7,080 research outputs found
Ghosts in asymmetric brane gravity and the decoupled stealth limit
We study the spectrum of gravitational perturbations around a vacuum de
Sitter brane in a 5D asymmetric braneworld model, with induced curvature on the
brane. This generalises the stealth acceleration model proposed by Charmousis,
Gregory and Padilla (CGP) which realises the Cardassian cosmology in which
power law cosmic acceleration can be driven by ordinary matter. Whenever the
bulk has infinite volume we find that there is always a perturbative ghost
propagating on the de Sitter brane, in contrast to the Minkowski brane case
analysed by CGP. We discuss the implication of this ghost for the stealth
acceleration model, and identify a limiting case where the ghost decouples as
the de Sitter curvature vanishes.Comment: 21 page
Infra-red modification of gravity from asymmetric branes
We consider a single Minkowski brane sandwiched in between two copies of
anti-de Sitter space. We allow the bulk Planck mass and cosmological constant
to differ on either side of the brane. Linearised perturbations about this
background reveal that gravity can be modified in the infra-red. At
intermediate scales, the braneworld propagator mimics four-dimensional GR in
that it has the correct momentum dependance. However it has the wrong tensor
structure. Beyond a source dependant scale, we show that quadratic brane
bending contributions become important, and conspire to correct the tensor
structure of the propagator. We argue that even higher order terms can
consistently be ignored up to very high energies, and suggest that there is no
problem with strong coupling. We also consider scalar and vector perturbations
in the bulk, checking for scalar ghosts.Comment: Version appearing in CQ
mm-Wave DRW Antenna Phase Centre Determination
This document presents an approach to the phase centre determination of a dielectric rod waveguide (DRW) antenna by means of measurements obtained with a planar measuring system at millimeter wave lengths. Phase centre determination by the least squares fit technique is described in this document for different DRW antennas (silicon and sapphire). Results at different operating frequencies are offered
Stealth Acceleration and Modified Gravity
We show how to construct consistent braneworld models which exhibit late time
acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our
models have the standard Minkowski vacuum and accelerate only in the presence
of matter, which we dub ``stealth-acceleration''. We use an effective action
for the brane which includes an induced gravity term, and allow for an
asymmetric set-up. We study the linear stability of flat brane vacua and find
the regions of parameter space where the set-up is stable. The 4-dimensional
graviton is only quasi-localised in this set-up and as a result gravity is
modified at late times. One of the two regions is strongly coupled and the
scalar mode is eaten up by an extra symmetry that arises in this limit. Having
filtered the well-defined theories we then focus on their cosmology. When the
graviton is quasi-localised we find two main examples of acceleration. In each
case, we provide an illustrative model and compare it to LambdaCDM.Comment: 32 pages, 5 figure
Galaxy interactions II: High density environments
With the aim to assess the role of dense environments in galaxy interactions,
properties we present an analysis of close galaxy pairs in groups and clusters,
obtained from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7). We
identified pairs that reside in groups by cross-correlating the total galaxy
pair catalogue with the SDSS-DR7 group catalogue from Zapata et al. (2009). We
classify pair galaxies according to the intensity of interaction. We analysed
the effect of high density environments on different classes of galaxy-galaxy
interactions and we have also studied the impact of the group global
environment on pair galaxies. We find that galaxy pairs are more concentrated
towards the group centres with respect to the other group galaxy members, and
disturbed pairs show a preference to contain the brightest galaxy in the
groups. The color-magnitude relation exhibits significant differences between
pair galaxies and the control sample, consisting in color tails with a clear
excess of extremely blue and red galaxies for merging systems. In addition,
pair galaxies show a significant excess of young stellar populations with
respect to galaxies in the control sample; this finding suggests that, in dense
environments, strong interactions produce an important effect in modifying
galaxy properties. We find that the fraction of star forming galaxies decreases
toward the group centre; however, galaxy pairs show a more efficient star
formation activity than galaxies without a close companion. We have also found
that pair galaxies prefer groups with low density global environments with
respect to galaxies of the corresponding control sample. Blue, young stellar
population galaxies prefer groups within low density global environments.Comment: 10 pages, 11 figures, accepted for publication in A&
Portable tool removes burrs from pipe and tubing
Portable tool cleanly removes burrs that remain on tubing when it is cut. It restores the cut end to its original configuration, and carries away all chips and pieces. This tool is used in places of limited access where a larger tool could not be used
Comparing apples and oranges (and blueberries and grapes): fruit type affects development and cold-susceptibility of immature Drosophila suzukii
Drosophila suzukii is a cosmopolitan polyphagous pest on unripe soft-skinned fruits. We sought to determine 1) temperature treatments that could be used to kill immature D. suzukii in fruit or packaging, and 2) whether development on different fruits led to differences in cold tolerance of immature D. suzukii. We reared animals from egg on a banana-based laboratory diet and diets made of apple, blueberry, cherry, grape, orange, raspberry, or strawberry homogenate in agar, and measured development time, adult body size, and cold tolerance. Diet type had complex effects on development time; in particular, flies reared on apple- or blueberry-based diets developed more slowly to a smaller adult body size than those on other diets. Cold exposure killed eggs and both first- and second-instar larvae. Survival of 24h at +4°C by feeding third-instar larvae was lowest in blueberry and cherry. Five days at +0.6°C killed all feeding third-instar larvae; this treatment is likely sufficient for targeting D. suzukii in fruit. Two hours at -5 or -6°C killed all wandering third-instar larvae and pupae; this exposure could be sufficient for sanitation of packaging
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