1,797 research outputs found
Supernovae data and perturbative deviation from homogeneity
We show that a spherically symmetric perturbation of a dust dominated
FRW universe in the Newtonian gauge can lead to an apparent
acceleration of standard candles and provide a fit to the magnitude-redshift
relation inferred from the supernovae data, while the perturbation in the
gravitational potential remains small at all scales. We also demonstrate that
the supernovae data does not necessarily imply the presence of some additional
non-perturbative contribution by showing that any Lemaitre-Tolman-Bondi model
fitting the supernovae data (with appropriate initial conditions) will be
equivalent to a perturbed FRW spacetime along the past light cone.Comment: 8 pages, 3 figures; v2: 1 figure added, references added/updated,
minor modifications and clarifications, matches published versio
CSF protein biomarkers predicting longitudinal reduction of CSF β-amyloid42 in cognitively healthy elders.
β-amyloid (Aβ) plaque accumulation is a hallmark of Alzheimer's disease (AD). It is believed to start many years prior to symptoms and is reflected by reduced cerebrospinal fluid (CSF) levels of the peptide Aβ1-42 (Aβ42). Here we tested the hypothesis that baseline levels of CSF proteins involved in microglia activity, synaptic function and Aβ metabolism predict the development of Aβ plaques, assessed by longitudinal CSF Aβ42 decrease in cognitively healthy people. Forty-six healthy people with three to four serial CSF samples were included (mean follow-up 3 years, range 2-4 years). There was an overall reduction in Aβ42 from a mean concentration of 211-195 pg ml(-1) after 4 years. Linear mixed-effects models using longitudinal Aβ42 as the response variable, and baseline proteins as explanatory variables (n=69 proteins potentially relevant for Aβ metabolism, microglia or synaptic/neuronal function), identified 10 proteins with significant effects on longitudinal Aβ42. The most significant proteins were angiotensin-converting enzyme (ACE, P=0.009), Chromogranin A (CgA, P=0.009) and Axl receptor tyrosine kinase (AXL, P=0.009). Receiver-operating characteristic analysis identified 11 proteins with significant effects on longitudinal Aβ42 (largely overlapping with the proteins identified by linear mixed-effects models). Several proteins (including ACE, CgA and AXL) were associated with Aβ42 reduction only in subjects with normal baseline Aβ42, and not in subjects with reduced baseline Aβ42. We conclude that baseline CSF proteins related to Aβ metabolism, microglia activity or synapses predict longitudinal Aβ42 reduction in cognitively healthy elders. The finding that some proteins only predict Aβ42 reduction in subjects with normal baseline Aβ42 suggest that they predict future development of the brain Aβ pathology at the earliest stages of AD, prior to widespread development of Aβ plaques
Plasma phosphorylated tau181 and neurodegeneration in Alzheimer's disease
We examined if plasma phosphorylated tau is associated with neurodegeneration in Alzheimer’s disease. We investigated 372 cognitively unimpaired participants, 554 mild cognitive impairment patients, and 141 Alzheimer’s disease dementia patients. Tau phosphorylated at threonine 181, regional cortical thickness (using magnetic resonance imaging) and hypometabolism (using fluorodeoxyglucose positron emission tomography) were measured longitudinally. High plasma tau was associated with hypometabolism and cortical atrophy at baseline and over time, and longitudinally increased tau was associated with accelerated atrophy, but these associations were only observed in Aβ‐positive participants. Plasma phosphorylated tau may identify and track processes linked to neurodegeneration in Alzheimer’s disease
Long-term trends (1975–2014) in the concentrations and export of carbon from Finnish rivers to the Baltic Sea: organic and inorganic components compared.
