1,730 research outputs found
On the Significance of the Weyl Curvature in a Relativistic Cosmological Model
The Weyl curvature includes the Newtonian field and an additional field, the
so-called anti-Newtonian. In this paper, we use the Bianchi and Ricci
identities to provide a set of constraints and propagations for the Weyl
fields. The temporal evolutions of propagations manifest explicit solutions of
gravitational waves. We see that models with purely Newtonian field are
inconsistent with relativistic models and obstruct sounding solutions.
Therefore, both fields are necessary for the nonlocal nature and radiative
solutions of gravitation.Comment: 15 pages, incorporating proof correction
Newtonian and Post-Newtonian approximations of the k = 0 Friedmann Robertson Walker Cosmology
In a previous paper we derived a post-Newtonian approximation to cosmology
which, in contrast to former Newtonian and post-Newtonian cosmological
theories, has a well-posed initial value problem. In this paper, this new
post-Newtonian theory is compared with the fully general relativistic theory,
in the context of the k = 0 Friedmann Robertson Walker cosmologies. It is found
that the post-Newtonian theory reproduces the results of its general
relativistic counterpart, whilst the Newtonian theory does not.Comment: 11 pages, Latex, corrected typo
A novel approach to identify blazar emission states using clustering algorithms
Even after decades of multi-wavelength (MWL) observations, blazars still
remain mysterious objects. Their extreme variability and variety of emission
characteristics observed during different time periods make it hard to
understand the fundamental processes behind their emission. Thus, a robust
identification and characterization of the different emission states among
blazars is vital to investigate the underlying processes causing the observed
emission. In this contribution, we present a novel technique to determine
emission states across MWL lightcurves (LCs) of blazars using a clustering
algorithm. Using the Extreme Deconvolution algorithm, we apply a Gaussian
Mixture model to the 12-year long-term LC of one of our archetypal blazars, Mrk
501. The two main advantages of the method are that, compared to more
conventional methods, such as the Bayesian block algorithm, it considers
multiple wavebands simultaneously and it is not dependent on the order in time
of the data points. This allows to assign data points to the same emission
state even though they are separated by other states in time. The well sampled
gamma-ray, X-ray and radio LCs used as input allow to identify six clusters.
The clustering is mainly driven by the X-ray flux, showing different levels of
quiescent, intermediate and high flux states. However, the radio flux reveals a
more complicated pattern, dividing some of the X-ray flux levels in low and
high-radio flux states. This suggests that multiple emission regions maybe
responsible for the radio to gamma-ray flux.Comment: 8 pages, 1 figure, Presented at the 38th International Cosmic Ray
Conference (ICRC 2023), 202
Tachyonic potential in Bianchi type-I universe
Motivated from recent string theoretic results, a tachyonic potential is
constructed for a spatially homogeneous and anisotropic background cosmology.Comment: 5 pages,LATEX,Typos in the text corrected, more references adde
Cosmological Models with Shear and Rotation
Cosmological models involving shear and rotation are considered, first in the
General Relat ivistic and then in the Newtonian framework with the aim of
investigating singularities in them by using numerical and analytical
techniques. The dynamics of these rotating models ar e studied. It is shown
that singularities are unavoidable in such models and that the centr ifugal
force arising due to rotation can never overcome the gravitational and shearing
forc e over a length of time.Comment: 17 pages, 6 figures Journal Ref: J. Astrophys. Astr. (1999) 20, 79-8
Post-Newtonian extension of the Newton-Cartan theory
The theory obtained as a singular limit of General Relativity, if the
reciprocal velocity of light is assumed to tend to zero, is known to be not
exactly the Newton-Cartan theory, but a slight extension of this theory. It
involves not only a Coriolis force field, which is natural in this theory
(although not original Newtonian), but also a scalar field which governs the
relation between Newtons time and relativistic proper time. Both fields are or
can be reduced to harmonic functions, and must therefore be constants, if
suitable global conditions are imposed. We assume this reduction of
Newton-Cartan to Newton`s original theory as starting point and ask for a
consistent post-Newtonian extension and for possible differences to usual
post-Minkowskian approximation methods, as developed, for example, by
Chandrasekhar. It is shown, that both post-Newtonian frameworks are formally
equivalent, as far as the field equations and the equations of motion for a
hydrodynamical fluid are concerned.Comment: 13 pages, LaTex, to appear in Class. Quantum Gra
BLISTER Regulates Polycomb-Target Genes, Represses Stress-Regulated Genes and Promotes Stress Responses in Arabidopsis thaliana
Polycomb group (PcG) proteins are key epigenetic regulators of development.
