2,441 research outputs found
Universal field equations for metric-affine theories of gravity
We show that almost all metric--affine theories of gravity yield Einstein
equations with a non--null cosmological constant . Under certain
circumstances and for any dimension, it is also possible to incorporate a Weyl
vector field and therefore the presence of an anisotropy. The viability
of these field equations is discussed in view of recent astrophysical
observations.Comment: 13 pages. This is a copy of the published paper. We are posting it
here because of the increasing interest in f(R) theories of gravit
Structure of Spinning Particle Suggested by Gravity, Supergravity and Low Energy String Theory
The structure of spinning particle suggested by the rotating Kerr-Newman
(black hole) solution, super-Kerr-Newman solution and the Kerr-Sen solution to
low energy string theory is considered. Main peculiarities of the Kerr spinning
particle are discussed: a vortex of twisting principal null congruence,
singular ring and the Kerr source representing a rotating relativistic disk of
the Compton size. A few stringy structures can be found in the real and complex
Kerr geometry.
Low-energy string theory predicts the existence of a heterotic string placed
on the sharp boundary of this disk. The obtained recently supergeneralization
of the Kerr-Newman solution suggests the existence of extra axial singular line
and fermionic traveling waves concentrating near these singularities.
We discuss briefly a possibility of experimental test of these predictions.Comment: Latex, 8 pages, talk at the International Workshop Spin'99, Prague,
5-11 September, 199
Singular shell embedded into a cosmological model
We generalize Israel's formalism to cover singular shells embedded in a
non-vacuum Universe. That is, we deduce the relativistic equation of motion for
a thin shell embedded in a Schwarzschild/Friedmann-Lemaitre-Robertson-Walker
spacetime. Also, we review the embedding of a Schwarzschild mass into a
cosmological model using "curvature" coordinates and give solutions with
(Sch/FLRW) and without the embedded mass (FLRW).Comment: 25 pages, 2 figure
The Application of the Newman-Janis Algorithm in Obtaining Interior Solutions of the Kerr Metric
In this paper we present a class of metrics to be considered as new possible
sources for the Kerr metric. These new solutions are generated by applying the
Newman-Janis algorithm (NJA) to any static spherically symmetric (SSS) ``seed''
metric. The continuity conditions for joining any two of these new metrics is
presented. A specific analysis of the joining of interior solutions to the Kerr
exterior is made. The boundary conditions used are those first developed by
Dormois and Israel. We find that the NJA can be used to generate new physically
allowable interior solutions. These new solutions can be matched smoothly to
the Kerr metric. We present a general method for finding such solutions with
oblate spheroidal boundary surfaces. Finally a trial solution is found and
presented.Comment: 11 pages, Latex, 4 postscript figures. To be published in Classical
and Quantum Gravity. Title and abstract are now on the same pag
Effective supergravity descriptions of superstring cosmology
This text is a review of aspects of supergravity theories that are relevant
in superstring cosmology. In particular, it considers the possibilities and
restrictions for `uplifting terms', i.e. methods to produce de Sitter vacua. We
concentrate on N=1 and N=2 supergravities, and the tools of superconformal
methods, which clarify the structure of these theories. Cosmic strings and
embeddings of target manifolds of supergravity theories in others are discussed
in short at the end.Comment: 12 pages, contribution to the proceedings of the 2nd international
conference on Quantum Theories and Renormalization Group in Gravity and
Cosmology, Barcelona, July 11-15, 2006, Journal of Physics
The Universality of Einstein Equations
It is shown that for a wide class of analytic Lagrangians which depend only
on the scalar curvature of a metric and a connection, the application of the
so--called ``Palatini formalism'', i.e., treating the metric and the connection
as independent variables, leads to ``universal'' equations. If the dimension
of space--time is greater than two these universal equations are Einstein
equations for a generic Lagrangian and are suitably replaced by other universal
equations at bifurcation points. We show that bifurcations take place in
particular for conformally invariant Lagrangians and prove
that their solutions are conformally equivalent to solutions of Einstein
equations. For 2--dimensional space--time we find instead that the universal
equation is always the equation of constant scalar curvature; the connection in
this case is a Weyl connection, containing the Levi--Civita connection of the
metric and an additional vectorfield ensuing from conformal invariance. As an
example, we investigate in detail some polynomial Lagrangians and discuss their
bifurcations.Comment: 15 pages, LaTeX, (Extended Version), TO-JLL-P1/9
Nonextensive Thermostatistics and the H-Theorem
The kinetic foundations of Tsallis' nonextensive thermostatistics are
investigated through Boltzmann's transport equation approach. Our analysis
follows from a nonextensive generalization of the ``molecular chaos
hypothesis". For , the -transport equation satisfies an -theorem
based on Tsallis entropy. It is also proved that the collisional equilibrium is
given by Tsallis' -nonextensive velocity distribution.Comment: 4 pages, no figures, corrected some typo
Nonextensivity and multifractality in low-dimensional dissipative systems
Power-law sensitivity to initial conditions at the edge of chaos provides a
natural relation between the scaling properties of the dynamics attractor and
its degree of nonextensivity as prescribed in the generalized statistics
recently introduced by one of us (C.T.) and characterized by the entropic index
. We show that general scaling arguments imply that , where and are the
extremes of the multifractal singularity spectrum of the attractor.
This relation is numerically checked to hold in standard one-dimensional
dissipative maps. The above result sheds light on a long-standing puzzle
concerning the relation between the entropic index and the underlying
microscopic dynamics.Comment: 12 pages, TeX, 4 ps figure
Power-Law Sensitivity to Initial Conditions within a Logistic-like Family of Maps: Fractality and Nonextensivity
Power-law sensitivity to initial conditions, characterizing the behaviour of
dynamical systems at their critical points (where the standard Liapunov
exponent vanishes), is studied in connection with the family of nonlinear 1D
logistic-like maps The main ingredient of our approach is the generalized deviation
law \lim_{\Delta x(0) -> 0} \Delta x(t) / \Delta x(0)} = [1+(1-q)\lambda_q
t]^{1/(1-q)} (equal to for q=1, and proportional, for large
t, to for is the entropic index appearing in
the recently introduced nonextensive generalized statistics). The relation
between the parameter q and the fractal dimension d_f of the onset-to-chaos
attractor is revealed: q appears to monotonically decrease from 1
(Boltzmann-Gibbs, extensive, limit) to -infinity when d_f varies from 1
(nonfractal, ergodic-like, limit) to zero.Comment: LaTeX, 6 pages , 5 figure
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