10 research outputs found
Cosmological dynamics of Scalar--Tensor Gravity
We study the phase--space of FLRW models derived from Scalar--Tensor Gravity
where the non--minimal coupling is and the effective
potential is . Our analysis allows to unfold many
feature of the cosmology of this class of theories. For example, the evolution
mechanism towards states indistinguishable from GR is recovered and proved to
depend critically on the form of the potential . Also, transient
almost--Friedmann phases evolving towards accelerated expansion and unstable
inflationary phases evolving towards stable ones are found. Some of our results
are shown to hold also for the String-Dilaton action.Comment: 25 pages, 4 figures, 12 tables, submitted to CQ
Compactifying the state space for alternative theories of gravity
In this paper we address important issues surrounding the choice of variables
when performing a dynamical systems analysis of alternative theories of
gravity. We discuss the advantages and disadvantages of compactifying the state
space, and illustrate this using two examples. We first show how to define a
compact state space for the class of LRS Bianchi type I models in -gravity
and compare to a non--compact expansion--normalised approach. In the second
example we consider the flat Friedmann matter subspace of the previous example,
and compare the compact analysis to studies where non-compact
non--expansion--normalised variables were used. In both examples we comment on
the existence of bouncing or recollapsing orbits as well as the existence of
static models.Comment: 18 pages, revised to match published versio
Cosmological dynamics of exponential gravity
We present a detailed investigation of the cosmological dynamics based on
gravity. We apply the dynamical system approach to both
the vacuum and matter cases and obtain exact solutions and their stability in
the finite and asymptotic regimes. The results show that cosmic histories exist
which admit a double de-Sitter phase which could be useful for describing the
early and the late-time accelerating universe.Comment: 17 pages LaTeX, 3 figure
Dynamics of f(R)-cosmologies containing Einstein static models
We study the dynamics of homogeneous isotropic FRW cosmologies with positive
spatial curvature in -gravity, paying special attention to the existence
of Einstein static models and only study forms of for which these
static models have been shown to exist. We construct a compact state space and
identify past and future attractors of the system and recover a previously
discovered future attractor corresponding to an expanding accelerating model.
We also discuss the existence of universes which have both a past and future
bounce, a phenomenon which is absent in General Relativity.Comment: 14 pages, 6 figure
The phase space view of f(R) gravity
We study the geometry of the phase space of spatially flat
Friedmann-Lemaitre-Robertson-Walker models in f(R) gravity, for a general form
of the function f(R). The equilibrium points (de Sitter spaces) and their
stability are discussed, and a comparison is made with the phase space of the
equivalent scalar-tensor theory. New effective Lagrangians and Hamiltonians are
also presented.Comment: 14 pages, 2 figures, published in Classical and Quantum Gravity;
references adde
Reconstruction of the Scalar-Tensor Lagrangian from a LCDM Background and Noether Symmetry
We consider scalar-tensor theories and reconstruct their potential U(\Phi)
and coupling F(\Phi) by demanding a background LCDM cosmology. In particular we
impose a background cosmic history H(z) provided by the usual flat LCDM
parameterization through the radiation (w_{eff}=1/3), matter (w_{eff}=0) and
deSitter (w_{eff}=-1) eras. The cosmological dynamical system which is
constrained to obey the LCDM cosmic history presents five critical points in
each era, one of which corresponding to the standard General Relativity (GR).
In the cases that differ from GR, the reconstructed coupling and potential are
of the form F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m where m is a constant.
This class of scalar tensor theories is also theoretically motivated by a
completely independent approach: imposing maximal Noether symmetry on the
scalar-tensor Lagrangian. This approach provides independently: i) the form of
the coupling and the potential as F(\Phi)\sim \Phi^2 and U(\Phi)\sim F(\Phi)^m,
ii) a conserved charge related to the potential and the coupling and iii)
allows the derivation of exact solutions by first integrals of motion.Comment: Added comments, discussion, references. 15 revtex pages, 5 fugure
Shear dynamics in bianchi i cosmologies with Rn-gravity
We consider the case of Rn-gravity and perform a detailed analysis of the dynamics in Bianchi I cosmologies which exhibit local rotational symmetry (LRS). We find exact solutions and study their behaviour and stability in terms of the values of the parameter n. In particular, we found a set of cosmic histories in which the universe is initially isotropic, then develops shear anisotropies which approaches a constant value. \ua9 2008 World Scientific Publishing Co. Pte. Ltd