462 research outputs found
Seasonal changes in the pelagic catch of two clupeid zooplanktivores in relation to the abundance of copepod zooplankton in the northern end of Lake Tanganyika
Catches of clupeid fish were recorded twice a week from February 2007 to May 2008 in the northern end of Lake Tanganyika, and allocated to species (Stolothrissa tanganicae and Limnothrissa miodon) according to representative catch samples from ten artisanal lift-net fishing units. In each sample, clupeids were measured and weighed for length frequency analysis. Age was estimated from length growth curves based on otolith weight. Copepod zooplankton was sampled twice a month from February 2007 to January 2008. Peaks of copepod zooplankton were recorded in the rainy season, and there was overall a tight positive correlation between monthly rainfall and copepod biomass. The clupeids appeared in the catch at 30-50mm length when they were two-three (S. tanganicae) or three-four months old (L. miodon). For S. tanganicae, three catch peaks were due to cohorts born when copepod food was abundant, but one catch peak was due to a cohort which originated in the dry season when copepods were scarce. Likewise, two of the L. miodon cohorts giving rise to high catches likely originated from the rainy season when food was abundant, but two cohorts apparently originated from the dry season with low food conditions. The success of several cohorts of both clupeids therefore seems to be linked to rainfall and abundance of copepods, but sometimes strong cohorts could arise even under poor food conditions. Both species were recruited in the catch far before the age of maturity, making them vulnerable to overfishing.</p
Chronology protection in stationary three-dimensional spacetimes
We study chronology protection in stationary, rotationally symmetric
spacetimes in 2+1 dimensional gravity, focusing especially on the case of
negative cosmological constant. We show that in such spacetimes closed timelike
curves must either exist all the way to the boundary or, alternatively, the
matter stress tensor must violate the null energy condition in the bulk. We
also show that the matter in the closed timelike curve region gives a negative
contribution to the conformal weight from the point of view of the dual
conformal field theory. We illustrate these properties in a class of examples
involving rotating dust in anti-de Sitter space, and comment on the use of the
AdS/CFT correspondence to study chronology protection.Comment: 20 pages. V2: minor corrections, Outlook expanded, references added,
published versio
Warm tachyonic inflation in warped background
We analyze warm tachyonic inflation, proposed in the literature, but from the
viewpoint of four dimensional effective action for tachyon field on a non-BPS
D3-brane. We find that consistency with observational data on density
perturbation and validity of effective action requires warped compactification.
The number of background branes which source the flux is found to be of the
order of 10 in contrast to the order of in the standard cold
inflationary scenario.Comment: 9 pages, RevTe
Open String Attractors
We present a simple example of a supersymmetric attractor mechanism in the
purely open string context of D-branes embedded in curved space-time. Our
example involves a class of D3-branes embedded in the 2-charge D1-D5 background
of type IIB whose worldvolume contains a 2-sphere. Turning on worldvolume
fluxes, these branes carry induced (p,q) string charges. Supersymmetric
configurations display a flow of the open string moduli towards an attractor
solution independent of their asymptotics. The equations governing this
mechanism closely resemble the attractor flow equations for supersymmetric
black holes in closed string theory. The BPS equations take the form of a
gradient flow and describe worldvolume solitons interpolating between an AdS_2
geometry where the two-sphere has collapsed, and an attractor solution with
AdS_2 x S^2 geometry. In these limiting solutions, the preserved supersymmetry
is enhanced from 4 to 8 supercharges. We also discuss the interpretation of our
solutions as intersecting brane configurations placed in the D1-D5 background,
as well as the S-duality transformation to the F1-NS5 background.Comment: 37 pages, 6 figures. v2: small corrections, figure and references
adde
Dilaton transformation under abelian and non-abelian T-duality in the path integral approach
We present a convenient method for deriving the transformation of the dilaton
under T-duality in the path-integral approach. Subtleties arising in performing
