453 research outputs found
The Einstein 3-form G_a and its equivalent 1-form L_a in Riemann-Cartan space
The definition of the Einstein 3-form G_a is motivated by means of the
contracted 2nd Bianchi identity. This definition involves at first the complete
curvature 2-form. The 1-form L_a is defined via G_a = L^b \wedge #(o_b \wedge
o_a). Here # denotes the Hodge-star, o_a the coframe, and \wedge the exterior
product. The L_a is equivalent to the Einstein 3-form and represents a certain
contraction of the curvature 2-form. A variational formula of Salgado on
quadratic invariants of the L_a 1-form is discussed, generalized, and put into
proper perspective.Comment: LaTeX, 13 Pages. To appear in Gen. Rel. Gra
Linear Einstein equations and Kerr-Schild maps
We prove that given a solution of the Einstein equations for the
matter field , an autoparallel null vector field and a solution
of the linearized Einstein equation on the
given background, the Kerr-Schild metric ( arbitrary constant) is an exact solution of the Einstein equation for the
energy-momentum tensor . The mixed form of the Einstein equation for
Kerr-Schild metrics with autoparallel null congruence is also linear. Some more
technical conditions hold when the null congruence is not autoparallel. These
results generalize previous theorems for vacuum due to Xanthopoulos and for
flat seed space-time due to G\"{u}rses and G\"{u}rsey.Comment: 9 pages, accepted by Class. Quant. Gra
A Categorical Equivalence between Generalized Holonomy Maps on a Connected Manifold and Principal Connections on Bundles over that Manifold
A classic result in the foundations of Yang-Mills theory, due to J. W.
Barrett ["Holonomy and Path Structures in General Relativity and Yang-Mills
Theory." Int. J. Th. Phys. 30(9), (1991)], establishes that given a
"generalized" holonomy map from the space of piece-wise smooth, closed curves
based at some point of a manifold to a Lie group, there exists a principal
bundle with that group as structure group and a principal connection on that
bundle such that the holonomy map corresponds to the holonomies of that
connection. Barrett also provided one sense in which this "recovery theorem"
yields a unique bundle, up to isomorphism. Here we show that something stronger
is true: with an appropriate definition of isomorphism between generalized
holonomy maps, there is an equivalence of categories between the category whose
objects are generalized holonomy maps on a smooth, connected manifold and whose
arrows are holonomy isomorphisms, and the category whose objects are principal
connections on principal bundles over a smooth, connected manifold. This result
clarifies, and somewhat improves upon, the sense of "unique recovery" in
Barrett's theorems; it also makes precise a sense in which there is no loss of
structure involved in moving from a principal bundle formulation of Yang-Mills
theory to a holonomy, or "loop", formulation.Comment: 20 page
Post-Newtonian extension of the Newton-Cartan theory
The theory obtained as a singular limit of General Relativity, if the
reciprocal velocity of light is assumed to tend to zero, is known to be not
exactly the Newton-Cartan theory, but a slight extension of this theory. It
involves not only a Coriolis force field, which is natural in this theory
(although not original Newtonian), but also a scalar field which governs the
relation between Newtons time and relativistic proper time. Both fields are or
can be reduced to harmonic functions, and must therefore be constants, if
suitable global conditions are imposed. We assume this reduction of
Newton-Cartan to Newton`s original theory as starting point and ask for a
consistent post-Newtonian extension and for possible differences to usual
post-Minkowskian approximation methods, as developed, for example, by
Chandrasekhar. It is shown, that both post-Newtonian frameworks are formally
equivalent, as far as the field equations and the equations of motion for a
hydrodynamical fluid are concerned.Comment: 13 pages, LaTex, to appear in Class. Quantum Gra
Mathisson-Papapetrou equations in metric and gauge theories of gravity in a Lagrangian formulation
We present a simple method to derive the semiclassical equations of motion
for a spinning particle in a gravitational field. We investigate the cases of
classical, rotating particles (pole-dipole particles), as well as particles
with intrinsic spin. We show that, starting with a simple Lagrangian, one can
derive equations for the spin evolution and momentum propagation in the
framework of metric theories of gravity and in theories based on a
Riemann-Cartan geometry (Poincare gauge theory), without explicitly referring
to matter current densities (spin and energy-momentum). Our results agree with
those derived from the multipole expansion of the current densities by the
