481 research outputs found
Brane Induced Gravity: Codimension-2
We review the results of arXiv:hep-th/0703190, on brane induced gravity (BIG)
in 6D. Among a large diversity of regulated codimension-2 branes, we find that
for near-critical tensions branes live inside very deep throats which
efficiently compactify the angular dimension. In there, 4D gravity first
changes to 5D, and only later to 6D. The crossover from 4D to 5D is independent
of the tension, but the crossover from 5D to 6D is not. This shows how the
vacuum energy problem manifests in BIG: instead of tuning vacuum energy to
adjust the 4D curvature, generically one must tune it to get the desired
crossover scales and the hierarchy between the scales governing the 4D \to 5D
\to 6D transitions. In the near-critical limit, linearized perturbation theory
remains under control below the crossover scale, and we find that linearized
gravity around the vacuum looks like a scalar-tensor theory.Comment: 16 pages latex, 2 .eps figs, based on the talks given at the "Sowers
Workshop", Virginia Tech, May 14-18, 2007, "Cosmology and Strings" workshop
at ICTP, Trieste, Italy, July 9-13, 2007, "Dark Energy In the Universe",
Hakone, Japan, Sep 1-4, 2007 and "Zagreb Workshop 2007", Zagreb, Croatia, Nov
9-11, 2007; v2: added reference
On Bouncing Brane-Worlds, S-branes and Branonium Cosmology
We present several higher-dimensional spacetimes for which observers living
on 3-branes experience an induced metric which bounces. The classes of examples
include boundary branes on generalised S-brane backgrounds and probe branes in
D-brane/anti D-brane systems. The bounces we consider normally would be
expected to require an energy density which violates the weak energy condition,
and for our co-dimension one examples this is attributable to bulk curvature
terms in the effective Friedmann equation. We examine the features of the
acceleration which provides the bounce, including in some cases the existence
of positive acceleration without event horizons, and we give a geometrical
interpretation for it. We discuss the stability of the solutions from the point
of view of both the brane and the bulk. Some of our examples appear to be
stable from the bulk point of view, suggesting the possible existence of stable
bouncing cosmologies within the brane-world framework.Comment: 35 pages, 7 figures, JHEP style. Title changed and references adde
General Axisymmetric Solutions and Self-Tuning in 6D Chiral Gauged Supergravity
We re-examine the properties of the axially-symmetric solutions to chiral
gauged 6D supergravity, recently found in refs. hep-th/0307238 and
hep-th/0308064. Ref. hep-th/0307238 finds the most general solutions having two
singularities which are maximally-symmetric in the large 4 dimensions and which
are axially-symmetric in the internal dimensions. We show that not all of these
solutions have purely conical singularities at the brane positions, and that
not all singularities can be interpreted as being the bulk geometry sourced by
neutral 3-branes. The subset of solutions for which the metric singularities
are conical precisely agree with the solutions of ref. hep-th/0308064.
Establishing this connection between the solutions of these two references
resolves a minor conflict concerning whether or not the tensions of the
resulting branes must be negative. The tensions can be both negative and
positive depending on the choice of parameters. We discuss the physical
interpretation of the non-conical solutions, including their significance for
the proposal for using 6-dimensional self-tuning to understand the small size
of the observed vacuum energy. In passing we briefly comment on a recent paper
by Garriga and Porrati which criticizes the realization of self-tuning in 6D
supergravity.Comment: 27 pages, 1 figure; JHEP3 style; Some references added, and
discussion of tension constraints and unwarped solutions made more explici
A new framework of spatial targeting for single-species conservation planning
Context: Organisations acting to conserve and protect species across large spatial scales prioritise to optimise use of resources. Spatial conservation prioritization tools typically focus on identifying areas containing species groups of interest, with few tools used to identify the best areas for single-species conservation, in particular, to conserve currently widespread but declining species. /
Objective: A single-species prioritization framework, based on temporal and spatial patterns of occupancy and abundance, was developed to spatially prioritize conservation action for widespread species by identifying smaller areas to work within to achieve predefined conservation objectives. /
Methods: We demonstrate our approach for 29 widespread bird species in the UK, using breeding bird atlas data from two periods to define distribution, relative abundance and change in relative abundance. We selected occupied 10-km squares with abundance trends that matched species conservation objectives relating to maintaining or increasing population size or range, and then identified spatial clusters of squares for each objective using a Getis-Ord-Gi* or near neighbour analysis. /
Results: For each species, the framework identified clusters of 20-km squares that enabled us to identify small areas in which species recovery action could be prioritized. /
Conclusions: Our approach identified a proportion of species’ ranges to prioritize for species recovery. This approach is a relatively quick process that can be used to inform single-species conservation for any taxa if sufficiently fine-scale occupancy and abundance information is available for two or more time periods. This is a relatively simple first step for planning single-species focussed conservation to help optimise resource use
Zero modes of six-dimensional Abelian vortices
We analyze the fluctuations of Nielsen-Olesen vortices arising in the
six-dimensional Abelian-Higgs model. The regular geometry generated by the
defect breaks spontaneously six-dimensional Poincar\'e symmetry leading to a
warped space-time with finite four-dimensional Planck mass. As a consequence,
the zero mode of the spin two fluctuations of the geometry is always localized
but the graviphoton fields, corresponding to spin one metric fluctuations, give
rise to zero modes which are not localized either because of their behaviour at
infinity or because of their behaviour near the core of the vortex. A similar
situation occurs for spin zero fluctuations. Gauge field fluctuations exhibit a
localized zero mode.Comment: 45 pages in Revtex style with 4 figure
Conservation equation on braneworlds in six dimensions
We study braneworlds in six-dimensional Einstein-Gauss-Bonnet gravity. The
Gauss-Bonnet term is crucial for the equations to be well-posed in six
dimensions when non-trivial matter on the brane is included (the also involved
induced gravity term is not significant for their structure), and the matching
conditions of the braneworld are known. We show that the energy-momentum of the
brane is always conserved, independently of any regular bulk energy-momentum
tensor, contrary to the situation of the five-dimensional case.Comment: References added, minor changes, 3 pages, RevTeX, to app. in Class.
