2,605 research outputs found
When Matter Matters
We study a recently proposed scenario for the early universe: Subluminal
Galilean Genesis. We prove that without any other matter present in the
spatially flat Friedmann universe, the perturbations of the Galileon scalar
field propagate with a speed at most equal to the speed of light. This proof
applies to all cosmological solutions -- to the whole phase space.
However, in a more realistic situation, when one includes any matter which is
not directly coupled to the Galileon, there always exists a region of phase
space where these perturbations propagate superluminally, indeed with
arbitrarily high speed. We illustrate our analytic proof with numerical
computations. We discuss the implications of this result for the possible UV
completion of the model.Comment: 16 pages, 3 figures. v2 reflects version accepted for publication in
JCAP. Changes include a reorganisation of section order, a new figure 1 and
additional reference
Dust of Dark Energy
We introduce a novel class of field theories where energy always flows along
timelike geodesics, mimicking in that respect dust, yet which possess non-zero
pressure. This theory comprises two scalar fields, one of which is a Lagrange
multiplier enforcing a constraint between the other's field value and
derivative. We show that this system possesses no wave-like modes but retains a
single dynamical degree of freedom. Thus, the sound speed is always identically
zero on all backgrounds. In particular, cosmological perturbations reproduce
the standard behaviour for hydrodynamics with vanishing sound speed. Using all
these properties we propose a model unifying Dark Matter and Dark Energy in a
single degree of freedom. In a certain limit this model exactly reproduces the
evolution history of Lambda-CDM, while deviations away from the standard
expansion history produce a potentially measurable difference in the evolution
of structure.Comment: 13 pages, 3 figures. Added references, corrected language
Models of f(R) Cosmic Acceleration that Evade Solar-System Tests
We study a class of metric-variation f(R) models that accelerates the
expansion without a cosmological constant and satisfies both cosmological and
solar-system tests in the small-field limit of the parameter space.
Solar-system tests alone place only weak bounds on these models, since the
additional scalar degree of freedom is locked to the high-curvature
general-relativistic prediction across more than 25 orders of magnitude in
density, out through the solar corona. This agreement requires that the
galactic halo be of sufficient extent to maintain the galaxy at high curvature
in the presence of the low-curvature cosmological background. If the galactic
halo and local environment in f(R) models do not have substantially deeper
potentials than expected in LCDM, then cosmological field amplitudes |f_R| >
10^{-6} will cause the galactic interior to evolve to low curvature during the
acceleration epoch. Viability of large-deviation models therefore rests on the
structure and evolution of the galactic halo, requiring cosmological
simulations of f(R) models, and not directly on solar-system tests. Even small
deviations that conservatively satisfy both galactic and solar-system
constraints can still be tested by future, percent-level measurements of the
linear power spectrum, while they remain undetectable to cosmological-distance
measures. Although we illustrate these effects in a specific class of models,
the requirements on f(R) are phrased in a nearly model-independent manner.Comment: 13 pages, 10 figures. Submitted to Phys. Rev.
Longitudinal impact of demographic and clinical variables on Health-Related Quality of Life in Cystic Fibrosis
Objectives: The insights that people with cystic fibrosis have concerning their health are important given that aspects of health-related quality of life (HRQoL) are independent predictors of survival and a decrease in lung function is associated with a decrease in HRQoL over time. Cross-sectional data suggest that key variables, other than lung function, are also associated with HRQoL - although study results are equivocal. This work evaluates the relationship between these key demographic and clinical variables and HRQoL longitudinally.
Design: Longitudinal observational study. Observations were obtained at seven time points: approximately every two years over a twelve year period.
Setting: Large Adult Cystic Fibrosis Centre in the UK.
Participants: 234 participants aged 14-48 years at recruitment.
Outcome measure: Nine domains of HRQoL (Cystic Fibrosis Quality of Life Questionnaire) in relation to demographic (age, gender) and clinical measures (FEV1% predicted, BMI, cystic fibrosis related diabetes, B. cepacia complex, totally implantable vascular access device, nutritional and transplant status).
Results: A total of 770 patient assessments were obtained for 234 patients. The results of random coefficients modelling indicated that demographic and clinical variables were identified as being significant for HRQoL over time. In addition to lung function, transplant status, age, having a totally implantable vascular access device, cystic fibrosis related diabetes, BMI and B. cepacia complex impacted on many HRQoL domains longitudinally. Gender was important for the domain of Body image.
