1,036 research outputs found
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
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
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
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
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
Post-Newtonian Cosmology
Newtonian Cosmology is commonly used in astrophysical problems, because of
its obvious simplicity when compared with general relativity. However it has
inherent difficulties, the most obvious of which is the non-existence of a
well-posed initial value problem. In this paper we investigate how far these
problems are met by using the post-Newtonian approximation in cosmology.Comment: 12 pages, Late
A phase 3 multicenter, prospective, open-label efficacy and safety study of immune globulin (human) 10% caprylate/chromatography purified in patients with myasthenia gravis exacerbations
Background: Myasthenia gravis (MG) is an autoimmune disorder affecting neuromuscular transmission. Exacerbations may involve increasing bulbar weakness and/or sudden respiratory failure, both of which can be critically disabling. Management of MG exacerbations includes plasma exchange and intravenous immunoglobulin (IVIG); they are equally effective, but patients experience fewer side effects with IVIG. The objective of this study was to assess the efficacy and safety of immune globulin caprylate/chromatography purified (IGIV-C) in subjects with MG exacerbations. Methods: This prospective, open-label, non-controlled 28-day clinical trial was conducted in adults with MG Foundation of America class IVb or V status. Subjects received IGIV-C 2 g/kg over 2 consecutive days (1 g/kg/day) and were assessed for efficacy/safety on Days 7, 14, 21, and 28. The primary efficacy endpoint was the change from Baseline in quantitative MG (QMG) score to Day 14. Secondary endpoints of clinical response, Baseline to Day 14, included at least a 3-point decrease in QMG and MG Composite and a 2-point decrease in MG-activities of daily living (MG-ADL). Results: Forty-nine subjects enrolled. The change in QMG score at Day 14 was significant (p < 0.001) in the Evaluable (-6.4, n = 43) and Safety (-6.7, n = 49) populations. Among evaluable subjects, Day 14 response rates were 77, 86, and 88% for QMG, MG Composite, and MG-ADL, respectively. IGIV-C showed good tolerability with no serious adverse events. Conclusions: The results of this study show that IGIV-C was effective, safe, and well tolerated in the treatment of MG exacerbations
Anisotropy and inflation in Bianchi I brane worlds
After a more general assumption on the influence of the bulk on the brane, we
extend some conclusions by Maartens et al. and Santos et al. on the asymptotic
behavior of Bianchi I brane worlds. As a consequence of the nonlocal
anisotropic stresses induced by the bulk, in most of our models, the brane does
not isotropize and the nonlocal energy does not vanish in the limit in which
the mean radius goes to infinity. We have also found the intriguing possibility
that the inflation due to the cosmological constant might be prevented by the
interaction with the bulk. We show that the problem for the mean radius can be
completely solved in our models, which include as particular cases those in the
references above.Comment: 10 pages, improved discussion on the likeliness of
non-isotropization, completed list of references, matches version to appear
in Class. Quantum Gra
CMB Anisotropy of Spherical Spaces
The first-year WMAP data taken at their face value hint that the Universe
might be slightly positively curved and therefore necessarily finite, since all
spherical (Clifford-Klein) space forms M^3 = S^3/Gamma, given by the quotient
of S^3 by a group Gamma of covering transformations, possess this property. We
examine the anisotropy of the cosmic microwave background (CMB) for all typical
groups Gamma corresponding to homogeneous universes. The CMB angular power
spectrum and the temperature correlation function are computed for the
homogeneous spaces as a function of the total energy density parameter
Omega_tot in the large range [1.01, 1.20] and are compared with the WMAP data.
We find that out of the infinitely many homogeneous spaces only the three
corresponding to the binary dihedral group T*, the binary octahedral group O*,
and the binary icosahedral group I* are in agreement with the WMAP
observations. Furthermore, if Omega_tot is restricted to the interval [1.00,
1.04], the space described by T* is excluded since it requires a value of
Omega_tot which is probably too large being in the range [1.06, 1.07]. We thus
conclude that there remain only the two homogeneous spherical spaces S^3/O* and
S^3/I* with Omega_tot of about 1.038 and 1.018, respectively, as possible
topologies for our Universe.Comment: A version with high resolution sky maps can be obtained at
http://www.physik.uni-ulm.de/theo/qc
- …