2,027 research outputs found
Unified model of baryonic matter and dark components
We investigate an interacting two-fluid cosmological model and introduce a
scalar field representation by means of a linear combination of the individual
energy densities. Applying the integrability condition to the scalar field
equation we show that this "exotic quintessence" is driven by an exponential
potential and the two-fluid mixture can be considered as a model of three
components. These components are associated with baryonic matter, dark matter
and dark energy respectively. We use the Simon, Verde & Jimenez (2005)
determination of the redshift dependence of the Hubble parameter to constrain
the current density parameters of this model. With the best fit density
parameters we obtain the transition redshift between non accelerated and
accelerated regimes z_{acc}=0.66 and the time elapsed since the initial
singularity t_0= 19.8 Gyr. We study the perturbation evolution of this model
and find that the energy density perturbation decreases with the cosmological
time.Comment: 8 pages, 6 figures A new section adde
Linear and non-linear perturbations in dark energy models
I review the linear and second-order perturbation theory in dark energy
models with explicit interaction to matter in view of applications to N-body
simulations and non-linear phenomena. Several new or generalized results are
obtained: the general equations for the linear perturbation growth; an
analytical expression for the bias induced by a species-dependent interaction;
the Yukawa correction to the gravitational potential due to dark energy
interaction; the second-order perturbation equations in coupled dark energy and
their Newtonian limit. I also show that a density-dependent effective dark
energy mass arises if the dark energy coupling is varying.Comment: 12 pages, submitted to Phys. Rev; v2: added a ref. and corrected a
typ
Observational constraints on an interacting dark energy model
We use observations of cosmic microwave background anisotropies, supernova
luminosities and the baryon acoustic oscillation signal in the galaxy
distribution to constrain the cosmological parameters in a simple interacting
dark energy model with a time-varying equation of state. Using a Monte Carlo
Markov Chain technique we determine the posterior likelihoods. Constraints from
the individual data sets are weak, but the combination of the three data sets
confines the interaction constant to be less than 23% of the expansion
rate of the Universe ; at 95% CL . The CMB
acoustic peaks can be well fitted even if the interaction rate is much larger,
but this requires a larger or smaller (depending on the sign of interaction)
matter density today than in the non-interacting model. Due to this degeneracy
between the matter density and the interaction rate, the only observable effect
on the CMB is a larger or smaller integrated Sachs-Wolfe (ISW) effect. While SN
or BAO data alone do not set any direct constraints on the interaction, they
exclude the models with very large matter density, and hence indirectly
constrain the interaction rate when jointly analysed with the CMB data. To
enable the analysis described in this paper, we present in a companion paper
[arXiv:0907.4981] a new systematic analysis of the early radiation era solution
to find the adiabatic initial conditions for the Boltzmann integration.Comment: 16 pages, 10 figures. V2: Improved typography (2-column format);
References and a motivation of CPL parametrization added; Accepted by MNRA
Perturbation evolution with a non-minimally coupled scalar field
We recently proposed a simple dilaton-derived quintessence model in which the
scalar field was non-minimally coupled to cold dark matter, but not to
`visible' matter. Such couplings can be attributed to the dilaton in the low
energy limit of string theory, beyond tree level. In this paper we discuss the
implications of such a model on structure formation, looking at its impact on
matter perturbations and CMB anisotropies. We find that the model only deviates
from CDM and minimally coupled theories at late times, and is well
fitted to current observational data. The signature left by the coupling, when
it breaks degeneracy at late times, presents a valuable opportunity to
constrain non-minimal couplings given the wealth of new observational data
promised in the near future.Comment: Version appearing in Physical Review D. 10 pages, 9 figs. Comparison
with SN1a and projected MAP results, and appendix adde
Observational constraints on the braneworld model with brane-bulk energy exchange
We investigate the viability of the braneworld model with energy exchange
between the brane and bulk, by using the most recent observational data related
to the background evolution. We show that this energy exchange behaves like a
source of dark energy and can alter the profile of the cosmic expansion. The
new Supernova Type Ia (SNIa) Gold sample, Supernova Legacy Survey (SNLS) data,
the position of the acoustic peak at the last scattering surface from the
Wilkinson Microwave Anisotropy Probe (WMAP) observations and the baryon
