712 research outputs found
The 1+1+2 formalism for Scalar-Tensor gravity
We use the 1+1+2 covariant approach to clarify a number of aspects of
spherically symmetric solutions of non-minimally coupled scalar tensor
theories. Particular attention is focused on the extension of Birkhoff's
theorem and the nature of quasi-local horizons in this context.Comment: 27 pages, accepted versio
A new approach to reconstruction methods in gravity
We present a new approach of the reconstruction method based on the use of
the cosmic parameters instead of a time law for the scale factor. This allows
the derivation and analysis of a set of new non-trivial cosmological solutions
for -gravity. A number of simple examples are given.Comment: 5 pages, Revtex
A detailed analysis of structure growth in theories of gravity
We investigate the connection between dark energy and fourth order gravity by
analyzing the behavior of scalar perturbations around a
Friedmann-Robertson-Walker background. The evolution equations for scalar
perturbation are derived using the covariant and gauge invariant approach and
applied to two widely studied gravity models. The structure of the
general fourth order perturbation equations and the analysis of scalar
perturbations lead to the discovery of a characteristic signature of fourth
order gravity in the matter power spectrum, the details of which have not seen
before in other works in this area. This could provide a crucial test for
fourth order gravity on cosmological scales.Comment: 27 pages and 35 figure
Global gravitational instability of FLRW backgrounds - interpreting the dark sectors
The standard model of cosmology is based on homogeneous-isotropic solutions
of Einstein's equations. These solutions are known to be gravitationally
unstable to local inhomogeneous perturbations, commonly described as evolving
on a background given by the same solutions. In this picture, the FLRW
backgrounds are taken to describe the average over inhomogeneous perturbations
for all times. We study in the present article the (in)stability of FLRW dust
backgrounds within a class of averaged inhomogeneous cosmologies. We examine
the phase portraits of the latter, discuss their fixed points and orbital
structure and provide detailed illustrations. We show that FLRW cosmologies are
unstable in some relevant cases: averaged models are driven away from them
through structure formation and accelerated expansion. We find support for the
proposal that the dark components of the FLRW framework may be associated to
these instability sectors. Our conclusion is that FLRW cosmologies have to be
considered critically as for their role to serve as reliable models for the
physical background.Comment: 15 pages, 13 figures, 1 table. Matches published version in CQ
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
A dynamical system approach to higher order gravity
The dynamical system approach has recently acquired great importance in the
investigation on higher order theories of gravity. In this talk I review the
main results and I give brief comments on the perspectives for further
developments.Comment: 6 pages, 1 figure, 2 tables, talk given at IRGAC 2006, July 200
Curvature driven acceleration : a utopia or a reality ?
The present work shows that a combination of nonlinear contribution from the
Ricci curvature in Einstein field equations can drive a late time acceleration
of expansion of the universe. The transit from the decelerated to the
accelerated phase of expansion takes place smoothly without having to resort to
a study of asymptotic behaviour. This result emphasizes the need for thorough
and critical examination of models with nonlinear contribution from the
curvature.Comment: 8 pages, 4 figure
Higher-order QED corrections to W-boson mass determination at hadron colliders
The impact of higher-order final-state photonic corrections on the precise
determination of the W-boson mass at the Tevatron and LHC colliders is
evaluated. In the presence of realistic selection criteria, the shift in the W
mass from a fit to the transverse mass distribution is found to be about 10 MeV
in the channel and almost negligible in the
channel. The calculation, which is implemented in a Monte Carlo event generator
for data analysis, can contribute to reduce the uncertainty associated to the W
mass measurement at future hadron collider experiments.Comment: 9 pages, 2 figures, 1 table, RevTe
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