2,322 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
$\sqrt{-{g}}d^{4}x$ and $\Phi d^{4}x$, where $\Phi$ is a metric independent
density, also curvature, curvature square terms, first order formalism and for
scale invariance a dilaton field $\phi$ 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 $\phi \rightarrow \infty$ relevant for the non singular
origin of the Universe and $\phi \rightarrow -\infty$, describing our present
Universe. Surprisingly, avoidance of singularities and stability as $\phi
\rightarrow \infty$ imply a positive but small vacuum energy as $\phi
\rightarrow -\infty$. 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 $\sqrt{-{g}}d^{4}x$ and $\Phi
d^{4}x$, where $\Phi$ is a metric independent density, used as an additional
measure of integration. Also curvature, curvature square terms and for scale
invariance a dilaton field $\phi$ 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 $\phi
\rightarrow \infty$ relevant for the non singular origin of the Universe,
followed by an inflationary phase and $\phi \rightarrow -\infty$, 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 $\phi \rightarrow -\infty$, 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

### Atom laser dynamics in a tight-waveguide

We study the transient dynamics that arise during the formation of an atom
laser beam in a tight waveguide. During the time evolution the density profile
develops a series of wiggles which are related to the diffraction in time
phenomenon. The apodization of matter waves, which relies on the use of smooth
aperture functions, allows to suppress such oscillations in a time interval,
after which there is a revival of the diffraction in time. The revival time
scale is directly related to the inverse of the harmonic trap frequency for the
atom reservoir.Comment: 6 pages, 5 figures, to be published in the Proceedings of the 395th
WE-Heraeus Seminar on "Time Dependent Phenomena in Quantum Mechanics ",
organized by T. Kramer and M. Kleber (Blaubeuren, Germany, September 2007

### 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

### Cosmology with Ricci-type dark energy

We consider the dynamics of a cosmological substratum of pressureless matter
and holographic dark energy with a cutoff length proportional to the Ricci
scale. Stability requirements for the matter perturbations are shown to single
out a model with a fixed relation between the present matter fraction
$\Omega_{m0}$ and the present value $\omega_{0}$ of the equation-of-state
parameter of the dark energy. This model has the same number of free parameters
as the $\Lambda$CDM model but it has no $\Lambda$CDM limit. We discuss the
consistency between background observations and the mentioned
stability-guaranteeing parameter combination.Comment: 6 pages, 3 figures, submitted to the Proceedings of the CosmoSurII
conference, Valpara\'iso, Chile, 27 - 31 May 201

- â€¦