998 research outputs found
Curvature singularities, tidal forces and the viability of Palatini f(R) gravity
In a previous paper we showed that static spherically symmetric objects
which, in the vicinity of their surface, are well-described by a polytropic
equation of state with 3/2<Gamma<2 exhibit a curvature singularity in Palatini
f(R) gravity. We argued that this casts serious doubt on the validity of
Palatini f(R) gravity as a viable alternative to General Relativity. In the
present paper we further investigate this characteristic of Palatini f(R)
gravity in order to clarify its physical interpretation and consequences.Comment: 15 pages. CQG in press. Part of the material moved to an appendix,
discussion on the meV scale predictions of Palatini f(R) gravity adde
Extending Sibgatullin's ansatz for the Ernst potential to generate a richer family of axially symmetric solutions of Einstein's equations
The scope of this talk is to present some preliminary results on an effort,
currently in progress, to generate an exact solution of Einstein's equation,
suitable for describing spacetime around a rotating compact object.
Specifically, the form of the Ernst potential on the symmetry axis and its
connection with the multipole moments is discussed thoroughly. The way to
calculate the multipole moments of spacetime directly from the value of the
Ernst potential on the symmetry axis is presented. Finally, a mixed ansatz is
formed for the Ernst potential including parameters additional to the ones
dictated by Sibgatullin. Thus, we believe that this talk can also serve as a
comment on choosing the appropriate ansatz for the Ernst potential.Comment: Talk given in the 11th Conference on Recent Developments in Gravity,
2-5 June 2004, Lesbos, Greec
f(R) gravity, torsion and non-metricity
For both f(R) theories of gravity with an independent symmetric connection
(no torsion), usually referred to as Palatini f(R) gravity theories, and for
f(R) theories of gravity with torsion but no non-metricity, called U4 theories,
it has been shown that the independent connection can actually be eliminated
algebraically, as long as this connection does not couple to matter.
Remarkably, the outcome in both case is the same theory, which is dynamically
equivalent with an \omega_0=-3/2 Brans--Dicke theory. It is shown here that
even for the most general case of an independent connection with both
non-metricity and torsion one arrives at exactly the same theory as in the more
restricted cases. This generalizes the previous results and explains why
assuming that either the torsion or the the non-metricity vanishes ultimately
leads to the same theory. It also demonstrates that f(R) actions cannot support
an independent connection which carries dynamical degrees of freedom,
irrespectively of how general this connection is, at least as long as there is
no connection-matter coupling.Comment: v2: slightly shortened version published in CQG as a Fast Track
Communicatio
Horava Gravity and Gravitons at a Conformal Point
Recently Horava proposed a renormalizable gravity theory with higher
derivatives by abandoning the Lorenz invariance in UV. Here, I study the Horava
model at , where an anisotropic Weyl symmetry exists in the UV
limit, in addition to the foliation-preserving diffeomorphism. By considering
linear perturbations around Minkowski vacuum, I show that the scalar graviton
mode is completely disappeared and only the usual tensor graviton modes remain
in the physical spectrum. The existence of the UV conformal symmetry is unique
to the theory with the detailed balance and it is quite probable that
be the UV fixed point. This situation is analogous to
, which is Lorentz invariant in the IR limit and is believed to be
the IR fixed point.Comment: Added comments and references, Accepted in GER
The phase portrait of a matter bounce in Horava-Lifshitz cosmology
The occurrence of a bounce in FRW cosmology requires modifications of general
relativity. An example of such a modification is the recently proposed
Horava-Lifshitz theory of gravity, which includes a ``dark radiation'' term
with a negative coefficient in the analog of the Friedmann equation. This paper
describes a phase space analysis of models of this sort with the aim of
determining to what extent bouncing solutions can occur. A simplification,
valid in the relevant region, allows a reduction of the dimension of phase
space so that visualization in three dimensions is possible. It is found that a
bounce is possible, but not generic in models under consideration. Apart from
previously known bouncing solutions some new ones are also described. Other
interesting solutions found include ones which describe a novel sort of
oscillating universes.Comment: 14 pages, 8 figure
f(R) Gravity with Torsion: The Metric-Affine Approach
The role of torsion in f(R) gravity is considered in the framework of
metric-affine formalism. We discuss the field equations in empty space and in
presence of perfect fluid matter taking into account the analogy with the
Palatini formalism. As a result, the extra curvature and torsion degrees of
freedom can be dealt as an effective scalar field of fully geometric origin.
From a cosmological point of view, such a geometric description could account
for the whole Dark Side of the Universe.Comment: 12 page
Neutron stars in generalized f(R) gravity
Quartic gravity theory is considered with the Einstein-Hilbert Lagrangean
being Ricci\'s tensor and R
the curvature scalar. The parameters and are taken of order 1 km
Arguments are given which suggest that the effective theory so obtained may be
a plausible approximation of a viable theory. A numerical integration is
performed of the field equations for a free neutron gas. As in the standard
Oppenheimer-Volkoff calculation the star mass increases with increasing central
density until about 1 solar mass and then decreases. However a dramatic
difference exists in the behaviour of the baryon number, which increases
monotonically. The calculation suggests that the theory allows stars in
equilibrium with arbitrary baryon number, no matter how large.Comment: Keywords: stars, neutron stars; gravity; modified gravity Accepted in
Astrophysics and Space Scienc
Jumping from Metric f(R) to Scalar-Tensor Theories and the relations between their post-Newtonian Parameters
We review the dynamical equivalence between gravity in the metric
formalism and scalar-tensor gravity, and use this equivalence to deduce the
post-Newtonian parameters and for a theory, obtaining a
result that is different with respect to that known in the literature. Then, we
obtain explicit expressions of these paremeters in terms of the mass of the
scalar field (or, differently speaking, the mass of the additional scalar
degree of freedom associated to a theory) which can be used to constrain
gravity by means of current observations.Comment: 10 pages, 1 table, no figures Accepted for publication in CQ
Acceleration of the universe in the Einstein frame of a metric-affine f(R) gravity
We show that inflation and current cosmic acceleration can be generated by a
metric-affine f(R) gravity formulated in the Einstein conformal frame, if the
gravitational Lagrangian L(R) contains both positive and negative powers of the
curvature scalar R. In this frame, we give the equations for the expansion of
the homogeneous and isotropic matter-dominated universe in the case
L(R)=R+{R^3}/{\beta^2}-{\alpha^2}/{3R}, where \alpha and \beta are constants.
We also show that gravitational effects of matter in such a universe at very
late stages of its expansion are weakened by a factor that tends to 3/4, and
the energy density of matter \epsilon scales the same way as in the \Lambda-CDM
model only when \kappa*\epsilon<<\alpha.Comment: 12 pages; published versio
theory and geometric origin of the dark sector in Horava-Lifshitz gravity
Inclusion of term in the action of Horava-Lifshitz quantum gravity
with projectability but without detailed balance condition is investigated,
where denotes the 3-spatial dimensional Ricci scalar. Conditions for the
spin-0 graviton to be free of ghosts and instability are studied. The
requirement that the theory reduce to general relativity in the IR makes the
scalar mode unstable in the Minkowski background but stable in the de Sitter.
It is remarkable that the dark sector, dark matter and dark energy, of the
universe has a naturally geometric origin in such a setup. Bouncing universes
can also be constructed. Scalar perturbations in the FRW backgrounds with
non-zero curvature are presented.Comment: Mod. Phys. Lett. A26, 387-398 (2011
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