8,809 research outputs found
Elastic turbulence in shear banding wormlike micelles
We study the dynamics of the Taylor-Couette flow of shear banding wormlike
micelles. We focus on the high shear rate branch of the flow curve and show
that for sufficiently high Weissenberg numbers, this branch becomes unstable.
This instability is strongly sub-critical and is associated with a shear stress
jump. We find that this increase of the flow resistance is related to the
nucleation of turbulence. The flow pattern shows similarities with the elastic
turbulence, so far only observed for polymer solutions. The unstable character
of this branch led us to propose a scenario that could account for the recent
observations of Taylor-like vortices during the shear banding flow of wormlike
micelles
Instanton Corrected Non-Supersymmetric Attractors
We discuss non-supersymmetric attractors with an instanton correction in Type
IIA string theory compactified on a Calabi-Yau three-fold at large volume. For
a stable non-supersymmetric black hole, the attractor point must minimize the
effective black hole potential. We study the supersymmetric as well as
non-supersymmetric attractors for the D0-D4 system with instanton corrections.
We show that in simple models, like the STU model, the flat directions of the
mass matrix can be lifted by a suitable choice of the instanton parameters.Comment: Minor modifications, Corrected typos, 38 pages, 1 figur
Quasinormal modes of d-dimensional spherical black holes with a near extreme cosmological constant
We derive an expression for the quasinormal modes of scalar perturbations in
near extreme d-dimensional Schwarzschild-de Sitter and Reissner-Nordstrom-de
Sitter black holes. We show that, in the near extreme limit, the dynamics of
the scalar field is characterized by a Poschl-Teller effective potential. The
results are qualitatively independent of the spacetime dimension and field
mass.Comment: 5 pages, REVTeX4, version to be published in Physical Review
BPS black holes, the Hesse potential, and the topological string
The Hesse potential is constructed for a class of four-dimensional N=2
supersymmetric effective actions with S- and T-duality by performing the
relevant Legendre transform by iteration. It is a function of fields that
transform under duality according to an arithmetic subgroup of the classical
dualities reflecting the monodromies of the underlying string compactification.
These transformations are not subject to corrections, unlike the
transformations of the fields that appear in the effective action which are
affected by the presence of higher-derivative couplings. The class of actions
that are considered includes those of the FHSV and the STU model. We also
consider heterotic N=4 supersymmetric compactifications. The Hesse potential,
which is equal to the free energy function for BPS black holes, is manifestly
duality invariant. Generically it can be expanded in terms of powers of the
modulus that represents the inverse topological string coupling constant,
, and its complex conjugate. The terms depending holomorphically on
are expected to correspond to the topological string partition function and
this expectation is explicitly verified in two cases. Terms proportional to
mixed powers of and are in principle present.Comment: 28 pages, LaTeX, added comment
Hartle-Hawking Wave-Function for Flux Compactifications
We argue that the topological string partition function, which has been known
to correspond to a wave-function, can be interpreted as an exact
``wave-function of the universe'' in the mini-superspace sector of physical
superstring theory. This realizes the idea of Hartle and Hawking in the context
of string theory, including all loop quantum corrections. The mini-superspace
approximation is justified as an exact description of BPS quantities. Moreover
this proposal leads to a conceptual explanation of the recent observation that
the black hole entropy is the square of the topological string wave-function.
This wave-function can be interpreted in the context of flux compactification
of all spatial dimensions as providing a physical probability distribution on
the moduli space of string compactification. Euclidean time is realized
holographically in this setup.Comment: 37 pages, 2 figure
Populações de minhocas (abundância e riqueza especĂfica) em um gradiente altitudinal no Pico Caratuva na Serra do Mar do Paraná.
Earthworm populations in an altitudinal gradient (1000-1850m) of the Coastal Atlantic Rainforest, Brazil.
Field propagation in de Sitter black holes
We present an exhaustive analysis of scalar, electromagnetic and
gravitational perturbations in the background of Schwarzchild-de Sitter and
Reissner-Nordstrom-de Sitter spacetimes. The field propagation is considered by
means of a semi-analytical (WKB) approach and two numerical schemes: the
characteristic and general initial value integrations. The results are compared
near the extreme cosmological constant regime, where analytical results are
presented. A unifying picture is established for the dynamics of different spin
fields.Comment: 15 pages, 16 figures, published versio
Quasi-normal modes of Schwarzschild-de Sitter black holes
The low-laying frequencies of characteristic quasi-normal modes (QNM) of
Schwarzschild-de Sitter (SdS) black holes have been calculated for fields of
different spin using the 6th-order WKB approximation and the approximation by
the P\"{o}shl-Teller potential. The well-known asymptotic formula for large
is generalized here on a case of the Schwarzchild-de Sitter black hole. In the
limit of the near extreme term the results given by both methods are
in a very good agreement, and in this limit fields of different spin decay with
the same rate.Comment: 9 pages, 1 ancillary Mathematica(R) noteboo
Gravitational quasinormal radiation of higher-dimensional black holes
We find the gravitational resonance (quasinormal) modes of the higher
dimensional Schwarzschild and Reissner-Nordstrem black holes. The effect on the
quasinormal behavior due to the presence of the term is investigated.
The QN spectrum is totally different for different signs of . In more
than four dimensions there excited three types of gravitational modes: scalar,
vector, and tensor. They produce three different quasinormal spectra, thus the
isospectrality between scalar and vector perturbations, which takes place for
D=4 Schwarzschild and Schwarzschild-de-Sitter black holes, is broken in higher
dimensions. That is the scalar-type gravitational perturbations, connected with
deformations of the black hole horizon, which damp most slowly and therefore
dominate during late time of the black hole ringing.Comment: 13 pages, 2 figures, several references are adde
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