992 research outputs found
Redshift drift in axially symmetric quasi-spherical Szekeres models
Models of inhomogeneous universes constructed with exact solutions of
Einstein's General Relativity have been proposed in the literature with the aim
of reproducing the cosmological data without any need for a dark energy
component. Besides large scale inhomogeneity models spherically symmetric
around the observer, Swiss-cheese models have also been studied. Among them,
Swiss-cheeses where the inhomogeneous patches are modeled by different
particular Szekeres solutions have been used for reproducing the apparent
dimming of the type Ia supernovae (SNIa). However, the problem of fitting such
models to the SNIa data is completely degenerate and we need other constraints
to fully characterize them. One of the tests which is known to be able to
discriminate between different cosmological models is the redshift-drift. This
drift has already been calculated by different authors for
Lema\^itre-Tolman-Bondi (LTB) models. We compute it here for one particular
axially symmetric quasi-spherical Szekeres (QSS) Swiss-cheese which has
previously been shown to reproduce to a good accuracy the SNIa data, and we
compare the results to the drift in the CDM model and in some LTB
models that can be found in the literature. We show that it is a good
discriminator between them. Then, we discuss our model's remaining degrees of
freedom and propose a recipe to fully constrain them.Comment: 15 pages, 7 figures, minor changes in title, text, figures and
references; conclusions unchanged, this version matches the published versio
The Fermi-Pasta-Ulam recurrence and related phenomena for 1D shallow-water waves in a finite basin
In this work, different regimes of the Fermi-Pasta-Ulam (FPU) recurrence are
simulated numerically for fully nonlinear "one-dimensional" potential water
waves in a finite-depth flume between two vertical walls. In such systems, the
FPU recurrence is closely related to the dynamics of coherent structures
approximately corresponding to solitons of the integrable Boussinesq system. A
simplest periodic solution of the Boussinesq model, describing a single soliton
between the walls, is presented in an analytical form in terms of the elliptic
Jacobi functions. In the numerical experiments, it is observed that depending
on a number of solitons in the flume and their parameters, the FPU recurrence
can occur in a simple or complicated manner, or be practically absent. For
comparison, the nonlinear dynamics of potential water waves over nonuniform
beds is simulated, with initial states taken in the form of several pairs of
colliding solitons. With a mild-slope bed profile, a typical phenomenon in the
course of evolution is appearance of relatively high (rogue) waves, while for
random, relatively short-correlated bed profiles it is either appearance of
tall waves, or formation of sharp crests at moderate-height waves.Comment: revtex4, 10 pages, 33 figure
You Can't Get Through Szekeres Wormholes - or - Regularity, Topology and Causality in Quasi-Spherical Szekeres Models
The spherically symmetric dust model of Lemaitre-Tolman can describe
wormholes, but the causal communication between the two asymptotic regions
through the neck is even less than in the vacuum
(Schwarzschild-Kruskal-Szekeres) case. We investigate the anisotropic
generalisation of the wormhole topology in the Szekeres model. The function
E(r, p, q) describes the deviation from spherical symmetry if \partial_r E \neq
0, but this requires the mass to be increasing with radius, \partial_r M > 0,
i.e. non-zero density. We investigate the geometrical relations between the
mass dipole and the locii of apparent horizon and of shell-crossings. We
present the various conditions that ensure physically reasonable
quasi-spherical models, including a regular origin, regular maxima and minima
in the spatial sections, and the absence of shell-crossings. We show that
physically reasonable values of \partial_r E \neq 0 cannot compensate for the
effects of \partial_r M > 0 in any direction, so that communication through the
neck is still worse than the vacuum.
We also show that a handle topology cannot be created by identifying
hypersufaces in the two asymptotic regions on either side of a wormhole, unless
a surface layer is allowed at the junction. This impossibility includes the
Schwarzschild-Kruskal-Szekeres case.Comment: zip file with LaTeX text + 6 figures (.eps & .ps). 47 pages. Second
replacement corrects some minor errors and typos. (First replacement prints
better on US letter size paper.
Experimental Study of Antimicrobial Properties and Acute Toxicity of the Peo-based Combined Suppositories
Aim. The research of antimicrobial and toxicological parameters of a promising pharmaceutical composition with indole-3-carbinol and meloxicam in the form of rectal suppositories.Materials and methods. The research of antimicrobial activity was carried out in vitro by diffusion in nutrient agar in the modification of "holes" on the reference strains of common pathogens Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Bacillus subtilis, Candida albicans.Adult rats were used for the study of acute toxicity. Suppository mass were administrated in the largest possible volume rectally or orally. The animals were periodically monitored according to the experimental plan – the assessment of physiological parameters before administration and after 6, 12, 18, 24 hours, at 3, 7 and 14 days. The animals were removed from experiment and necropsy provided after 1, 3, 7, and 14 days.Results. The sizes of zones of inhibition of the microorganisms growth were most significant (from 19.27±0.61 mm of E. coli to 40.80±0.42 mm of S. aureus) near sample of the combined composition suppository compared with other combination of active substances and excipients.During the observation of animals for 14 days and the study of internal organs after autopsy, deviations in physiological (weight, temperature, activity, respiratory rate) and macroscopic morphological indicators of animals from reference values were not detected.Conclusion. According to the results of microbiological studies, a moderate antimicrobial effect of suppositories of combined composition in relation to all the studied pathogens was revealed. The absence of manifestations of acute toxicity allows us to conclude that the pharmaceutical composition can be classified as practically non-toxic substances.The obtained results allow us to recommend a pharmaceutical composition with indole-3-carbinol and meloxicam on a polyethylene oxide basis in the form of suppositories for further preclinical studies of specific pharmacological effects as a prostate protective agent
Current-sheet formation in incompressible electron magnetohydrodynamics
The nonlinear dynamics of axisymmetric, as well as helical, frozen-in vortex
structures is investigated by the Hamiltonian method in the framework of ideal
incompressible electron magnetohydrodynamics. For description of current-sheet
formation from a smooth initial magnetic field, local and nonlocal nonlinear
approximations are introduced and partially analyzed that are generalizations
of the previously known exactly solvable local model neglecting electron
inertia. Finally, estimations are made that predict finite-time singularity
formation for a class of hydrodynamic models intermediate between that local
model and the Eulerian hydrodynamics.Comment: REVTEX4, 5 pages, no figures. Introduction rewritten, new material
and references adde
Cosmic String in Scalar-Tensor Gravity
The gravitational properties of a local cosmic string in the framework of
scalar-tensor gravity are examined. We find the metric in the weak-field
approximation and we show that, contrary to the General Relativity case, the
cosmic string in scalar-tensor gravitation exerces a force on non-relativistic,
neutral test particle. This force is proportional to the derivative of the
conformal factor and it is always attractive. Moreover, this
force could have played an important role at the Early Universe, although
nowadays it can be neglegible. It is also shown that the angular separation
remains unaltered for scalar-tensor cosmic strings.Comment: 15 pages, LATEX, no figure
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