Quant. Gra
piRNA-directed cleavage of meiotic transcripts regulates spermatogenesis.
MIWI catalytic activity is required for spermatogenesis, indicating that piRNA-guided cleavage is critical for germ cell development. To identify meiotic piRNA targets, we augmented the mouse piRNA repertoire by introducing a human meiotic piRNA cluster. This triggered a spermatogenesis defect by inappropriately targeting the piRNA machinery to mouse mRNAs essential for germ cell development. Analysis of such de novo targets revealed a signature for pachytene piRNA target recognition. This enabled identification of both transposable elements and meiotically expressed protein-coding genes as targets of native piRNAs. Cleavage of genic targets began at the pachytene stage and resulted in progressive repression through meiosis, driven at least in part via the ping-pong cycle. Our data support the idea that meiotic piRNA populations must be strongly selected to enable successful spermatogenesis, both driving the response away from essential genes and directing the pathway toward mRNA targets that are regulated by small RNAs in meiotic cells.This work was supported by the National Institutes of Health R37 grant GM062534-14 to G.J.H. iTRAQ was performed with assistance from the Cold Spring Harbor Laboratory Proteomics Shared Resource, which is supported by Cancer Center support grant 5P30CA045508. W.S.S.G. is a McClintock Fellow of the Watson School of Biological Sciences and is supported by the NSS Scholarship from the Agency for Science, Technology and Research, Singapore. O.H.T. is supported by a fellowship of the Human Frontier Science Program. R.B. is supported by the Starr Centennial Scholarship from the Watson School of Biological Sciences. G.J.H. is a Howard Hughes Medical Institute Investigator.This is the final version of the article. It first appeared from Cold Spring Harbor Laboratory Press via http://dx.doi.org/10.1101/gad.260455.11
Galileon Hairs of Dyson Spheres, Vainshtein's Coiffure and Hirsute Bubbles
We study the fields of spherically symmetric thin shell sources, a.k.a. Dyson
spheres, in a {\it fully nonlinear covariant} theory of gravity with the
simplest galileon field. We integrate exactly all the field equations once,
reducing them to first order nonlinear equations. For the simplest galileon,
static solutions come on {\it six} distinct branches. On one, a Dyson sphere
surrounds itself with a galileon hair, which far away looks like a hair of any
Brans-Dicke field. The hair changes below the Vainshtein scale, where the extra
galileon terms dominate the minimal gradients of the field. Their hair looks
more like a fuzz, because the galileon terms are suppressed by the derivative
of the volume determinant. It shuts off the `hair bunching' over the `angular'
2-sphere. Hence the fuzz remains dilute even close to the source. This is
really why the Vainshtein's suppression of the modifications of gravity works
close to the source. On the other five branches, the static solutions are all
{\it singular} far from the source, and shuttered off from asymptotic infinity.
One of them, however, is really the self-accelerating branch, and the
singularity is removed by turning on time dependence. We give examples of
regulated solutions, where the Dyson sphere explodes outward, and its
self-accelerating side is nonsingular. These constructions may open channels
for nonperturbative transitions between branches, which need to be addressed
further to determine phenomenological viability of multi-branch gravities.Comment: 29+1 pages, LaTeX, 2 .pdf figure
The tip-link antigen, a protein associated with the transduction complex of sensory hair cells, is protocadherin-15
Sound and acceleration are detected by hair bundles, mechanosensory structures located at the apical pole of hair cells in the inner ear. The different elements of the hair bundle, the stereocilia and a kinocilium, are interconnected by a variety of link types. One of these links, the tip link, connects the top of a shorter stereocilium with the lateral membrane of an adjacent taller stereocilium and may gate the mechanotransducer channel of the hair cell. Mass spectrometric and Western blot analyses identify the tip-link antigen, a hitherto unidentified antigen specifically associated with the tip and kinocilial links of sensory hair bundles in the inner ear and the ciliary calyx of photoreceptors in the eye, as an avian ortholog of human protocadherin-15, a product of the gene for the deaf/blindness Usher syndrome type 1F/DFNB23 locus. Multiple protocadherin-15 transcripts are shown to be expressed in the mouse inner ear, and these define four major isoform classes, two with entirely novel, previously unidentified cytoplasmic domains. Antibodies to the three cytoplasmic domain-containing isoform classes reveal that each has a different spatiotemporal expression pattern in the developing and mature inner ear. Two isoforms are distributed in a manner compatible for association with the tip-link complex. An isoform located at the tips of stereocilia is sensitive to calcium chelation and proteolysis with subtilisin and reappears at the tips of stereocilia as transduction recovers after the removal of calcium chelators. Protocadherin-15 is therefore associated with the tip-link complex and may be an integral component of this structure and/or required for its formatio
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