826 research outputs found
Does a relativistic metric generalization of Newtonian gravity exist in 2+1 dimensions?
It is shown that, contrary to previous claims, a scalar tensor theory of
Brans-Dicke type provides a relativistic generalization of Newtonian gravity in
2+1 dimensions. The theory is metric and test particles follow the space-time
geodesics. The static isotropic solution is studied in vacuum and in regions
filled with an incompressible perfect fluid. It is shown that the solutions can
be consistently matched at the matter vacuum interface, and that the Newtonian
behavior is recovered in the weak field regime.Comment: 6 pages, no figures, Revtex4. Some discussions on the physical nature
of the interior solution and on the omega->infinity limit and some references
added. Version to appear in Phys. Rev.
Intrinsic Geometry of a Null Hypersurface
We apply Cartan's method of equivalence to construct invariants of a given
null hypersurface in a Lorentzian space-time. This enables us to fully classify
the internal geometry of such surfaces and hence solve the local equivalence
problem for null hypersurface structures in 4-dimensional Lorentzian
space-times
Modelling the rotational curves of spiral galaxies with a scalar field
In a previous work (Mbelek 2001), we modelled the rotation curves (RC) of
spiral galaxies by including in the equation of motion of the stars the
dynamical terms from an external real self-interacting scalar field, ,
minimally coupled to gravity and which respects the equivalence principle in
the weak fields and low velocity approximation. This model appeared to have
three free parameters : the turnover radius, , the maximum tangential
velocity, , plus a strictly positive
integer, . Here, we propose a new improved version where the coupling of the
-field to dark matter is emphasized at the expense of its
self-interaction. This reformulation presents the very advantageous possibility
that the same potential is used for all galaxies. Using at the same time a
quasi-isothermal dark matter density and the scalar field helps to better fit
the RC of spiral galaxies. In addition, new correlations are established.Comment: Latex, 5 pages with 3 Postscript figure
Extended Inflation with an Exponential Potential
In this paper we investigate extended inflation with an exponential potential
, which provides a simple cosmological
scenario where the distribution of the constants of Nature is mostly determined
by . In particular, we show that this theory predicts a uniform
distribution for the Planck mass at the end of inflation, for the entire
ensemble of universes that undergo stochastic inflation. Eternal inflation
takes place in this scenario for a broad family of initial conditions, all of
which lead up to the same value of the Planck mass at the end of inflation. The
predicted value of the Planck mass is consistent with the observed value within
a comfortable range of values of the parameters involved.Comment: 6 pages, 2 figure
General Relativity as an Attractor in Scalar-Tensor Stochastic Inflation
Quantum fluctuations of scalar fields during inflation could determine the
very large-scale structure of the universe. In the case of general
scalar-tensor gravity theories these fluctuations lead to the diffusion of
fundamental constants like the Planck mass and the effective Brans--Dicke
parameter, . In the particular case of Brans--Dicke gravity, where
is constant, this leads to runaway solutions with infinitely large
values of the Planck mass. However, in a theory with variable we find
stationary probability distributions with a finite value of the Planck mass
peaked at exponentially large values of after inflation. We conclude
that general relativity is an attractor during the quantum diffusion of the
fields.Comment: LaTeX (with RevTex) 11 pages, 2 uuencoded figures appended, also
available on WWW via http://star.maps.susx.ac.uk/index.htm
Extended Gravity Theories and the Einstein-Hilbert Action
I discuss the relation between arbitrarily high-order theories of gravity and
scalar-tensor gravity at the level of the field equations and the action. I
show that -order gravity is dynamically equivalent to Brans-Dicke
gravity with an interaction potential for the Brans-Dicke field and further
scalar fields. This scalar-tensor action is then conformally equivalent to the
Einstein-Hilbert action with scalar fields. This clarifies the nature and
extent of the conformal equivalence between extended gravity theories and
general relativity with many scalar fields.Comment: 12 pages, Plain Latex, SUSSEX-AST-93/7-
Spinless Matter in Transposed-Equi-Affine Theory of Gravity
We derive and discus the equations of motion for spinless matter:
relativistic spinless scalar fields, particles and fluids in the recently
proposed by A. Saa model of gravity with covariantly constant volume with
respect to the transposed connection in Einstein-Cartan spaces.
A new interpretation of this theory as a theory with variable Plank
"constant" is suggested.
We show that the consistency of the semiclassical limit of the wave equation
and classical motion dictates a new definite universal interaction of torsion
with massive fields.Comment: 29 pages, latex, no figures. New Section on semiclassical limit of
wave equation added; old references rearranged; new references, remarks,
comments, and acknowledgments added; typos correcte
Nielsen-Olesen vortex in varying-alpha theories
We consider soliton solutions to Bekenstein's theory, for which the fine
structure constant is allowed to vary due to the
presence of a dielectric field pervading the vacuum. More specifically we
investigate the effects of a varying upon a complex scalar field with
a U(1) electromagnetic gauge symmetry subject to spontaneous symmetry breaking.
We find vortex solutions to this theory, similar to the Nielsen-Olesen vortex.
Near the vortex core the electric charge is typically much larger than far away
from the string, lending these strings a superconducting flavour. In general
the dielectric field coats the usual local string with a global string
envelope. We discuss the cosmological implications of networks of such strings,
with particular emphasis on their ability to generate inhomogeneous
recombination scenarios. We also consider the possibility of the dielectric
being a charged free field. Even though the vacuum of such a field is trivial,
we find that the dielectric arranges itself in the shape of a local string,
with a quantized magnetic flux at the core -- presumably borrowing these
topological features from the underlying Nielsen-Olesen vortex.Comment: 7 pages, 1 figur
Isotropization of Bianchi type models and a new FRW solution in Brans-Dicke theory
Using scaled variables we are able to integrate an equation valid for
isotropic and anisotropic Bianchi type I, V, IX models in Brans-Dicke (BD)
theory. We analyze known and new solutions for these models in relation with
the possibility that anisotropic models asymptotically isotropize, and/or
possess inflationary properties. In particular, a new solution of curve
() Friedmann-Robertson-Walker (FRW) cosmologies in Brans-Dicke theory
is analyzed.Comment: 15 pages, 4 postscript figures, to appear in Gen. Rel. Grav., special
issue dedicated in honour of Prof. H. Dehne
Black holes in the Brans-Dicke-Maxwell theory
The black hole solutions in the higher dimensional Brans-Dicke-Maxwell theory
are investigated. We find that the presence of the nontrivial scalar field
depends on the spacetime dimensions (D). When D=4, the solution corresponds to
the Reissner-Nordstr\"{o}m black hole with a constant scalar field. In higher
dimensions (D>4), one finds the charged black hole solutions with the
nontrivial scalar field. The thermal properties of the charged black holes are
discussed and the reason why the nontrivial scalar field exists are explained.
Also the solutions for higher dimensional Brans-Dicke theory are given for
comparison.Comment: Revtex, 5 pages, no figures, contents were rewritten and new
references were adde
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