conventional Papapetrou method and from the WKB analysis for elementary
particles.Comment: 28 page
3-dimensional Cauchy-Riemann structures and 2nd order ordinary differential equations
The equivalence problem for second order ODEs given modulo point
transformations is solved in full analogy with the equivalence problem of
nondegenerate 3-dimensional CR structures. This approach enables an analog of
the Feffereman metrics to be defined. The conformal class of these (split
signature) metrics is well defined by each point equivalence class of second
order ODEs. Its conformal curvature is interpreted in terms of the basic point
invariants of the corresponding class of ODEs
On certain relationships between cosmological observables in the Einstein-Cartan gravity
We show that in the Einstein-Cartan gravity it is possible to obtain a
relation between Hubble's expansion and the global rotation (vorticity) of the
Universe. Gravitational coupling can be reduced to dimensionless quantity of
order unity, fixing the scalar mass density and the resulting negative
cosmological constant at spacelike infinity. Current estimates of the expansion
and rotation (see also astro-ph/9703082) of the Universe favour the massive
spinning particles as candidate particles for cold and hot dark matter. Nodland
and Ralston vorticity (Phys. Rev. Lett. 78 (1997) 3043) overestimates the value
favoured by the Einstein-Cartan gravity for three orders of magnitude.Comment: 7 pages, LaTeX styl
Observables, gauge invariance, and the role of the observers in the limit from general relativity to special relativity
Some conceptual issues concerning general invariant theories, with special
emphasis on general relativity, are analyzed. The common assertion that
observables must be required to be gauge invariant is examined in the light of
the role played by a system of observers. Some features of the reduction of the
gauge group are discussed, including the fact that in the process of a partial
gauge fixing the reduction at the level of the gauge group and the reduction at
the level of the variational principle do not commute. Distinctions between the
mathematical and the physical concept of gauge symmetry are discussed and
illustrated with examples. The limit from general relativity to special
relativity is considered as an example of a gauge group reduction that is
allowed in some specific physical circumstances. Whether and when the
Poincar\'e group must be considered as a residual gauge group will come out as
a result of our analysis, that applies, in particular, to asymptotically flat
spaces.Comment: 17 page
Plane torsion waves in quadratic gravitational theories
The definition of the Riemann-Cartan space of the plane wave type is given.
The condition under which the torsion plane waves exist is found. It is
expressed in the form of the restriction imposed on the coupling constants of
the 10-parametric quadratic gravitational Lagrangian. In the mathematical
appendix the formula for commutator of the variation operator and Hodge
operator is proved. This formula is applied for the variational procedure when
the gravitational field equations are obtained in terms of the exterior
differential forms.Comment: 3 May 1998. - 11
Volterra Distortions, Spinning Strings, and Cosmic Defects
Cosmic strings, as topological spacetime defects, show striking resemblance
to defects in solid continua: distortions, which can be classified into
disclinations and dislocations, are line-like defects characterized by a delta
function-valued curvature and torsion distribution giving rise to rotational
and translational holonomy. We exploit this analogy and investigate how
distortions can be adapted in a systematic manner from solid state systems to
Einstein-Cartan gravity. As distortions are efficiently described within the
framework of a SO(3) {\rlap{\supset}\times}} T(3) gauge theory of solid
continua with line defects, we are led in a straightforward way to a Poincar\'e
gauge approach to gravity which is a natural framework for introducing the
notion of distorted spacetimes. Constructing all ten possible distorted
spacetimes, we recover, inter alia, the well-known exterior spacetime of a
spin-polarized cosmic string as a special case of such a geometry. In a second
step, we search for matter distributions which, in Einstein-Cartan gravity, act
as sources of distorted spacetimes. The resulting solutions, appropriately
matched to the distorted vacua, are cylindrically symmetric and are interpreted
as spin-polarized cosmic strings and cosmic dislocations.Comment: 24 pages, LaTeX, 9 eps figures; remarks on energy conditions added,
discussion extended, version to be published in Class. Quantum Gra
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