the integral over the gauge fields are carefully analysed using Pauli-Villars
regularization, thereby clarifying existing ambiguities in the literature. The
formalism can not only be applied to the abelian case, but, and this for the
first time, to the non-abelian case as well. Furthermore, by choosing a
particular gauge, we directly obtain the target-space covariant expression for
the dual geometry in the abelian case. Finally it is shown that the conditions
for gauging non-abelian isometries are weaker than those generally found in the
literature.Comment: latex, 20 pages, no figure
Simple holographic duals to boundary CFTs
By relaxing the regularity conditions imposed in arXiv:1107.1722 on half-BPS
solutions to six-dimensional Type~4b supergravity, we enlarge the space of
solutions to include two new half-BPS configurations, which we refer to as the
\kap\ and the \funnel. We give evidence that the \kap\ and \funnel\ can be
interpreted as fully back-reacted brane solutions with respectively and
world volumes. \kap\ and \funnel\ solutions with a single
asymptotic region are constructed analytically. We argue
that \kap\ solutions provide simple examples of holographic duals to boundary
CFTs in two dimensions and present calculations of their holographic boundary
entropy to support the BCFT dual picture.Comment: 37 pages, pdflatex, 5 figure
What is needed of a tachyon if it is to be the dark energy?
We study a dark energy scenario in the presence of a tachyon field
with potential and a barotropic perfect fluid. The cosmological
dynamics crucially depends on the asymptotic behavior of the quantity
. If is a constant, which corresponds to
an inverse square potential , there exists one
stable critical point that gives an acceleration of the universe at late times.
When asymptotically, we can have a viable dark energy scenario
in which the system approaches an ``instantaneous'' critical point that
dynamically changes with . If approaches infinity
asymptotically, the universe does not exhibit an acceleration at late times. In
this case, however, we find an interesting possibility that a transient
acceleration occurs in a regime where is smaller than of order
unity.Comment: 11 pages and 3 figures, minor clarifications added; final version to
appear in PR
Vanishing Cosmological Constant in Modified Gauss-Bonnet Gravity with Conformal Anomaly
We consider dark energy cosmology in a de Sitter universe filled with quantum
conformal matter. Our model represents a Gauss-Bonnet model of gravity with
contributions from quantum effects. To the General Relativity action an
arbitrary function of the GB invariant, f(G), is added, and taking into account
quantum effects from matter the cosmological constant is studied. For the
considered model the conditions for a vanishing cosmological constant are
considered. Creation of a de Sitter universe by quantum effects in a GB
modified gravity is discussed.Comment: 8 pages latex, 1 figure. To appear in Int. J. Mod. Phys.
Black Hole Meiosis
The enumeration of BPS bound states in string theory needs refinement.
Studying partition functions of particles made from D-branes wrapped on
algebraic Calabi-Yau 3-folds, and classifying states using split attractor flow
trees, we extend the method for computing a refined BPS index, arXiv:0810.4301.
For certain D-particles, a finite number of microstates, namely polar states,
exclusively realized as bound states, determine an entire partition function
(elliptic genus). This underlines their crucial importance: one might call them
the `chromosomes' of a D-particle or a black hole. As polar states also can be
affected by our refinement, previous predictions on elliptic genera are
modified. This can be metaphorically interpreted as `crossing-over in the
meiosis of a D-particle'. Our results improve on hep-th/0702012, provide
non-trivial evidence for a strong split attractor flow tree conjecture, and
thus suggest that we indeed exhaust the BPS spectrum. In the D-brane
description of a bound state, the necessity for refinement results from the
fact that tachyonic strings split up constituent states into `generic' and
`special' states. These are enumerated separately by topological invariants,
which turn out to be partitions of Donaldson-Thomas invariants. As modular
predictions provide a check on many of our results, we have compelling evidence
that our computations are correct.Comment: 46 pages, 8 figures. v2: minor changes. v3: minor changes and
reference adde
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