16 research outputs found
Dark Energy Dominance and Cosmic Acceleration in First Order Formalism
The current accelerated universe could be produced by modified gravitational
dynamics as it can be seen in particular in its Palatini formulation. We
analyze here a specific non-linear gravity-scalar system in the first order
Palatini formalism which leads to a FRW cosmology different from the purely
metric one. It is shown that the emerging FRW cosmology may lead either to an
effective quintessence phase (cosmic speed-up) or to an effective phantom
phase. Moreover, the already known gravity assisted dark energy dominance
occurs also in the first order formalism. Finally, it is shown that a dynamical
theory able to resolve the cosmological constant problem exists also in this
formalism, in close parallel with the standard metric formulation.Comment: 21 pages, LaTeX file, no figures. Replaced version to be published on
Phys. Rev.
Post-Newtonian Parameters from Alternative Theories of Gravity
Alternative theories of gravity have been recently studied in connection with
their cosmological applications, both in the Palatini and in the metric
formalism. The aim of this paper is to propose a theoretical framework (in the
Palatini formalism) to test these theories at the solar system level and
possibly at the galactic scales. We exactly solve field equations in vacuum and
find the corresponding corrections to the standard general relativistic
gravitational field. On the other hand, approximate solutions are found in
matter cases starting from a Lagrangian which depends on a phenomenological
parameter. Both in the vacuum case and in the matter case the deviations from
General Relativity are controlled by parameters that provide the Post-Newtonian
corrections which prove to be in good agreement with solar system experiments.Comment: 17 pages, no figure
Accelerated Cosmological Models in Ricci squared Gravity
Alternative gravitational theories described by Lagrangians depending on
general functions of the Ricci scalar have been proven to give coherent
theoretical models to describe the experimental evidence of the acceleration of
universe at present time. In this paper we proceed further in this analysis of
cosmological applications of alternative gravitational theories depending on
(other) curvature invariants. We introduce Ricci squared Lagrangians in minimal
interaction with matter (perfect fluid); we find modified Einstein equations
and consequently modified Friedmann equations in the Palatini formalism. It is
striking that both Ricci scalar and Ricci squared theories are described in the
same mathematical framework and both the generalized Einstein equations and
generalized Friedmann equations have the same structure. In the framework of
the cosmological principle, without the introduction of exotic forms of dark
energy, we thus obtain modified equations providing values of w_{eff}<-1 in
accordance with the experimental data. The spacetime bi-metric structure plays
a fundamental role in the physical interpretation of results and gives them a
clear and very rich geometrical interpretation.Comment: New version: 26 pages, 1 figure (now included), Revtex
Accelerated Cosmological Models in First-Order Non-Linear Gravity
The evidence of the acceleration of universe at present time has lead to
investigate modified theories of gravity and alternative theories of gravity,
which are able to explain acceleration from a theoretical viewpoint without the
need of introducing dark energy. In this paper we study alternative
gravitational theories defined by Lagrangians which depend on general functions
of the Ricci scalar invariant in minimal interaction with matter, in view of
their possible cosmological applications. Structural equations for the
spacetimes described by such theories are solved and the corresponding field
equations are investigated in the Palatini formalism, which prevents
instability problems. Particular examples of these theories are also shown to
provide, under suitable hypotheses, a coherent theoretical explanation of
earlier results concerning the present acceleration of the universe and
cosmological inflation. We suggest moreover a new possible Lagrangian,
depending on the inverse of sinh(R), which gives an explanation to the present
acceleration of the universe.Comment: 23 pages, Revtex4 fil
Conformal aspects of Palatini approach in Extended Theories of Gravity
The debate on the physical relevance of conformal transformations can be
faced by taking the Palatini approach into account to gravitational theories.
We show that conformal transformations are not only a mathematical tool to
disentangle gravitational and matter degrees of freedom (passing from the
Jordan frame to the Einstein frame) but they acquire a physical meaning
considering the bi-metric structure of Palatini approach which allows to
distinguish between spacetime structure and geodesic structure. Examples of
higher-order and non-minimally coupled theories are worked out and relevant
cosmological solutions in Einstein frame and Jordan frames are discussed
showing that also the interpretation of cosmological observations can
drastically change depending on the adopted frame