7,481 research outputs found
Non Singular Origin of the Universe and the Cosmological Constant Problem (CCP)
We consider a non singular origin for the Universe starting from an Einstein
static Universe in the framework of a theory which uses two volume elements
and , where is a metric independent
density, also curvature, curvature square terms, first order formalism and for
scale invariance a dilaton field are considered in the action. In the
Einstein frame we also add a cosmological term that parametrizes the zero point
fluctuations. The resulting effective potential for the dilaton contains two
flat regions, for relevant for the non singular
origin of the Universe and , describing our present
Universe. Surprisingly, avoidance of singularities and stability as imply a positive but small vacuum energy as . Zero vacuum energy density for the present universe is
the "threshold" for universe creation.Comment: awarded an honorable mention in the Gravity Research Foundation 2011
Awards for Essays in Gravitation for 201
Non Singular Origin of the Universe and its Present Vacuum Energy Density
We consider a non singular origin for the Universe starting from an Einstein
static Universe, the so called "emergent universe" scenario, in the framework
of a theory which uses two volume elements and , where is a metric independent density, used as an additional
measure of integration. Also curvature, curvature square terms and for scale
invariance a dilaton field are considered in the action. The first order
formalism is applied. The integration of the equations of motion associated
with the new measure gives rise to the spontaneous symmetry breaking (S.S.B) of
scale invariance (S.I.). After S.S.B. of S.I., it is found that a non trivial
potential for the dilaton is generated. In the Einstein frame we also add a
cosmological term that parametrizes the zero point fluctuations. The resulting
effective potential for the dilaton contains two flat regions, for relevant for the non singular origin of the Universe,
followed by an inflationary phase and , describing
our present Universe. The dynamics of the scalar field becomes non linear and
these non linearities are instrumental in the stability of some of the emergent
universe solutions, which exists for a parameter range of values of the vacuum
energy in , which must be positive but not very big,
avoiding the extreme fine tuning required to keep the vacuum energy density of
the present universe small. Zero vacuum energy density for the present universe
defines the threshold for the creation of the universe.Comment: 28 pages, short version of this paper awarded an honorable mention by
the Gravity Research Foundation, 2011, accepted for publication in
International Journal of Modern Physics
Curvature in causal BD-type inflationary cosmology
We study a closed model of the universe filled with viscous fluid and
quintessence matter components in a Brans-Dicke type cosmological model. The
dynamical equations imply that the universe may look like an accelerated flat
Friedmann-Robertson-Walker universe at low redshift. We consider here
dissipative processes which follow a causal thermodynamics. The theory is
applied to viscous fluid inflation, where accepted values for the total entropy
in the observable universe is obtained.Comment: 11 pages, revtex 4. For a festschrift honoring Alberto Garcia. To be
publishen in Gen. Rel. Gra
Is the cosmological dark sector better modeled by a generalized Chaplygin gas or by a scalar field?
Both scalar fields and (generalized) Chaplygin gases have been widely used
separately to characterize the dark sector of the Universe. Here we investigate
the cosmological background dynamics for a mixture of both these components and
quantify the fractional abundances that are admitted by observational data from
supernovae of type Ia and from the evolution of the Hubble rate. Moreover, we
study how the growth rate of (baryonic) matter perturbations is affected by the
dark-sector perturbations.Comment: 20 pages, 9 figures, substantially revised, section on matter
perturbations added, accepted for publication in EPJ
Diffraction in time of a confined particle and its Bohmian paths
Diffraction in time of a particle confined in a box which its walls are
removed suddenly at is studied. The solution of the time-dependent
Schr\"{o}dinger equation is discussed analytically and numerically for various
initial wavefunctions. In each case Bohmian trajectories of the particles are
computed and also the mean arrival time at a given location is studied as a
function of the initial state.Comment: 8 pages, 6 figure
- …