3,373 research outputs found
Extreme objects with arbitrary large mass, or density, and arbitrary size
We consider a generalization of the interior Schwarzschild solution that we
match to the exterior one to build global C^1 models that can have arbitrary
large mass, or density, with arbitrary size. This is possible because of a new
insight into the problem of localizing the center of symmetry of the models and
the use of principal transformations to understand the structure of space.Comment: 20 pages, 6 figures. Fixed one reference. Added a new equatio
Frame dragging and super-energy
We show that the vorticity appearing in stationary vacuum spacetimes is
always related to the existence of a flow of super-energy on the plane
orthogonal to the vorticity vector. This result, toghether with the previously
established link between vorticity and super--energy in radiative (Bondi-Sachs)
spacetimes strength further the case for this latter quantity as the cause of
frame dragging.Comment: 12 pages Latex. To appear in Phys.Rev. D. Typos correcte
Comparing metrics at large: harmonic vs quo-harmonic coordinates
To compare two space-times on large domains, and in particular the global
structure of their manifolds, requires using identical frames of reference and
associated coordinate conditions. In this paper we use and compare two classes
of time-like congruences and corresponding adapted coordinates: the harmonic
and quo-harmonic classes. Besides the intrinsic definition and some of their
intrinsic properties and differences we consider with some detail their
differences at the level of the linearized approximation of the field
equations. The hard part of this paper is an explicit and general determination
of the harmonic and quo-harmonic coordinates adapted to the stationary
character of three well-know metrics, Schwarzschild's, Curzon's and Kerr's, to
order five of their asymptotic expansions. It also contains some relevant
remarks on such problems as defining the multipoles of vacuum solutions or
matching interior and exterior solutions.Comment: 27 pages, no figure
Why does gravitational radiation produce vorticity?
We calculate the vorticity of world--lines of observers at rest in a
Bondi--Sachs frame, produced by gravitational radiation, in a general Sachs
metric. We claim that such an effect is related to the super--Poynting vector,
in a similar way as the existence of the electromagnetic Poynting vector is
related to the vorticity in stationary electrovacum spacetimes.Comment: 9 pages; to appear in Classical and Quantum Gravit
Bel-Robinson tensor and dominant energy property in the Bianchi type I Universe
Within the framework of Bianchi type-I space-time we study the Bel-Robinson
tensor and its impact on the evolution of the Universe. We use different
definitions of the Bel-Robinson tensor existing in the literature and compare
the results. Finally we investigate the so called "dominant super-energy
property" for the Bel-Robinson tensor as a generalization of the usual dominant
energy condition for energy momentum tensors.
Keywords: Bianchi type I model, super-energy tensors
Pacs: 03.65.Pm and 04.20.HaComment: 15 pages, revised version, no figure
On the structure of the new electromagnetic conservation laws
New electromagnetic conservation laws have recently been proposed: in the
absence of electromagnetic currents, the trace of the Chevreton superenergy
tensor, is divergence-free in four-dimensional (a) Einstein spacetimes
for test fields, (b) Einstein-Maxwell spacetimes. Subsequently it has been
pointed out, in analogy with flat spaces, that for Einstein spacetimes the
trace of the Chevreton superenergy tensor can be rearranged in the
form of a generalised wave operator acting on the energy momentum
tensor of the test fields, i.e., . In this
letter we show, for Einstein-Maxwell spacetimes in the full non-linear theory,
that, although, the trace of the Chevreton superenergy tensor can
again be rearranged in the form of a generalised wave operator
acting on the electromagnetic energy momentum tensor, in this case the result
is also crucially dependent on Einstein's equations; hence we argue that the
divergence-free property of the tensor has
significant independent content beyond that of the divergence-free property of
Conserved superenergy currents
We exploit once again the analogy between the energy-momentum tensor and the
so-called ``superenergy'' tensors in order to build conserved currents in the
presence of Killing vectors. First of all, we derive the divergence-free
property of the gravitational superenergy currents under very general
circumstances, even if the superenergy tensor is not divergence-free itself.
The associated conserved quantities are explicitly computed for the
Reissner-Nordstrom and Schwarzschild solutions. The remaining cases, when the
above currents are not conserved, lead to the possibility of an interchange of
some superenergy quantities between the gravitational and other physical fields
in such a manner that the total, mixed, current may be conserved. Actually,
this possibility has been recently proved to hold for the Einstein-Klein-Gordon
system of field equations. By using an adequate family of known exact
solutions, we present explicit and completely non-obvious examples of such
mixed conserved currents.Comment: LaTeX, 19 pages; improved version adding new content to the second
section and some minor correction
Type I vacuum solutions with aligned Papapetrou fields: an intrinsic characterization
We show that Petrov type I vacuum solutions admitting a Killing vector whose
Papapetrou field is aligned with a principal bivector of the Weyl tensor are
the Kasner and Taub metrics, their counterpart with timelike orbits and their
associated windmill-like solutions, as well as the Petrov homogeneous vacuum
solution. We recover all these metrics by using an integration method based on
an invariant classification which allows us to characterize every solution. In
this way we obtain an intrinsic and explicit algorithm to identify them.Comment: 14 pages; v2: added new section, references and tabl
Conserved Matter Superenergy Currents for Hypersurface Orthogonal Killing Vectors
We show that for hypersurface orthogonal Killing vectors, the corresponding
Chevreton superenergy currents will be conserved and proportional to the
Killing vectors. This holds for four-dimensional Einstein-Maxwell spacetimes
with an electromagnetic field that is sourcefree and inherits the symmetry of
the spacetime. A similar result also holds for the trace of the Chevreton
tensor. The corresponding Bel currents have previously been proven to be
conserved and our result can be seen as giving further support to the concept
of conserved mixed superenergy currents. The analogous case for a scalar field
has also previously been proven to give conserved currents and we show, for
completeness, that these currents also are proportional to the Killing vectors.Comment: 13 page
Dynamical laws of superenergy in General Relativity
The Bel and Bel-Robinson tensors were introduced nearly fifty years ago in an
attempt to generalize to gravitation the energy-momentum tensor of
electromagnetism. This generalization was successful from the mathematical
point of view because these tensors share mathematical properties which are
remarkably similar to those of the energy-momentum tensor of electromagnetism.
However, the physical role of these tensors in General Relativity has remained
obscure and no interpretation has achieved wide acceptance. In principle, they
cannot represent {\em energy} and the term {\em superenergy} has been coined
for the hypothetical physical magnitude lying behind them. In this work we try
to shed light on the true physical meaning of {\em superenergy} by following
the same procedure which enables us to give an interpretation of the
electromagnetic energy. This procedure consists in performing an orthogonal
splitting of the Bel and Bel-Robinson tensors and analysing the different parts
resulting from the splitting. In the electromagnetic case such splitting gives
rise to the electromagnetic {\em energy density}, the Poynting vector and the
electromagnetic stress tensor, each of them having a precise physical
interpretation which is deduced from the {\em dynamical laws} of
electromagnetism (Poynting theorem). The full orthogonal splitting of the Bel
and Bel-Robinson tensors is more complex but, as expected, similarities with
electromagnetism are present. Also the covariant divergence of the Bel tensor
is analogous to the covariant divergence of the electromagnetic energy-momentum
tensor and the orthogonal splitting of the former is found. The ensuing {\em
equations} are to the superenergy what the Poynting theorem is to
electromagnetism. See paper for full abstract.Comment: 27 pages, no figures. Typos corrected, section 9 suppressed and more
acknowledgments added. To appear in Classical and Quantum Gravit
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