122 research outputs found
Alfven modes driven non-linearly by metric perturbations in anisotropic magnetized cosmologies
We consider anisotropic magnetized cosmologies filled with conductive plasma
fluid and study the implications of metric perturbations that propagate
parallel to the ambient magnetic field. It is known that in the first order
(linear) approximation with respect to the amplitude of the perturbations no
electric field and density perturbations arise. However, when we consider the
non-linear coupling of the metric perturbations with their temporal
derivatives, certain classes of solutions can induce steeply increasing in time
electric field perturbations. This is verified both numerically and
analytically. The source of these perturbations can be either high-frequency
quantum vacuum fluctuations, driven by the cosmological pump field, in the
early stages of the evolution of the Universe or astrophysical processes or a
non-linear isotropization process of an initially anisotropic cosmological
spacetime.Comment: 7 pages, RevTex, 3 figures ps, accepted for publication to IJMP
Gravito-magnetic instabilities in anisotropically expanding fluids
Gravitational instabilities in a magnetized Friedman - Robertson - Walker
(FRW) Universe, in which the magnetic field was assumed to be too weak to
destroy the isotropy of the model, are known and have been studied in the past.
Accordingly, it became evident that the external magnetic field disfavors the
perturbations' growth, suppressing the corresponding rate by an amount
proportional to its strength. However, the spatial isotropy of the FRW Universe
is not compatible with the presence of large-scale magnetic fields. Therefore,
in this article we use the general-relativistic (GR) version of the
(linearized) perturbed magnetohydrodynamic equations with and without
resistivity, to discuss a generalized Jeans criterion and the potential
formation of density condensations within a class of homogeneous and
anisotropically expanding, self-gravitating, magnetized fluids in curved
space-time. We find that, for a wide variety of anisotropic cosmological
models, gravito-magnetic instabilities can lead to sub-horizonal, magnetized
condensations. In the non-resistive case, the power spectrum of the unstable
cosmological perturbations suggests that most of the power is concentrated on
large scales (small k), very close to the horizon. On the other hand, in a
resistive medium, the critical wave-numbers so obtained, exhibit a delicate
dependence on resistivity, resulting in the reduction of the corresponding
Jeans lengths to smaller scales (well bellow the horizon) than the
non-resistive ones, while increasing the range of cosmological models which
admit such an instability.Comment: 10 pages RevTex, 4 figures, accepted for publication in the
International Journal of Modern Physics
Plasma waves driven by gravitational waves in an expanding universe
In a Friedmann-Robertson-Walker (FRW) cosmological model with zero spatial
curvature, we consider the interaction of the gravitational waves with the
plasma in the presence of a weak magnetic field. Using the relativistic
hydromagnetic equations it is verified that large amplitude magnetosonic waves
are excited, assuming that both, the gravitational field and the weak magnetic
field do not break the homogeneity and isotropy of the considered FRW
spacetime.Comment: 14 page
Composite p-Branes in Diverse Dimensions
We use a simple algebraic method to find a special class of composite p-brane
solutions of higher dimensional gravity coupled with matter. These solutions
are composed of n constituent p-branes corresponding n independent harmonic
functions. A simple algebraic criteria of existence of such solutions is
presented. Relations with D=11,10 known solutions are discussed.Comment: 11 pages, latex, emlines2.sty; corrections in Sect.
Galaxy formation and cosmic-ray acceleration in a magnetized universe
We study the linear magneto-hydrodynamical behaviour of a Newtonian cosmology
with a viscous magnetized fluid of finite conductivity and generalise the Jeans
instability criterion. The presence of the field favors the anisotropic
collapse of the fluid, which in turn leads to further magnetic amplification
and to an enhanced current-sheet formation in the plane normal to the ambient
magnetic field. When the currents exceed a certain threshold, the resulting
electrostatic turbulence can dramatically amplify the resistivity of the medium
(anomalous resistivity). This could trigger strong electric fields and
subsequently the acceleration of ultra-high energy cosmic rays (UHECRs) during
the formation of protogalactic structures.Comment: 10 pages, ApJL in pres
Dynamo effects in magnetized ideal-plasma cosmologies
The excitation of cosmological perturbations in an anisotropic cosmological
model and in the presence of a homogeneous magnetic field has been studied,
using the ideal magnetohydrodynamic (MHD) equations. In this case, the system
of partial differential equations which governs the evolution of the magnetized
cosmological perturbations can be solved analytically. Our results verify that
fast-magnetosonic modes propagating normal to the magnetic field, are excited.
