886 research outputs found
Modified gravity and the stability of de Sitter space
Within the context of modified gravity and dark energy scenarios of the
accelerating universe, we study the stability of de Sitter space with respect
to inhomogeneous perturbations using a gauge-independent formalism. In modified
gravity the stability condition is exactly the same that one obtains from a
homogeneous perturbation analysis, while the stability condition in
scalar-tensor gravity is more restrictive.Comment: 5 pages, no figures, RevTeX, to appear in Phys. Rev. D (Rapid
Communications section
Negative energy and stability in scalar-tensor gravity
Linearized gravitational waves in Brans-Dicke and scalar-tensor theories
carry negative energy. A gauge-invariant analysis shows that the background
Minkowski space is stable at the classical level with respect to linear scalar
and tensor inhomogeneous perturbations.Comment: 9 pages, latex, to appear in Phys. Rev.
de Sitter space and the equivalence between f(R) and scalar-tensor gravity
It is shown that, when f'' is non-vanishing, metric f(R) gravity is
completely equivalent to a scalar-tensor theory (with zero Brans-Dicke
parameter) with respect to perturbations of de Sitter space, contrary to
previous expectations. Moreover, the stability conditions of de Sitter space
with respect to homogeneous and inhomogeneous perturbations coincide in most
scalar-tensor theories, as is the case in metric f(R) gravity.Comment: 4 pages, revtex, to appear in Phys. Rev. D. Revised version contains
additional and updated reference
Solar System experiments do not yet veto modified gravity models
The dynamical equivalence between modified and scalar-tensor gravity theories
is revisited and it is concluded that it breaks down in the limit to general
relativity. A gauge-independent analysis of cosmological perturbations in both
classes of theories lends independent support to this conclusion. As a
consequence, the PPN formalism of scalar-tensor gravity and Solar System
experiments do not veto modified gravity, as previously thought.Comment: 7 pages, latex, submitted to Phys. Rev.
Matter instability in modified gravity
The Dolgov-Kawasaki instability discovered in the matter sector of the
modified gravity scenario incorporating a 1/R correction to Einstein gravity is
studied in general f(R) theories. A stability condition is found in the metric
version of these theories to help ruling out models that are unviable from the
theoretical point of view.Comment: 4 pages, revtex, to appear in Phys. Rev. D. In the revised version,
an error concerning the Palatini version of these theories has been corrected
and the references update
Extended Planck Scale
Traditional derivations of the Planck mass ignore the role of charge and spin
in general relativity. From the Kerr-Newman null surface and horizon radii,
quantized charge and spin dependence are introduced in an extended Planck scale
of mass. Spectra emerge with selection rules dependent upon the choice of
Kerr-Newman radius to link with the Compton wavelength. The appearance of the
fine structure constant suggests the possibility of a variation in time of the
extended Planck mass, which may be much larger than the variation in the
traditional one. There is a suggestion of a connection with the value
governing high-energy radiation in Z-boson production and decay.Comment: 9 page
The New Planck Scale: Quantized Spin and Charge Coupled to Gravity
In the standard approach to defining a Planck scale where gravity is brought
into the quantum domain, the Schwarzschild gravitational radius is set equal to
the Compton wavelength. However, ignored thereby are the charge and spin, the
fundamental quantized aspects of matter. The gravitational and null-surface
radii of the Kerr-Newman metric are used to introduce spin and charge into a
new extended Planck scale. The fine structure constant appears in the extended
Planck mass and the recent discovery of the variation with the
evolution of the universe adds further significance. An extended Planck charge
and Planck spin are derived. There is an intriguing suggestion of a connection
with the value governing high-energy radiation in Z-boson production
and decay.Comment: 8 pages, This essay received an "honourable mention" in the 2003
Essay Competition of the Gravity Research Foundatio
Gauge-invariant gravitational wave modes in pre-big bang cosmology
The t<0 branch of pre-big bang cosmological scenarios is subject to a
gravitational wave instability. The unstable behaviour of tensor perturbations
is derived in a very simple way in Hwang's covariant and gauge-invariant
formalism developed for extended theories of gravity. A simple interpretation
of this instability as the effect of an "antifriction" is given, and it is
argued that a universe must eventually enter the expanding phase.Comment: 4 pages, latex, to appear in Eur. Phys. J.
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