acoustic oscillation peak found in the Sloan Digital Sky Survey (SDSS) are used
to constrain the free parameters of this model. To infer its consistency with
the age of the Universe, we compare the age of old cosmological objects with
what computed using the best fit values for the model parameters. At 68% level
of confidence, the combination of Gold sample SNIa, Cosmic Microwave Background
(CMB) shift parameter and SDSS databases provide
, and
, hence a spatially flat Universe with
. The same combination with SNLS supernova
observation give ,
and consequently
provides a spatially flat Universe . These
results obviously seem to be compatible with the most recent WMAP results
indicating a flat Universe.Comment: 17 pages and 18 figures, V2: Added comments, references, explained
some topics related to the matter power spectrum as a robust constraint,
accepted for publication in Mon. Not. R. Astron. So
Cluster number counts dependence on dark energy inhomogeneities and coupling to dark matter
Cluster number counts can be used to test dark energy models. We investigate
dark energy candidates which are coupled to dark matter. We analyze the cluster
number counts dependence on the amount of dark matter coupled to dark energy.
Further more, we study how dark energy inhomogeneities affect cluster
abundances. It is shown that increasing the coupling reduces significantly the
cluster number counts, and that dark energy inhomogeneities increases cluster
abundances. Wiggles in cluster number counts are shown to be a specific
signature of coupled dark energy models. Future observations will possibly
detect such oscillations and discriminate among the different dark energy
models.Comment: 9 pages, 8 figures. Further extensions on section on discriminating
models with future surveys. Accepted for publication in Mon. Not. Roy. Astro.
So
Cosmology and Astrophysical Constraints of Gauss-Bonnet Dark Energy
Cosmological consequences of a string-motivated dark energy scenario
featuring a scalar field coupled to the Gauss-Bonnet invariant are
investigated. We study the evolution of the universe in such a model,
identifying its key properties. The evolution of the homogeneous background and
cosmological perturbations, both at large and small scales, are calculated. The
impact of the coupling on galaxy distributions and the cosmic microwave
background is examined. We find the coupling provides a mechanism to viably
onset the late acceleration, to alleviate the coincidence problem, and
furthermore to effectively cross the phantom divide at the present while
avoiding a Big Rip in the future. We show the model could explain the present
cosmological observations, and discuss how various astrophysical and
cosmological data, from the Solar system, supernovae Ia, cosmic microwave
background radiation and large scale structure constrain it.Comment: 6 pages, 3 figures. References added. Accepted for publication in
Phys. Lett.
Number counts in homogeneous and inhomogeneous dark energy models
In the simple case of a constant equation of state, redshift distribution of
collapsed structures may constrain dark energy models. Different dark energy
models having the same energy density today but different equations of state
give quite different number counts. Moreover, we show that introducing the
possibility that dark energy collapses with dark matter (``inhomogeneous'' dark
energy) significantly complicates the picture. We illustrate our results by
comparing four dark energy models to the standard -model. We
investigate a model with a constant equation of state equal to -0.8, a phantom
energy model and two scalar potentials (built out of a combination of two
exponential terms). Although their equations of state at present are almost
indistinguishable from a -model, both scalar potentials undergo quite
different evolutions at higher redshifts and give different number counts. We
show that phantom dark energy induces opposite departures from the
-model as compared with the other models considered here. Finally, we
find that inhomogeneous dark energy enhances departures from the
-model with maximum deviations of about 15% for both number counts and
integrated number counts. Larger departures from the -model are
obtained for massive structures which are rare objects making it difficult to
statistically distinguish between models.Comment: 10 pages, 11 figures. Version accepted for publication in A&
Carotid artery dissections from TCAR as reported by the Food and Drug Administration
BACKGROUND: Transcarotid artery revascularization (TCAR) is hybrid procedure that allows carotid stenting using direct surgical access of the carotid artery to restore blood flow through the carotid artery. It has shown the lowest perioperative stroke rate when compared with any prospective trial of transfemoral carotid artery stenting. However, intraoperative injuries related to the procedure and its management are not well characterized. We anticipate that this analysis will add qualitative insight in further characterizing adverse outcomes of this novel technology.