Conclusion: Demographic and changes in clinical variables were independently associated with a change in health-related quality of life over time. Compared with these longitudinal data, cross-sectional data are inadequate when evaluating the relationships between HRQoL domains and key demographic and clinical variables, as they fail to recognise the full impact of the CF disease trajectory and its treatments on quality of life
Quantitative Determination of the Adiabatic Condition Using Force-Detected Nuclear Magnetic Resonance
The adiabatic condition governing cyclic adiabatic inversion of proton spins
in a micron-sized ammonium chloride crystal was studied using room temperature
nuclear magnetic resonance force microscopy. A systematic degradation of
signal-to-noise was observed as the adiabatic condition became violated. A
theory of adiabatic following applicable to cyclic adiabatic inversion is
reviewed and implemented to quantitatively determine an adiabaticity threshold
from our experimental results.Comment: 5 pages, 3 fig
Critical sets of the total variance of state detect all SLOCC entanglement classes
We present a general algorithm for finding all classes of pure multiparticle
states equivalent under Stochastic Local Operations and Classsical
Communication (SLOCC). We parametrize all SLOCC classes by the critical sets of
the total variance function. Our method works for arbitrary systems of
distinguishable and indistinguishable particles. We also discuss the Morse
indices of critical points which have the interpretation of the number of
independent non-local perturbations increasing the variance and hence
entanglement of a state. We illustrate our method by two examples.Comment: 4 page
Multipartite quantum correlations: symplectic and algebraic geometry approach
We review a geometric approach to classification and examination of quantum
correlations in composite systems. Since quantum information tasks are usually
achieved by manipulating spin and alike systems or, in general, systems with a
finite number of energy levels, classification problems are usually treated in
frames of linear algebra. We proposed to shift the attention to a geometric
description. Treating consistently quantum states as points of a projective
space rather than as vectors in a Hilbert space we were able to apply powerful
methods of differential, symplectic and algebraic geometry to attack the
problem of equivalence of states with respect to the strength of correlations,
or, in other words, to classify them from this point of view. Such
classifications are interpreted as identification of states with `the same
correlations properties' i.e. ones that can be used for the same information
purposes, or, from yet another point of view, states that can be mutually
transformed one to another by specific, experimentally accessible operations.
It is clear that the latter characterization answers the fundamental question
`what can be transformed into what \textit{via} available means?'. Exactly such
an interpretations, i.e, in terms of mutual transformability can be clearly
formulated in terms of actions of specific groups on the space of states and is
the starting point for the proposed methods.Comment: 29 pages, 9 figures, 2 tables, final form submitted to the journa
A SCUBA Map in the Spitzer First Look Survey: Source Catalog and Number Counts
Using the SCUBA instrument on the JCMT, we have made a submillimeter mosaic
at 850um of a subarea of the Spitzer First Look Survey (FLS). Our image covers
the central 151 square arcmin of the northern extragalactic Continuous Viewing
Zone (CVZ) field of the FLS to a median 3sigma depth of 9.7 mJy. The image
contains ten 850um sources detected at 3.5sigma or higher significance, of
which five are detected at greater than or equal 4sigma. We make the catalog of
these SCUBA-selected FLS sources available to the community. After correcting
for incompleteness and flux bias, we find that the density of sources brighter
that 10mJy in our field is (1.3 +1.1 -0.7)x10^2 deg^-2 (95% Poisson confidence
limits), which is consistent with other surveys that probe the bright end of
the submillimeter population.Comment: ApJL, in press. Higher quality versions of Figs 1 and 3 can be
obtained from the author
G-Bounce
We present a wide class of models which realise a bounce in a spatially flat
Friedmann universe in standard General Relativity. The key ingredient of the
theories we consider is a noncanonical, minimally coupled scalar field
belonging to the class of theories with Kinetic Gravity Braiding /
Galileon-like self-couplings. In these models, the universe smoothly evolves
from contraction to expansion, suffering neither from ghosts nor gradient
instabilities around the turning point. The end-point of the evolution can be a
standard radiation-domination era or an inflationary phase. We formulate
necessary restrictions for Lagrangians needed to obtain a healthy bounce and
illustrate our results with phase portraits for simple systems including the
recently proposed Galilean Genesis scenario.Comment: 28 pages. v2 reflects version accepted for publication in JCAP.
References and minor comments adde
The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field
The Hubble Deep Field (HDF) is the deepest set of multicolor optical
photometric observations ever undertaken, and offers a valuable data set with
which to study galaxy evolution. Combining the optical WFPC2 data with
ground-based near-infrared photometry, we derive photometrically estimated
redshifts for HDF galaxies with J<23.5. We demonstrate that incorporating the
near-infrared data reduces the uncertainty in the estimated redshifts by
approximately 40% and is required to remove systematic uncertainties within the
redshift range 1<z<2. Utilizing these photometric redshifts, we determine the
evolution of the comoving ultraviolet (2800 A) luminosity density (presumed to
be proportional to the global star formation rate) from a redshift of z=0.5 to
z=2. We find that the global star formation rate increases rapidly with
redshift, rising by a factor of 12 from a redshift of zero to a peak at z~1.5.
For redshifts beyond 1.5, it decreases monotonically. Our measures of the star
formation rate are consistent with those found by Lilly et al. (1996) from the
CFRS at z 2, and
bridge the redshift gap between those two samples. The overall star formation
or metal enrichment rate history is consistent with the predictions of Pei and
Fall (1995) based on the evolving HI content of Lyman-alpha QSO absorption line
systems.Comment: Latex format, 10 pages, 3 postscript figures. Accepted for
publication in Ap J Letter
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