595 research outputs found
Quintessence and cosmic acceleration
A cosmological model with perfect fluid and self-interacting quintessence
field is considered in the framework of the spatially flat
Friedmann-Robertson-Walker (FRW) geometry. By assuming that all physical
quantities depend on the volume scale factor of the Universe, the general
solution of the gravitational field equations can be expressed in an exact
parametric form. The quintessence field is a free parameter. With an
appropriate choice of the scalar field a class of exact solutions is obtained,
with an exponential type scalar field potential fixed via the gravitational
field equations. The general physical behavior of the model is consistent with
the recent cosmological scenario favored by supernova Type Ia observations,
indicating an accelerated expansion of the Universe.Comment: 6 pages, 3 figures, to appear in Int. J. Mod. Phys.
Density perturbations in Kaluza--Klein theories during a de Sitter phase
In the context of Kaluza-Klein theories, we consider a model in which the
universe is filled with a perfect fluid described by a barotropic equation of
state. An analysis of density perturbations employing the synchronous gauge
shows that there are cases where these perturbations have an exponential growth
during a de Sitter phase evolution in the external space.Comment: LaTex file, 10 pages. To be published in Classical and Quantum
Gravit
Is Random Close Packing of Spheres Well Defined?
Despite its long history, there are many fundamental issues concerning random
packings of spheres that remain elusive, including a precise definition of
random close packing (RCP). We argue that the current picture of RCP cannot be
made mathematically precise and support this conclusion via a molecular
dynamics study of hard spheres using the Lubachevsky-Stillinger compression
algorithm. We suggest that this impasse can be broken by introducing the new
concept of a maximally random jammed state, which can be made precise.Comment: 6 pages total, 2 figure
A smooth bouncing cosmology with scale invariant spectrum
We present a bouncing cosmology which evolves from the contracting to the
expanding phase in a smooth way, without developing instabilities or
pathologies and remaining in the regime of validity of 4d effective field
theory. A nearly scale invariant spectrum of perturbations is generated during
the contracting phase by an isocurvature scalar with a negative exponential
potential and then converted to adiabatic. The model predicts a slightly blue
spectrum, n_S >~ 1, no observable gravitational waves and a high (but model
dependent) level of non-Gaussianities with local shape. The model represents an
explicit and predictive alternative to inflation, although, at present, it is
clearly less compelling.Comment: 20 pages, 1 fig. v2: references added, JCAP published versio
The spherical collapse model in time varying vacuum cosmologies
We investigate the virialization of cosmic structures in the framework of
flat FLRW cosmological models, in which the vacuum energy density evolves with
time. In particular, our analysis focuses on the study of spherical matter
perturbations, as they decouple from the background expansion, "turn around"
and finally collapse. We generalize the spherical collapse model in the case
when the vacuum energy is a running function of the Hubble rate,
. A particularly well motivated model of this type is the
so-called quantum field vacuum, in which is a quadratic function,
, with . This model was previously studied
by our team using the latest high quality cosmological data to constrain its
free parameters, as well as the predicted cluster formation rate. It turns out
that the corresponding Hubble expansion history resembles that of the
traditional CDM cosmology. We use this CDM framework to
illustrate the fact that the properties of the spherical collapse model (virial
density, collapse factor, etc.) depend on the choice of the considered vacuum
energy (homogeneous or clustered). In particular, if the distribution of the
vacuum energy is clustered, then, under specific conditions, we can produce
more concentrated structures with respect to the homogeneous vacuum energy
case.Comment: 14 pages, 4 figures, minor changes, accepted for publication in Phys.
Rev.