The highly conserved Polycomb repressive complex 2 (PRC2) represses thousands
of target genes by trimethylating H3K27 (H3K27me3). Plant specific PcG
components and functions are largely unknown, however, we previously
identified the plant-specific protein BLISTER (BLI) as a PRC2 interactor. BLI
regulates PcG target genes and promotes cold stress resistance. To further
understand the function of BLI, we analyzed the transcriptional profile of
bli-1 mutants. Approximately 40% of the up-regulated genes in bli are PcG
target genes, however, bli-1 mutants did not show changes in H3K27me3 levels
at all tested genes, indicating that BLI regulates PcG target genes downstream
of or in parallel to PRC2. Interestingly, a significant number of BLI
regulated H3K27me3 target genes is regulated by the stress hormone absciscic
acid (ABA). We further reveal an overrepresentation of genes responding to
abiotic stresses such as drought, high salinity, or heat stress among the up-
regulated genes in bli mutants. Consistently, bli mutants showed reduced
desiccation stress tolerance. We conclude that the PRC2 associated protein BLI
is a key regulator of stress-responsive genes in Arabidopsis: it represses
ABA-responsive PcG target genes, likely downstream of PRC2, and promotes
resistance to several stresses such as cold and drought
Sensory neuropathy and metabolic risk factors in human immune deficiency virus infected South Africans receiving protease inhibitors
BackgroundProtease inhibitors (PI)s have been associated with distal sensory polyneuropathy (DSP) and metabolic complications in high-income countries. No data exist in Africans where second-line antiretroviral therapy (ART) often include PIs.MethodWe performed a cross-sectional study to assess the DSP frequency and metabolic risk factors in community-based South Africans taking ritonavir-boosted lopinavir as PI. Examination findings categorized subjects as having DSP (≥1 neuropathic sign) or symptomatic DSP [DSP with symptom(s)]. Fasting-state glucose and lipid profiles were assessed. We compared the ritonavir/lopinavir-group to a nested group on first-line ART [dideoxy-nucleoside reverse transcriptase inhibitors (d-drugs)] selected from a dataset collected at the same time and matched for d-drug exposure.ResultsThe ritonavir/lopinavir-group (n=86) consisted predominantly of women (84%) with a median age of 36years (IQR 32–41). The median current CD4+ count was 489cells/μL (IQR 291–665). The median exposure time to ritonavir/lopinavir was 18months (IQR 10–26) and to d-drugs, 24months (IQR 16–38). DSP was present in 78% and symptomatic DSP in 48%; symptoms were most frequently of moderate intensity. Only age independently associated with DSP and symptomatic DSP (p=0.08 and p=0.04, respectively). None of the metabolic syndrome components showed associations with DSP or symptomatic DSP despite a trend towards hypertriglyceridemia overall. The ritonavir/lopinavir-group had less DSP compared to the d-drug only group (p=0.002) but the frequency of symptomatic DSP was similar (p=0.49).ConclusionRitonavir-boosted lopinavir did not add additional risk to developing DSP in this community-based African cohort after a median of 18months on second-line ART.Electronic supplementary materialThe online version of this article (doi:10.1186/s12981-015-0073-8) contains supplementary material, which is available to authorized users
Validation of the self regulation questionnaire as a measure of health in quality of life research
<p>Abstract</p> <p>Objectives</p> <p>Several epidemiological studies address psychosomatic 'self regulation' as a measure of quality of life aspects. However, although widely used in studies with a focus on complementary cancer treatment, and recognized to be associated with better survival of cancer patients, it is unclear what the 'self regulation' questionnaire exactly measures.</p> <p>Design and setting</p> <p>In a sample of 444 individuals (27% healthy, 33% cancer, 40% other internal diseases), we performed reliability and exploratory factor analyses, and correlated the 16-item instrument with external measures such as the Hospital Anxiety and Depression Scale, the Herdecke Quality of Life questionnaire, and autonomic regulation questionnaire.</p> <p>Results</p> <p>The 16-item pool had a very good internal consistency (Cronbach's alpha = 0.948) and satisfying/good (r<sub>rt </sub>= 0.796) test-retest reliability after 3 months. Exploratory factor analysis indicated 2 sub-constructs: (1) Ability to change behaviour in order to reach goals, and (2) Achieve satisfaction and well-being. Both sub-scales correlated well with quality of life aspects, particularly with Initiative Power/Interest, Social Interactions, Mental Balance, and negatively with anxiety and depression.</p> <p>Conclusions</p> <p>The Self Regulation Questionnaire (SRQ) was found to be a valid and reliable tool which measures unique psychosomatic abilities. Self regulation deals with competence and autonomy and can be regarded as a problem solving capacity in terms of an active adaptation to stressful situations to restore wellbeing. The tool is an interesting option to be used particularly in complementary medicine research with a focus on behavioural modification.</p
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