1,404 research outputs found
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
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
User requirements for multimedia indexing and retrieval of unedited audio-visual footage - RUSHES
Multimedia analysis and reuse of raw un-edited audio visual content known as rushes is gaining acceptance by a large number of research labs and companies. A set of research projects are considering multimedia indexing, annotation, search and retrieval in the context of European funded research, but only the FP6 project RUSHES is focusing on automatic semantic annotation, indexing and retrieval of raw and un-edited audio-visual content. Even professional content creators and providers as well as home-users are dealing with this type of content and therefore novel technologies for semantic search and retrieval are required. As a first result of this project, the user requirements and possible user-scenarios are presented in this paper. These results lay down the foundation for the research and development of a multimedia search engine particularly dedicated to the specific needs of the users and the content
Corrections and Comments on the Multipole Moments of Axisymmetric Electrovacuum Spacetimes
Following the method of Hoenselaers and Perj\'{e}s we present a new corrected
and dimensionally consistent set of multipole gravitational and electromagnetic
moments for stationary axisymmetric spacetimes. Furthermore, we use our results
to compute the multipole moments, both gravitational and electromagnetic, of a
Kerr-Newman black hole.Comment: This is a revised and corrected versio
Reply to "Can gravitational dynamics be obtained by diffeomorphism invariance of action?"
In a previous work we showed that, in a suitable setting, one can use
diffeomorphism invariance in order to derive gravitational field equations from
boundary terms of the gravitational action. Standing by our results we reply
here to a recent comment questioning their validity.Comment: Accepted for publication in PR
Covariant conservation of energy momentum in modified gravities
An explicit proof of the vanishing of the covariant divergence of the
energy-momentum tensor in modified theories of gravity is presented. The
gravitational action is written in arbitrary dimensions and allowed to depend
nonlinearly on the curvature scalar and its couplings with a scalar field. Also
the case of a function of the curvature scalar multiplying a matter Lagrangian
is considered. The proof is given both in the metric and in the first-order
formalism, i.e. under the Palatini variational principle. It is found that the
covariant conservation of energy-momentum is built-in to the field equations.
This crucial result, called the generalized Bianchi identity, can also be
deduced directly from the covariance of the extended gravitational action.
Furthermore, we demonstrate that in all of these cases, the freely falling
world lines are determined by the field equations alone and turn out to be the
geodesics associated with the metric compatible connection. The independent
connection in the Palatini formulation of these generalized theories does not
have a similar direct physical interpretation. However, in the conformal
Einstein frame a certain bi-metricity emerges into the structure of these
theories. In the light of our interpretation of the independent connection as
an auxiliary variable we can also reconsider some criticisms of the Palatini
formulation originally raised by Buchdahl.Comment: 8 pages. v2: more discussio
Projectable Horava-Lifshitz gravity in a nutshell
Approximately one year ago Horava proposed a power-counting renormalizable
theory of gravity which abandons local Lorentz invariance. The proposal has
been received with growing interest and resulted in various different versions
of Horava-Lifshitz gravity theories, involving a colourful potpourri of new
terminology. In this proceedings contribution we first motivate and briefly
overview the various different approaches, clarifying their differences and
similarities. We then focus on a model referred to as projectable
Horava-Lifshitz gravity and summarize the key results regarding its viability.Comment: 8 pages, no figures, to appear in the proceedings of First
Mediterranean Conference on Classical and Quantum Gravity Conference (MCCQG),
Kolymbari (Crete, Greece), September 14-18, 200
Pathological behaviour of the scalar graviton in Ho\v{r}ava-Lifshitz gravity
We confirm the recent claims that, in the infrared limit of
Ho\v{r}ava-Lifshitz gravity, the scalar graviton becomes a ghost if the sound
speed squared is positive on the flat de Sitter and Minkowski background. In
order to avoid the ghost and tame the instability, the sound speed squared
should be negative and very small, which means that the flow parameter
should be very close to its General Relativity (GR) value. We
calculate the cubic interactions for the scalar graviton which are shown to
have a similar structure with those of the curvature perturbation in
k-inflation models. The higher order interactions become increasing important
for a smaller sound speed squared, that is, when the theory approaches GR. This
invalidates any linearized analysis and any predictability is lost in this
limit as quantum corrections are not controllable. This pathological behaviour
of the scalar graviton casts doubt on the validity of the projectable version
of the theory.Comment: 7 pages, references added; v3: Typos corrected, minor changes to text
and precise determination of the strong coupling scale. Replaced to match
published version
Horava-Lifshitz gravity: a status report
This is intended to be a brief introduction and overview of Horava-Lifshitz
gravity. The motivation and all of the various version of the theory (to date)
are presented. The dynamics of the theory are discussed in some detail, with a
focus on low energy viability and consistency, as these have been the issues
that attracted most of the attention in the literature so far. Other properties
of the theory and developments within its framework are also covered, such as:
its relation to Einstein-aether theory, cosmology, and future perspectives.Comment: 17 pages, no figures, based on talk given at the 14th Conference on
Recent Developments in Gravity (NEBXIV), Ioannina, Greece, 8-11 Jun 2010; v2:
minor changes to match published version, references adde
The present universe in the Einstein frame, metric-affine R+1/R gravity
We study the present, flat isotropic universe in 1/R-modified gravity. We use
the Palatini (metric-affine) variational principle and the Einstein
(metric-compatible connected) conformal frame. We show that the energy density
scaling deviates from the usual scaling for nonrelativistic matter, and the
largest deviation occurs in the present epoch. We find that the current
deceleration parameter derived from the apparent matter density parameter is
consistent with observations. There is also a small overlap between the
predicted and observed values for the redshift derivative of the deceleration
parameter. The predicted redshift of the deceleration-to-acceleration
transition agrees with that in the \Lambda-CDM model but it is larger than the
value estimated from SNIa observations.Comment: 11 pages; published versio
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