663 research outputs found
Gravitational Instability in Presence of Bulk Viscosity: the Jeans Mass and the Quasi-Isotropic Solution
This paper focuses on the analysis of the gravitational instability in
presence of bulk viscosity both in Newtonian regime and in the
fully-relativistic approach. The standard Jeans Mechanism and the
Quasi-Isotropic Solution are treated expressing the bulk-viscosity coefficient
as a power-law of the fluid energy density , i.e.,
\zeta=\zo\rho^{s}. In the Newtonian regime, the perturbation evolution is
founded to be damped by viscosity and the top-down mechanism of structure
fragmentation is suppressed. The value of the Jeans Mass remains unchanged also
in presence of viscosity. In the relativistic approach, we get a power-law
solution existing only in correspondence to a restricted domain of \zo.Comment: 3 pages, Proceedings of The XII Marcel Grossmann Meetin
Heterotic Resolved Conifolds with Torsion, from Supergravity to CFT
We obtain a family of heterotic supergravity backgrounds describing
non-Kahler warped conifolds with three-form flux and an Abelian gauge bundle,
preserving N=1 supersymmetry in four dimensions. At large distance from the
singularity the usual Ricci-flat conifold is recovered. By performing a Z_2
orbifold of the T^{1,1} base, the conifold singularity can be blown-up to a
four-cycle, leading to a completely smooth geometry. Remarkably, the throat
regions of the solutions, which can be isolated from the asymptotic Ricci-flat
geometry using a double-scaling limit, possess a worldsheet CFT description in
terms of heterotic cosets whose target space is the warped resolved
orbifoldized conifold. Thus this construction provides exact solutions of the
modified Bianchi identity. By solving algebraically these CFTs we compute the
exact tree-level heterotic string spectrum and describe worldsheet
non-perturbative effects. The holographic dual of these solutions, in
particular their confining behavior, and the embedding of these fluxed
singularities into heterotic compactifications with torsion are also discussed.Comment: 52 pages, 5 figures; v2: small correction
Heterotic warped Eguchi-Hanson spectra with five-branes and line bundles
We consider heterotic strings on a warped Eguchi-Hanson space with five-brane
and line bundle gauge fluxes. The heterotic string admits an exact CFT
description in terms of an asymmetrically gauged SU(2)xSL(2,R) WZW model, in a
specific double scaling limit in which the blow-up radius and the string scale
are sent to zero simultaneously. This allows us to compute the perturbative 6D
spectra for these models in two independent fashions: i) Within the
supergravity approximation we employ a representation dependent index; ii) In
the double scaling limit we determine all marginal vertex operators of the
coset CFT. To achieve agreement between the supergravity and the CFT spectra,
we conjecture that the untwisted and the twisted CFT states correspond to the
same set of hyper multiplets in supergravity. This is in a similar spirit as a
conjectured duality between asymptotically linear dilaton CFTs and little
string theory living on NS-five-branes. As the five-brane charge is
non-vanishing, heterotic (anti-)five-branes have to be added in order to cancel
irreducible gauge anomalies. The local spectra can be combined in such a way
that supersymmetry is preserved on the compact resolved T^4/Z_2 orbifold by
choosing the local gauge fluxes appropriately.Comment: 1+36 pages LaTe
Quasi-linear model for the beam-plasma instability: analysis of the self-consistent evolution
We re-analyze the quasi-linear self consistent dynamics for the beam-plasma
instability, by comparing the theory predictions to numerical simulations of
the corresponding Hamiltonian system. While the diffusive features of the
asymptotic dynamics are reliably predicted, the early temporal mesoscale
transport appears less efficient in reproducing the convective feature of the
self-consistent scenario. As a result, we identify the origin of the observed
discrepancy in the underlying quasi-linear model assumption that the
distribution function is quasi-stationary. Furthermore, we provide a correction
to the instantaneous quasi-linear growth rate based on a linear expansion of
the distribution function time dependence, and we successfully test this
revised formulation for the spectral evolution during the temporal mesoscale.Comment: 12 pages, 5 figure
Coexistence of Magneto-Rotational and Jeans Instabilities in an Axisymmetric Nebula
We analyze the magneto-rotational instability (MRI) effects on gravitational
collapse and its influence on the instability critical scale. In particular, we
study an axisymmetric nonstratified differentially rotating cloud, embedded in
a small magnetic field, and we perform a local linear stability analysis,
including the self gravity of the system. We demonstrate that the linear
evolution of the perturbations is characterized by the emergence of an
anisotropy degree of the perturbed mass densities. Starting with spherical
growing overdensities, we see that they naturally acquire an anisotropy of
order unity in their shape. Despite the linear character of our analysis, we
infer that such a seed of anisotropy can rapidly grow in a nonlinear regime,
leading to the formation of filament-like structures. However, we show how such
an anisotropy is essentially an intrinsic feature of the Jean instability, and
how MRI only plays a significant role in fixing the critical scale of the mode
spectrum. We then provide a characterization of the present analysis in terms
of the cosmological setting, in order to provide an outlook of how the present
results could concern the formation of large-scale structures across the
Universe.Comment: 8 pages, 4 figure
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