METHODS: The FDA maintains a database called the MAUDE (Manufacturer and User Facility Device Experience) for surveillance of all medical devices approved for use. This database was queried for all cases associated with Silk Road Medical’s ENROUTE Transcarotid Neuroprotection System from September 2016 to October 2020.. Case narratives related to patient injuries were individually analyzed to determine type (carotid artery dissection) and time of injury (intraoperative, recovery, post-discharge follow- up). Carotid artery dissection (CD) reporting was further analyzed for associated procedural event at the time of injury, number of access attempts to CD repair, and type of CD repair.
RESUTS: Of the 115 unique incidents in the database, there were 58 CDs. Most were identified intraoperatively (n=55), while 3 were incidentally identified postoperatively. Overall, sheath placement was the most common procedural event attributed to CD (n=34). There was adequate narrative information about CD repair in 54 patients where 52 of them were performed intraoperatively. There were total of 28 endovascular repair and 24 open surgical repairs of CDs from TCAR procedure.
There was no significant difference in rate of endovascular and open surgical repair of CDs that did not need additional access attempts. However, rate of open surgical repair was significantly higher in CDs with persistent failure to engage the true lumen in 2 or more additional access attempts.
Total of 4 strokes were associated with CD. Two occurred during recovery from TCAR admission where one was not intervened per physician’s discretion despite evidence of dissection during the procedure. The other was associated with a fall from a hypotensive event 7 hours after an endovascular CD repair. One incident of stroke occurred intraoperatively during a conversion to CEA as a result of CD. One incident of stroke occurred 4 days after TCAR procedure in which a CD was identified during the stroke evaluation
Conclusion: Carotid artery dissection is the most common injury related to TCAR as reported on MAUDE database. Most common procedural event associated CD was sheath placement. Rate of open surgical repair was significantly higher than endovascular repair in dissections with persistent failure to engage true lumen despite additional access attempts. This should add to qualitative insight among vascular surgery community regarding intraoperative management of carotid artery dissections from a TCAR procedure.https://scholarscompass.vcu.edu/gradposters/1144/thumbnail.jp
Cosmic microwave background: polarization and temperature anisotropies from symmetric structures
I consider the case of anisotropies in the Cosmic Microwave Background (CMB)
from one single ordered perturbation source, or seed, existing well before
decoupling between matter and radiation. Such structures could have been left
by high energy symmetries breaking in the early universe.
I focus on the cases of spherical and cylindrical symmetry of the seed. I
give general analytic expressions for the polarization and temperature linear
perturbations, factoring out of the Fourier integral the dependence on the
photon propagation direction and on the geometric coordinates describing the
seed. I show how the CMB perturbations manifestly reflect the symmetries of
their seeds. CMB anisotropies are obtained with a line of sight integration.
This treatment highlights the undulatory properties of the CMB. I show with
numerical examples how the polarization and temperature perturbations propagate
beyond the size of their seeds, reaching the CMB sound horizon at the time
considered. Just like the waves from a pebble thrown in a pond, CMB anisotropy
from a seed intersecting the last scattering surface appears as a series of
temperature and polarization waves surrounding the seed, extending on the scale
of the CMB sound horizon at decoupling, roughly in the sky. Each wave
is characterized by its own value of the CMB perturbation, with the same mean
amplitude of the signal coming from the seed interior.
These waves could allow to distinguish relics from high energy processes of
the early universe from point-like astrophysical sources, because of their
angular extension and amplitude. Also, the marked analogy between polarization
and temperature signals offers cross correlation possibilities for the future
Planck Surveyor observations.Comment: 21 pages, seven postscript figures, final version accepted for
publication in Phys.Rev.
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