
situations. This invalidates the conclusions of several papers that make
inappropriate use of this expansion. For the stable models that modify gravity
only at small curvatures we find that when the asymptotic background curvature
is large we approximately recover the solutions of Einstein gravity through the
so-called Chameleon mechanism, as a result of the non-linear dynamics of the
extra scalar degree of freedom contained in the metric. In these models one
would observe a transition from Einstein to scalar-tensor gravity as the
Universe expands and the background curvature diminishes. Assuming an adiabatic
evolution we estimate the redshift at which this transition would take place
for a source with given mass and radius. We also show that models of dynamical
Dark Energy claimed to be compatible with tests of gravity because the mass of
the scalar is large in vacuum (e.g. those that also include R^2 corrections in
the action), are not viable.Comment: 26 page
Scalar-Tensor Models of Normal and Phantom Dark Energy
We consider the viability of dark energy (DE) models in the framework of the
scalar-tensor theory of gravity, including the possibility to have a phantom DE
at small redshifts as admitted by supernova luminosity-distance data. For
small , the generic solution for these models is constructed in the form of
a power series in without any approximation. Necessary constraints for DE
to be phantom today and to cross the phantom divide line at small
are presented. Considering the Solar System constraints, we find for the
post-Newtonian parameters that and for
the model to be viable, and (but very close to 1) if the model
has a significantly phantom DE today. However, prospects to establish the
phantom behaviour of DE are much better with cosmological data than with Solar
System experiments. Earlier obtained results for a -dominated universe
with the vanishing scalar field potential are extended to a more general DE
equation of state confirming that the cosmological evolution of these models
rule them out. Models of currently fantom DE which are viable for small can
be easily constructed with a constant potential; however, they generically
become singular at some higher . With a growing potential, viable models
exist up to an arbitrary high redshift.Comment: 30 pages, 4 figures; Matches the published version containing an
expanded discussion of various point
On compatibility of string effective action with an accelerating universe
In this paper, we fully investigate the cosmological effects of the moduli
dependent one-loop corrections to the gravitational couplings of the string
effective action to explain the cosmic acceleration problem in early (and/or
late) universe. These corrections comprise a Gauss-Bonnet (GB) invariant
multiplied by universal non-trivial functions of the common modulus
and the dilaton . The model exhibits several features of cosmological
interest, including the transition between deceleration and acceleration
phases. By considering some phenomenologically motivated ansatzs for one of the
scalars and/or the scale factor (of the universe), we also construct a number
of interesting inflationary potentials. In all examples under consideration, we
find that the model leads only to a standard inflation () when the
numerical coefficient associated with modulus-GB coupling is positive,
while the model can lead also to a non-standard inflation (), if
is negative. In the absence of (or trivial) coupling between the GB term and
the scalars, there is no crossing between the phases, while
this is possible with non-trivial GB couplings, even for constant dilaton phase
of the standard picture. Within our model, after a sufficient amount of e-folds
of expansion, the rolling of both fields and can be small. In
turn, any possible violation of equivalence principle or deviations from the
standard general relativity may be small enough to easily satisfy all
astrophysical and cosmological constraints.Comment: 30 pages, 8 figures; v2 significant changes in notations, appendix
and refs added; v3 significant revisions, refs added; v4 appendix extended,
new refs, published versio
The accelerating universe and a limiting curvature proposal
We consider the hypothesis of a limiting minimal curvature in gravity as a
way to construct a class of theories exhibiting late-time cosmic acceleration.
Guided by the minimal curvature conjecture (MCC) we are naturally lead to a set
of scalar tensor theories in which the scalar is non-minimally coupled both to
gravity and to the matter Lagrangian. The model is compared to the Lambda Cold
Dark Matter concordance model and to the observational data using the gold
SNeIa sample of Riess et. al. (2004). An excellent fit to the data is achieved.
We present a toy model designed to demonstrate that such a new, possibly
fundamental, principle may be responsible for the recent period of cosmological
acceleration. Observational constraints remain to be imposed on these models.Comment: 22 pages, 7 figures; revised version to appear in JCAP; references
adde
f(R) Gravity and scalar-tensor theory
In the present paper we will investigate the relation between scalar-tensor
theory and theories of gravity. Such studies have been performed in the
past for the metric formalism of gravity; here we will consider mainly
the Palatini formalism, where the metric and the connections are treated as
independent quantities. We will try to investigate under which circumstances
theories of gravity are equivalent to scalar-tensor theory and examine
the implications of this equivalence, when it exists.Comment: minor changes to match published version, references adde
The growth of matter perturbations in some scalar-tensor DE models
We consider asymptotically stable scalar-tensor dark energy (DE) models for
which the equation of state parameter tends to zero in the past. The
viable models are of the phantom type today, however this phantomness is milder
than in General Relativity if we take into account the varying gravitational
constant when dealing with the SNIa data. We study further the growth of matter
perturbations and we find a scaling behaviour on large redshifts which could
provide an important constraint. In particular the growth of matter
perturbations on large redshifts in our scalar-tensor models is close to the
standard behaviour , while it is substantially different
for the best-fit model in General Relativity for the same parametrization of
the background expansion. As for the growth of matter perturbations on small
redshifts, we show that in these models the parameter can take absolute values much larger than in models inside
General Relativity. Assuming a constant when is large
would lead to a poor fit of the growth function . This provides another
characteristic discriminative signature for these models.Comment: 13 pages, 7 figures, matches version published in JCA