3,053 research outputs found
The formation of voids in a universe with cold dark matter and a cosmological constant
A spherical Lagrangian hydrodynamical code has been written to study the
formation of cosmological structures in the early Universe. In this code we
take into account the presence of collisionless non-baryonic cold dark matter
(CDM), the cosmological constant and a series of physical processes present
during and after the recombination era, such as photon drag resulting from the
cosmic background radiation and hydrogen molecular production. We follow the
evolution of the structure since the recombination era until the present epoch.
As an application of this code we study the formation of voids starting from
negative density perturbations which evolved during and after the recombination
era. We analyse a set of COBE-normalized models, using different spectra to see
their influence on the formation of voids. Our results show that large voids
with diameters ranging from 10h^{-1} Mpc up to 50h^{-1} Mpc can be formed in a
universe model dominated by the cosmological constant (\Omega_\Lambda ~ 0.8).
This particular scenario is capable of forming large and deep empty regions
(with density contrasts \delta < -0.6). Our results also show that the physical
processes acting on the baryonic matter produce a transition region where the
radius of the dark matter component is greater than the baryonic void radius.
The thickness of this transition region ranges from about tens of kiloparsecs
up to a few megaparsecs, depending on the spectrum considered. Putative objects
formed near voids and within the transition region would have a different
amount of baryonic/dark matter when compared with \Omega_b/\Omega_d. If one
were to use these galaxies to determine, by dynamical effects or other
techniques, the quantity of dark matter present in the Universe, the result
obtained would be only local and not representative of the Universe as a whole.Comment: MNRAS (in press); 9 pages, no figure
A hierarchy of voids: Much ado about nothing
We present a model for the distribution of void sizes and its evolution in
the context of hierarchical scenarios of gravitational structure formation. We
find that at any cosmic epoch the voids have a size distribution which is
well-peaked about a characteristic void size which evolves self-similarly in
time. This is in distinct contrast to the distribution of virialized halo
masses which does not have a small-scale cut-off.
In our model, the fate of voids is ruled by two processes. The first process
affects those voids which are embedded in larger underdense regions: the
evolution is effectively one in which a larger void is made up by the mergers
of smaller voids, and is analogous to how massive clusters form from the
mergers of less massive progenitors. The second process is unique to voids, and
occurs to voids which happen to be embedded within a larger scale overdensity:
these voids get squeezed out of existence as the overdensity collapses around
them. It is this second process which produces the cut-off at small scales.
In the excursion set formulation of cluster abundance and evolution, solution
of the cloud-in-cloud problem, i.e., counting as clusters only those objects
which are not embedded in larger clusters, requires study of random walks
crossing one barrier. We show that a similar formulation of void evolution
requires study of a two-barrier problem: one barrier is required to account for
voids-in-voids, and the other for voids-in-clouds. Thus, in our model, the void
size distribution is a function of two parameters, one of which reflects the
dynamics of void formation, and the other the formation of collapsed objects.Comment: 23 pages, 9 figures, submitted to MNRA
Cosmological Black Holes as Seeds of Voids in Galaxy Distribution
Deep surveys indicate a bubbly structure of cosmological large scale which
should be the result of evolution of primordial density perturbations. Several
models have been proposed to explain origin and dynamics of such features but,
till now, no exhaustive and fully consistent theory has been found. We discuss
a model where cosmological black holes, deriving from primordial perturbations,
are the seeds for large-scale-structure voids. We give details of dynamics and
accretion of the system voids-cosmological black holes from the epochs
till now finding that void of of diameter and
under-density of -0.9 will fits the observations without conflicting with the
homogeneity and isotropy of cosmic microwave background radiation.Comment: to appear in Astronomy & Astrophysic
Nursing Diagnosis Risk for falls: prevalence and clinical profile of hospitalized patients
Objectives: to identify the prevalence of the Nursing Diagnosis (ND) Risk for falls in the hospitalizations of adult patients in clinical and surgical units, to characterize the clinical profile and to identify the risk factors of the patients with this ND. Method: a cross-sectional study with 174 patients. The data was collected from the computerized nursing care prescriptions system and on-line hospital records, and analyzed statistically. Results: the prevalence of the ND Risk for falls was 4%. The patients’ profile indicated older adults, males (57%), those hospitalized in the clinical units (63.2%), with a median length of hospitalization of 20 (10-24) days, with neurological illnesses (26%), cardio-vascular illnesses (74.1%) and various co-morbidities (3±1.8). The prevalent risk factors were neurological alterations (43.1%), impaired mobility (35.6%) and extremes of age (10.3%). Conclusion: the findings contributed to evidencing the profile of the patients with a risk of falling hospitalized in clinical and surgical wards, which favors the planning of interventions for preventing this adverse event
Bond Strength and Cytotoxicity of a Universal Adhesive According to the Hybridization Strategies to Dentin
Predicting gene expression in T cell differentiation from histone modifications and transcription factor binding affinities by linear mixture models
Voids in a CDM Universe
We study the formation and evolution of voids in the dark matter distribution
using various simulations of the popular Cold Dark Matter cosmogony.
We identify voids by requiring them to be regions of space with a mean
overdensity of -0.8 or less. Each of the simulations contains thousands of
voids. The distribution of void sizes in the different simulations shows good
agreement. Voids very clearly correspond to minima in the smoothed initial
density field. We find a universal void mass profile of the form
where is the effective radius of a void and . The mass function
of haloes in voids is steeper than that of haloes that populate denser regions.
The abundances of void haloes seem to evolve somewhat more strongly between
redshifts and 0 than the global abundances of haloes. (abridged)Comment: 12 pages, 16 figures, submitted to MNRAS; 13 pages, 17 figures,
revised version, accepted for publicatio
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