12,102 research outputs found
A Tale of Two Distances
There are two basic ways to measure physical distances in cosmology: One
based on standard candles and one based on standard rulers. Comparing current
data for each method allows us to rule out axion-photon mixing and
dust-extinction as the sources of supernova dimming and generally protects the
case for cosmic acceleration from attacks based on loss of photons. The
combined data constrains the energy densities in a LCDM model to 0.19 < Omega_m
< 0.32 and 0.47 < Omega_Lambda < 0.82 (at 2\sigma) without recourse to any
further data sets. Future data will improve on these limits and allow us to
place constraints on more exotic physics.Comment: 4 pages, 1 figure, uses moriond.sty. To be published in the
Proceedings of the XXXIX Rencontres de Moriond 'Exploring the Universe
New Spontaneous Model of Fibrodysplasia Ossificans Progressiva
We report the first known example of spontaneous, naturally occurring fibrodysplasia ossificans progressiva (FOP) in a mammal. The Southeast Asian mouse deer of the genus _Tragulus_ (Artiodactyla: Tragulidae) have an osseous sheath covering the lower back and upper thigh region consistent with the clinical definition of FOP. This heterotophic bone deposition is sex related apparently with a genetic basis - it only occurs in males and is lacking in females; it is present in all adults males, including both wild obtained and zoo bred animals. _Tragulus_ may offer the opportunity to examine many of the disease's most significant attributes experimentally
Promotion on oscillating and alternating tableaux and rotation of matchings and permutations
Using Henriques' and Kamnitzer's cactus groups, Sch\"utzenberger's promotion
and evacuation operators on standard Young tableaux can be generalised in a
very natural way to operators acting on highest weight words in tensor products
of crystals.
For the crystals corresponding to the vector representations of the
symplectic groups, we show that Sundaram's map to perfect matchings intertwines
promotion and rotation of the associated chord diagrams, and evacuation and
reversal. We also exhibit a map with similar features for the crystals
corresponding to the adjoint representations of the general linear groups.
We prove these results by applying van Leeuwen's generalisation of Fomin's
local rules for jeu de taquin, connected to the action of the cactus groups by
Lenart, and variants of Fomin's growth diagrams for the Robinson-Schensted
correspondence
Young people and ICT 2002: findings from a survey conducted in Autumn 2002
This report describes a survey that explored the attitudes and experiences of young people aged 5-18 and their parents, in relation to the use of information and communications technology (ICT) at home and at schoo
Mathematical Model for the Mineralization of Bone
A mathematical model is presented for the transport and precipitation of mineral in refilling osteons. One goal of this model was to explain calcification 'halos,' in which the bone near the haversian canal is more highly mineralized than the more peripheral lamellae, which have been mineralizing longer. It was assumed that the precipitation rate of mineral is proportional to the difference between the local concentration of calcium ions and an equilibrium concentration and that the transport of ions is by either diffusion or some other concentration gradient-dependent process. Transport of ions was assumed to be slowed by the accumulation of mineral in the matrix along the transport path. ne model also mimics bone apposition, slowing of apposition during refilling, and mineralization lag time. It was found that simple diffusion cannot account for the transport of calcium ions into mineralizing bone, because the diffusion coefficient is two orders of magnitude too low. If a more rapid concentration gradient-driven means of transport exists, the model demonstrates that osteonal geometry and variable rate of refilling work together to produce calcification halos, as well as the primary and secondary calcification effect reported in the literature
The essence of quintessence and the cost of compression
Standard two-parameter compressions of the infinite dimensional dark energy
model space show crippling limitations even with current SN-Ia data. Firstly
they cannot cope with rapid evolution - the best-fit to the latest SN-Ia data
shows late and very rapid evolution to w_0 = -2.85. However all of the standard
parametrisations (incorrectly) claim that this best-fit is ruled out at more
than 2-sigma, primarily because they track it well only at very low redshifts,
z < 0.2. Further they incorrectly rule out the observationally acceptable
region w 1. Secondly the parametrisations give wildly different
estimates for the redshift of acceleration, which vary from z_{acc}=0.14 to
z_{acc}=0.59. Although these failings are largely cured by including
higher-order terms (3 or 4 parameters) this results in new degeneracies which
open up large regions of previously ruled-out parameter space. Finally we test
the parametrisations against a suite of theoretical quintessence models. The
widely used linear expansion in z is generally the worst, with errors of up to
10% at z=1 and 20% at z > 2. All of this casts serious doubt on the usefulness
of the standard two-parameter compressions in the coming era of high-precision
dark energy cosmology and emphasises the need for decorrelated compressions
with at least three parameters.Comment: 7 pages, 4 colour figures, EmulateApJ; v2: includes Bayesian evidence
analysis and table that were only present in published version, because of
increased interest in Bayesian model comparison (no new material beyond the
one in the published ApJL of 2004
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