But, what's most important, is that, at late times, the magnetic-induction
contrast grows, resulting in the enhancement of the ambient magnetic field.
This process can be particularly favored by condensations, formed within the
plasma fluid due to gravitational instabilities.Comment: 7 pages, RevTex, accepted for publication to IJMP
Exactly Solvable Model of Superstring in Plane-wave Background with Linear Null Dilaton
In this paper, we study an exactly solvable model of IIB superstring in a
time-dependent plane-wave backgound with a constant self-dual Ramond-Ramond
5-form field strength and a linear dilaton in the light-like direction. This
background keeps sixteen supersymmetries. In the light-cone gauge, the action
is described by the two-dimensional free bosons and fermions with
time-dependent masses. The model could be canonically quantized and its
Hamiltonian is time-dependent with vanishing zero point energy. The spectrum of
the excitations is symmetric between the bosonic and fermionic sector. The
string mode creation turns out to be very small.Comment: 35 pages, Latex; Acknowledgement added; Published versio
Intersecting Branes
BPS configurations of intersecting branes have many applications in string
theory. We attempt to provide an introductory and pedagogical review of
supergravity solutions corresponding to orthogonal BPS intersections of branes
with an emphasis on eleven and ten space-time dimensions. Recent work on BPS
solutions corresponding to non-orthogonally intersecting branes is also
discussed. These notes are based on lectures given at the APCTP Winter School
"Dualities of Gauge and String Theories", Korea, February 1997.Comment: Latex, 48 pages. Typos corrected. Various minor improvement
Toda p-brane black holes and polynomials related to Lie algebras
Black hole generalized p-brane solutions for a wide class of intersection
rules are obtained. The solutions are defined on a manifold that contains a
product of n - 1 Ricci-flat internal spaces. They are defined up to a set of
functions H_s obeying non-linear differential equations equivalent to Toda-type
equations with certain boundary conditions imposed. A conjecture on polynomial
structure of governing functions H_s for intersections related to semisimple
Lie algebras is suggested. This conjecture is proved for Lie algebras: A_m,
C_{m+1}, m > 0. For simple Lie algebras the powers of polynomials coincide with
the components of twice the dual Weyl vector in the basis of simple coroots.
The coefficients of polynomials depend upon the extremality parameter \mu >0.
In the extremal case \mu = 0 such polynomials were considered previously by H.
L\"u, J. Maharana, S. Mukherji and C.N. Pope. Explicit formulas for
A_2-solution are obtained. Two examples of A_2-dyon solutions, i.e. dyon in D =
11 supergravity with M2 and M5 branes intersecting at a point and Kaluza-Klein
dyon, are considered.Comment: 24 pages, Latex, typos are eliminated, a correct relation on
parameters of special block-orthogonal solution is added in third line after
eq. (4.10
Baldness/delocalization in intersecting brane systems
Marginally bound systems of two types of branes are considered, such as the
prototypical case of Dp+4 branes and Dp branes. As the transverse separation
between the two types of branes goes to zero, different behaviour occurs in the
supergravity solutions depending on p; no-hair theorems result for p<=1 only.
Within the framework of the AdS/CFT correspondence, these supergravity no-hair
results are understood as dual manifestations of the Coleman-Mermin-Wagner
theorem. Furthermore, the rates of delocalization for p<=1 are matched in a
scaling analysis. Talk given at ``Strings '99''; based on hep-th/9903213 with
D. Marolf.Comment: LaTeX, 10 pages, 1 figure; contribution to Strings'99 proceeding
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