A Coherent Study of Emission Lines from Broad-Band Photometry: Specific Star-Formation Rates and [OIII]/H{\beta} Ratio at 3 < z < 6
We measure the H{\alpha} and [OIII] emission line properties as well as
specific star-formation rates (sSFR) of spectroscopically confirmed 3<z<6
galaxies in COSMOS from their observed colors vs. redshift evolution. Our model
describes consistently the ensemble of galaxies including intrinsic properties
(age, metallicity, star-formation history), dust-attenuation, and optical
emission lines. We forward-model the measured H{\alpha} equivalent-widths (EW)
to obtain the sSFR out to z~6 without stellar mass fitting. We find a strongly
increasing rest-frame H{\alpha} EW that is flattening off above z~2.5 with
average EWs of 300-600A at z~6. The sSFR is increasing proportional to
(1+z)^2.4 at z<2.2 and (1+z)^1.5 at higher redshifts, indicative of a fast mass
build-up in high-z galaxies within e-folding times of 100-200Myr at z~6. The
redshift evolution at z>3 cannot be fully explained in a picture of cold
accretion driven growth. We find a progressively increasing
[OIII]{\lambda}5007/H{\beta} ratio out to z~6, consistent with the ratios in
local galaxies selected by increasing H{\alpha} EW (i.e., sSFR). This
demonstrates the potential of using "local high-z analogs" to investigate the
spectroscopic properties and relations of galaxies in the re-ionization epoch.Comment: 18 pages, 11 figures, 3 table
Chiral Quasicrystalline Order and Dodecahedral Geometry in Exceptional Families of Viruses
On the example of exceptional families of viruses we i) show the existence of
a completely new type of matter organization in nanoparticles, in which the
regions with a chiral pentagonal quasicrystalline order of protein positions
are arranged in a structure commensurate with the spherical topology and
dodecahedral geometry, ii) generalize the classical theory of quasicrystals
(QCs) to explain this organization, and iii) establish the relation between
local chiral QC order and nonzero curvature of the dodecahedral capsid faces.Comment: 8 pages, 3 figure
Constraints on Cold Dark Matter Accelerating Cosmologies and Cluster Formation
We discuss the properties of homogeneous and isotropic flat cosmologies in
which the present accelerating stage is powered only by the gravitationally
induced creation of cold dark matter (CCDM) particles (). For
some matter creation rates proposed in the literature, we show that the main
cosmological functions such as the scale factor of the universe, the Hubble
expansion rate, the growth factor and the cluster formation rate are
analytically defined. The best CCDM scenario has only one free parameter and
our joint analysis involving BAO + CMB + SNe Ia data yields
() where
is the observed matter density parameter. In particular, this implies that the
model has no dark energy but the part of the matter that is effectively
clustering is in good agreement with the latest determinations from large scale
structure. The growth of perturbation and the formation of galaxy clusters in
such scenarios are also investigated. Despite the fact that both scenarios may
share the same Hubble expansion, we find that matter creation cosmologies
predict stronger small scale dynamics which implies a faster growth rate of
perturbations with respect to the usual CDM cosmology. Such results
point to the possibility of a crucial observational test confronting CCDM with
CDM scenarios trough a more detailed analysis involving CMB, weak
lensing, as well as the large scale structure.Comment: 12 pages, 3 figures, Accepted for publication by Physical Rev.
Bulk Scale Factor at Very Early Universe
In this paper we propose a higher dimensional Cosmology based on FRW model
and brane-world scenario. We consider the warp factor in the brane-world
scenario as a scale factor in 5-dimensional generalized FRW metric, which is
called as bulk scale factor, and obtain the evolution of it with space-like and
time-like extra dimensions. It is then showed that, additional space-like
dimensions can produce exponentially bulk scale factor under repulsive strong
gravitational force in the empty universe at a very early stage.Comment: 7 pages, October 201
Chiral bosons and improper constraints
We argue that a consistent quantization of the Floreanini-Jackiw model, as a
constrained system, should start by recognizing the improper nature of the
constraints. Then each boundary conditon defines a problem which must be
treated sparately. The model is settled on a compact domain which allows for a
discrete formulation of the dynamics; thus, avoiding the mixing of local with
collective coordinates. For periodic boundary conditions the model turns out to
be a gauge theory whose gauge invariant sector contains only chiral
excitations. For antiperiodoc boundary conditions, the mode is a second-class
theory where the excitations are also chiral. In both cases, the equal-time
algebra of the quantum energy-momentum densities is a Virasoro algebra. The
Poincar\'e symmetry holds for the finite as well as for the infinite domain.Comment: 13 pages, Revtex file, IF.UFRGS Preprin
- …