7 research outputs found
Symmetry analysis and exact solutions of modified Brans-Dicke cosmological equations
We perform a symmetry analysis of modified Brans-Dicke cosmological equations
and present exact solutions. We discuss how the solutions may help to build
models of cosmology where, for the early universe, the expansion is linear and
the equation of state just changes the expansion velocity but not the
linearity. For the late universe the expansion is exponential and the effect of
the equation of state on the rate of expansion is just to change the constant
Hubble parameter.Comment: LaTeX2e source file, 14 pages, 7 reference
Can hyperbolic phase of Brans-Dicke field account for Dark Matter?
We show that the introduction of a hyperbolic phase for Brans-Dicke (BD)
field results in a flat vacuum cosmological solution of Hubble parameter H and
fractional rate of change of BD scalar field, F which asymptotically approach
constant values. At late stages, hyperbolic phase of BD field behaves like dark
matter
Geometry and Matter Reduction in a 5D Kaluza-Klein Framework
In this paper we consider the Kaluza-Klein fields equations in presence of a
generic 5D matter tensor which is governed by a conservation equation due to 5D
Bianchi identities. Following a previous work, we provide a consistent approach
to matter where the problem of huge massive modes is removed, without relaxing
the compactification hypotheses; therefore we perform the dimensional reduction
either for metric fields and for matter, thus identifying a pure 4D tensor
term, a 4D vector term and a scalar one. Hence we are able to write down a
consistent set of equations for the complete dynamics of matter and fields ;
with respect to the pure Einstein-Maxwell system we now have two additional
scalar field: the usual dilaton one plus a scalar source term. Some significant
scenarios involving these terms are discussed and perspectives for cosmological
applications are suggested.Comment: to appear on IJMP
Dynamics of Matter in a Compactified Kaluza-Klein Model
A longstanding problem in Kaluza-Klein models is the description of matter
dynamics. Within the 5D model, the dimensional reduction of the geodesic motion
for a 5D free test particle formally restores electrodynamics, but the reduced
4D particle shows a charge-mass ratio that is upper bounded, such that it
cannot fit to any kind of elementary particle. At the same time, from the
quantum dynamics viewpoint, there is the problem of the huge massive modes
generation. We present a criticism against the 5D geodesic approach and face
the hypothesis that in Kaluza-Klein space the geodesic motion does not deal
with the real dynamics of test particle. We propose a new approach: starting
from the conservation equation for the 5D matter tensor, within the Papapetrou
multipole expansion, we prove that the 5D dynamical equation differs from the
5D geodesic one. Our new equation provides right coupling terms without
bounding and in such a scheme the tower of massive modes is removed.Comment: 21 pages, to appear on IJMP
Chiral spinors and gauge fields in noncommutative curved space-time
The fundamental concepts of Riemannian geometry, such as differential forms,
vielbein, metric, connection, torsion and curvature, are generalized in the
context of non-commutative geometry. This allows us to construct the
Einstein-Hilbert-Cartan terms, in addition to the bosonic and fermionic ones in
the Lagrangian of an action functional on non-commutative spaces. As an
example, and also as a prelude to the Standard Model that includes
gravitational interactions, we present a model of chiral spinor fields on a
curved two-sheeted space-time with two distinct abelian gauge fields. In this
model, the full spectrum of the generalized metric consists of pairs of tensor,
vector and scalar fields. They are coupled to the chiral fermions and the gauge
fields leading to possible parity violation effects triggered by gravity.Comment: 50 pages LaTeX, minor corrections and references adde
On the ADM decomposition of the 5-D Kaluza-Klein model
Our purpose is to recast KK model in terms of ADM variables. We examine and
solve the problem of the consistency of this approach, with particular care
about the role of the cylindricity hypothesis. We show in details how the KK
reduction commutes with the ADM slicing procedure and how this leads to a well
defined and unique ADM reformulation. This allows us to consider the
hamiltonian formulation of the model and can be the first step for the Ashtekar
reformulation of the KK scheme. Moreover we show how the time component of the
gauge vector arises naturally from the geometrical constraints of the dynamics;
this is a positive check for the autoconsistency of the KK theory and for an
hamiltonian description of the dynamics which wants to take into account the
compactification scenario: this result enforces the physical meaning of KK
model.Comment: 24 pages, no figures, to appear on IJMP