8 research outputs found
Non-chaotic dynamics in general-relativistic and scalar-tensor cosmology
In the context of scalar-tensor models of dark energy and inflation, the
dynamics of vacuum scalar-tensor cosmology are analysed without specifying the
coupling function or the scalar field potential. A conformal transformation to
the Einstein frame is used and the dynamics of general relativity with a
minimally coupled scalar field are derived for a generic potential. It is shown
that the dynamics are non-chaotic, thus settling an existing debate.Comment: 20 pages, LaTeX, to appear in Class. Quantum Gra
Extended quintessence, inflation, and stable de Sitter spaces
A new gauge-invariant criterion for stability against inhomogeneous
perturbations of de Sitter space is applied to scenarios of dark energy and
inflation in scalar-tensor gravity. The results extend previous studies.Comment: 16 pages, LaTeX, to appear in Class. Quantum Gra
Early Universe Dynamics in Semi-Classical Loop Quantum Cosmology
Within the framework of loop quantum cosmology, there exists a semi-classical
regime where spacetime may be approximated in terms of a continuous manifold,
but where the standard Friedmann equations of classical Einstein gravity
receive non-perturbative quantum corrections. An approximate, analytical
approach to studying cosmic dynamics in this regime is developed for both
spatially flat and positively-curved isotropic universes sourced by a
self-interacting scalar field. In the former case, a direct correspondence
between the classical and semi-classical field equations can be established
together with a scale factor duality that directly relates different expanding
and contracting universes. Some examples of non-singular, bouncing cosmologies
are presented together with a scaling, power-law solution.Comment: 14 pages, In Press, JCA
Cosmological constraints on the dark energy equation of state and its evolution
We have calculated constraints on the evolution of the equation of state of
the dark energy, w(z), from a joint analysis of data from the cosmic microwave
background, large scale structure and type-Ia supernovae. In order to probe the
time-evolution of w we propose a new, simple parametrization of w, which has
the advantage of being transparent and simple to extend to more parameters as
better data becomes available. Furthermore it is well behaved in all asymptotic
limits. Based on this parametrization we find that w(z=0)=-1.43^{+0.16}_{-0.38}
and dw/dz(z=0) = 1.0^{+1.0}_{-0.8}. For a constant w we find that -1.34 < w <
-0.79 at 95% C.L. Thus, allowing for a time-varying w shifts the best fit
present day value of w down. However, even though models with time variation in
w yield a lower chi^2 than pure LambdaCDM models, they do not have a better
goodness-of-fit. Rank correlation tests on SNI-a data also do not show any need
for a time-varying w.Comment: 19 pages, 11 figures, JCAP format, typos corrected, references
update
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