Probing the interiors of the ice giants: Shock compression of water to 700 GPa and 3.8 g/ccm
Recently there has been tremendous increase in the number of identified
extra-solar planetary systems. Our understanding of their formation is tied to
exoplanet internal structure models, which rely upon equations of state of
light elements and compounds like water. Here we present shock compression data
for water with unprecedented accuracy that shows water equations of state
commonly used in planetary modeling significantly overestimate the
compressibility at conditions relevant to planetary interiors. Furthermore, we
show its behavior at these conditions, including reflectivity and isentropic
response, is well described by a recent first-principles based equation of
state. These findings advocate this water model be used as the standard for
modeling Neptune, Uranus, and "hot Neptune" exoplanets, and should improve our
understanding of these types of planets.Comment: Accepted to Phys. Rev. Lett.; supplementary material attached
including 2 figures and 2 tables; to view attachments, please download and
extract the gzipped tar source file listed under "Other formats
Higher order finite difference schemes for the magnetic induction equations
We describe high order accurate and stable finite difference schemes for the
initial-boundary value problem associated with the magnetic induction
equations. These equations model the evolution of a magnetic field due to a
given velocity field. The finite difference schemes are based on Summation by
Parts (SBP) operators for spatial derivatives and a Simultaneous Approximation
Term (SAT) technique for imposing boundary conditions. We present various
numerical experiments that demonstrate both the stability as well as high order
of accuracy of the schemes.Comment: 20 page
Hubble flow variance and the cosmic rest frame
We characterize the radial and angular variance of the Hubble flow in the
COMPOSITE sample of 4534 galaxies, on scales in which much of the flow is in
the nonlinear regime. With no cosmological assumptions other than the existence
of a suitably averaged linear Hubble law, we find with decisive Bayesian
evidence (ln B >> 5) that the Hubble constant averaged in independent spherical
radial shells is closer to its asymptotic value when referred to the rest frame
of the Local Group, rather than the standard rest frame of the Cosmic Microwave
Background. An exception occurs for radial shells in the range 40/h-60/h Mpc.
Angular averages reveal a dipole structure in the Hubble flow, whose amplitude
changes markedly over the range 32/h-62/h Mpc. Whereas the LG frame dipole is
initially constant and then decreases significantly, the CMB frame dipole
initially decreases but then increases. The map of angular Hubble flow
variation in the LG rest frame is found to coincide with that of the residual
CMB temperature dipole, with correlation coefficient -0.92. These results are
difficult to reconcile with the standard kinematic interpretation of the motion
of the Local Group in response to the clustering dipole, but are consistent
with a foreground non-kinematic anisotropy in the distance-redshift relation of
0.5% on scales up to 65/h Mpc. Effectively, the differential expansion of space
produced by nearby nonlinear structures of local voids and denser walls and
filaments cannot be reduced to a local boost. This hypothesis suggests a
reinterpretation of bulk flows, which may potentially impact on calibration of
supernovae distances, anomalies associated with large angles in the CMB
anisotropy spectrum, and the dark flow inferred from the kinematic
Sunyaev-Zel'dovich effect. It is consistent with recent studies that find
evidence for a non-kinematic dipole in the distribution of distant radio
sources.Comment: 37 pages, 9 tables, 13 figures; v2 adds extensive new analysis
(including additional subsections, tables, figures); v3 adds a Monte Carlo
analysis (with additional table, figure) which further tightens the
statistical robustness of the dipole results; v4 adds further clarifications,
small corrections, references and discussion of Planck satellite results; v5
typos fixed, matches published versio
From dilute polyelectrolyte solutions to entangled polyelectrolyte networks: a study of sodium carboxymethyl cellulose in water by light scattering and rheology
Sodium carboxymethyl cellulose (Na CMC) is widely used in industry for its thickening and swelling properties. Applications are very broad and include pharmaceutical, food, home and personal care products as well as the paper industry, water treatment and mineral processing. Na CMC is a linear negatively charged water-soluble polymer derived from cellulose. Its behaviour in water is known to be very complex and a function of several parameters including the characteristics of the polymer itself [1] such as molecular weight and degree of substitution as well as the solution concentration and dissolution conditions [2] (e.g. addition order of the system components) [3]. While Dynamic Light Scattering (DLS) has been widely used to study the behaviour of polyelectrolytes, relatively few DLS studies have been conducted on Na CMC and, to our knowledge, none in pure water; this is most likely due to the difficulty of preparing salt-free Na CMC solutions of DLS grade. Indeed, the presence of even a few poorly substituted Na CMC fibres suffices to prevent proper DLS data from being collected. The aim of the present study was to investigate the behaviour of Na CMC (Mw = 700,000 g/mol; DS = 0.9) in pure water using both DLS and rheology measurements. A method was developed to prepare samples of appropriate quality for DLS measurements, which could then be successfully run over a wide range of concentrations. Rheology measurements were run in parallel to identify the different concentration regimes, facilitating comparisons to the behaviour typically found for polyelectrolytes (see Figure 1). Both DLS and rheology measurements were combined to look at the relationships between the structure of the Na CMC solutions and their rheological properties.
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