20 research outputs found
Gravitational excitons from extra dimensions
Inhomogeneous multidimensional cosmological models with a higher dimensional
space-time manifold are investigated under dimensional reduction. In the
Einstein conformal frame, small excitations of the scale factors of the
internal spaces near minima of an effective potential have a form of massive
scalar fields in the external space-time. Parameters of models which ensure
minima of the effective potentials are obtained for particular cases and masses
of gravitational excitons are estimated.Comment: Revised version --- 12 references added, Introduction enlarged, 20
pages, LaTeX, to appear in Phys.Rev.D56 (15.11.97
Some Cosmological Consequences of Non Trivial PPN Parameters and
We study homogeneous isotropic universe in a graviton-dilaton theory
obtained, in a previous paper, by a simple requirement that the theory be able
to predict non trivial values for and/or for a charge neutral
point star, without any naked singularities. We find that in this universe the
physical time can be continued indefinitely into the past or future, and that
all the physical curvature invariants are always finite, showing the absence of
big bang singularity. Adding a dilaton potential, we find again the same
features. As a surprising bonus, there emerges naturally a Brans-Dicke
function, which has precisely the kind of behaviour needed to make in hyperextended inflation.Comment: 14 pages. Latex file. Results unchanged, more detailed discussion
provide
Primordial and asymptotic inflation in Brans-Dicke cosmology
The basic motivation of this work is to attempt to explain the rapid
primordial inflation and the observed slow late-time inflation by using the
Brans-Dicke theory of gravity. We show that the ratio of these two inflation
parameters is proportional to the square root of the Brans-Dicke parameter . We also calculate the Hubble parameter and the
time variation of the time dependent Newtonian gravitational constant for
both regimes. The variation of the Hubble parameter predicted by Brans-Dicke
cosmology is shown to be consistent with recent measurements: The value of
in the late-time future is predicted as 0.86 times the present value of .Comment: 7 pages, no figure
Multidimensional cosmological models: cosmological and astrophysical implications and constraints
We investigate four-dimensional effective theories which are obtained by
dimensional reduction of multidimensional cosmological models with factorizable
geometry and consider the interaction between conformal excitations of the
internal space (geometrical moduli excitations) and Abelian gauge fields. It is
assumed that the internal space background can be stabilized by minima of an
effective potential. The conformal excitations over such a background have the
form of massive scalar fields (gravitational excitons) propagating in the
external spacetime. We discuss cosmological and astrophysical implications of
the interaction between gravexcitons and four-dimensional photons as well as
constraints arising on multidimensional models of the type considered in our
paper. In particular, we show that due to the experimental bounds on the
variation of the fine structure constant, gravexcitons should decay before
nucleosynthesis starts. For a successful nucleosynthesis the masses of the
decaying gravexcitons should be m>10^4 GeV. Furthermore, we discuss the
possible contribution of gravexcitons to UHECR. It is shown that, at energies
of about 10^{20}eV, the decay length of gravexcitons with masses m>10^4 GeV is
very small, but that for m <10^2 GeV it becomes much larger than the
Greisen-Zatsepin-Kuzmin cut-off distance. Finally, we investigate the
possibility for gravexciton-photon oscillations in strong magnetic fields of
astrophysical objects. The corresponding estimates indicate that even the high
magnetic field strengths of magnetars are not sufficient for an efficient and
copious production of gravexcitons.Comment: 16 pages, LaTeX2e, minor changes, improved references, to appear in
PR