226 research outputs found
Effect of inhomogeneities on the expansion rate of the Universe
While the expansion rate of a homogeneous isotropic Universe is simply
proportional to the square-root of the energy density, the expansion rate of an
inhomogeneous Universe also depends on the nature of the density
inhomogeneities. In this paper we calculate to second order in perturbation
variables the expansion rate of an inhomogeneous Universe and demonstrate
corrections to the evolution of the expansion rate. While we find that the mean
correction is small, the variance of the correction on the scale of the Hubble
radius is sensitive to the physical significance of the unknown spectrum of
density perturbations beyond the Hubble radius.Comment: 19 pages, 2 figures Version 2 includes some changes in numerical
factors and corrected typos. It is the version accepted for publication in
Physical review
Lessons and perspectives for applications of stochastic models in biological and cancer research
The effects of randomness, an unavoidable feature of intracellular environments, are observed at higher hierarchical levels of living matter organization, such as cells, tissues, and organisms. Additionally, the many compounds interacting as a well-orchestrated network of reactions increase the difficulties of assessing these systems using only experiments. This limitation indicates that elucidation of the dynamics of biological systems is a complex task that will benefit from the establishment of principles to help describe, categorize, and predict the behavior of these systems. The theoretical machinery already available, or ones to be discovered to help solve biological problems, might play an important role in these processes. Here, we demonstrate the application of theoretical tools by discussing some biological problems that we have approached mathematically: fluctuations in gene expression and cell proliferation in the context of loss of contact inhibition. We discuss the methods that have been employed to provide the reader with a biologically motivated phenomenological perspective of the use of theoretical methods. Finally, we end this review with a discussion of new research perspectives motivated by our results
CMB polarization from secondary vector and tensor modes
We consider a novel contribution to the polarization of the Cosmic Microwave
Background induced by vector and tensor modes generated by the non-linear
evolution of primordial scalar perturbations. Our calculation is based on
relativistic second-order perturbation theory and allows to estimate the
effects of these secondary modes on the polarization angular power-spectra. We
show that a non-vanishing B-mode polarization unavoidably arises from pure
scalar initial perturbations, thus limiting our ability to detect the signature
of primordial gravitational waves generated during inflation. This secondary
effect dominates over that of primordial tensors for an inflationary
tensor-to-scalar ratio . The magnitude of the effect is smaller than
the contamination produced by the conversion of polarization of type E into
type B, by weak gravitational lensing. However the lensing signal can be
cleaned, making the secondary modes discussed here the actual background
limiting the detection of small amplitude primordial gravitational waves.Comment: 14 pages, 3 figures, minor changes matching the version to be
published in Phys. Rev.
Cosmic Microwave Background anisotropies from second order gravitational perturbations
This paper presents a complete analysis of the effects of second order
gravitational perturbations on Cosmic Microwave Background anisotropies, taking
explicitly into account scalar, vector and tensor modes. We also consider the
second order perturbations of the metric itself obtaining them, for a universe
dominated by a collision-less fluid, in the Poisson gauge, by transforming the
known results in the synchronous gauge. We discuss the resulting second order
anisotropies in the Poisson gauge, and analyse the possible relevance of the
different terms. We expect that, in the simplest scenarios for structure
formation, the main effect comes from the gravitational lensing by scalar
perturbations, that is known to give a few percent contribution to the
anisotropies at small angular scales.Comment: 15 pages, revtex, no figures. Version to be published in Phys. Rev.
Diet and primary prevention of stroke: Systematic review and dietary recommendations by the ad hoc Working Group of the Italian Society of Human Nutrition
Abstract Background and aims To systematically review the latest evidence on established and emerging nutrition-related risk factors for incidence of and mortality from total, ischemic and haemorrhagic strokes. The present review was conducted in the framework of the work carried out through 2015 and 2016 for the preparation of the Italian Guidelines for the Prevention and Treatment of Stroke, 8th Edition, by ISO-SPREAD (Italian Stroke Organization and the Stroke Prevention and Educational Awareness Diffusion). Methods and results Systematic review of articles focused on primary prevention of stroke published between January 2013 to May 2016 through an extensive search of the literature using MEDLINE/PUBMED, EMBASE and the Cochrane Library. Articles were ranked according to the SIGN methodology while the GRADE system was used to establish the strength of recommendations. As a result of our literature search, we examined 87 meta-analyses overall (mainly of prospective studies), a few isolated more recent prospective studies not included in the meta-analyses, and a smaller number of available randomized controlled trials and case–control studies. Based on the analysis of the above articles, 36 Syntheses of the available evidence and 36 Recommendations were eventually prepared. The present document was developed by organizing the available evidence into three individual areas (nutrients, food groups and dietary patterns) to provide a systematic and user-friendly overview of the available evidence on the relationship between nutrition and primary prevention of stroke. Yet analysis of foods and food patterns allowed translating the information about nutrients in a tool more amenable to use in daily life also in the light of the argument that people eat foods rather than nutrients. Conclusions The present literature review and dietary recommendations provide healthcare professionals and all interested readers with a useful overview for the reduction of the risk of total, ischemic and haemorrhagic stroke through dietary modifications
Seroprevalence of Kaposi Sarcoma–associated Herpesvirus and Other Serologic Markers in the Brazilian Amazon
To determine the presence of Kaposi sarcoma–associated herpesvirus (KSHV) and other serologic markers, we tested serum specimens of 339 Amerindians, 181 rural non-Amerindians, and 1,133 urban blood donors (13 Amerindians) in the Brazilian Amazon. High KSHV seroprevalence in children and inverse association with herpes simplex virus type 2 indicates predominant nonsexual transmission among Amerindians
Generation of representative primary virus isolates from blood plasma after isolation of HIV-1 with CD44 MicroBeads
Infection of cell cultures with cell-free virus isolated from HIV-infected patients is notoriously difficult and results in a loss of viral variation. Here, we describe viral sequences from PBMC, U87.CD4.CCR5 and U87.CD4.CXCR4 cell cultures and compare them to those from blood plasma from 12 patients from whom virus particles were isolated using CD44 MicroBeads. In both PBMC and U87.CD4.CCR5 cultures, 66% of the plasma viral strains were retrieved after culturing. In addition, coreceptor use was predicted based on the env-V3 sequence and tested in U87.CD4 cells expressing either CCR5 or CXCR4. Recovery was lower for the CXCR4-using viruses. Only 50% of the virus clusters predicted to use CXCR4 could be retrieved from cell cultures, while 71% of CCR5-using strains were found in U87.CCR5 cultures. Therefore, isolation of primary viruses with CD44 MicroBeads results in a good representation in cell culture of the in vivo divergence
Halos of Unified Dark Matter Scalar Field
We investigate the static and spherically symmetric solutions of Einstein's
equations for a scalar field with non-canonical kinetic term, assumed to
provide both the dark matter and dark energy components of the Universe. In
particular, we give a prescription to obtain solutions (dark halos) whose
rotation curve v_c(r) is in good agreement with observational data. We show
that there exist suitable scalar field Lagrangians that allow to describe the
cosmological background evolution and the static solutions with a single dark
fluid.Comment: 19 pages LaTeX file; minor corrections made affecting Eqs.(52)-(56
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