440 research outputs found
Photometric Properties of Void Galaxies in the Sloan Digital Sky Survey DR7 Data Release
Using the sample presented in Pan:2011, we analyse the photometric properties
of 88,794 void galaxies and compare them to galaxies in higher density
environments with the same absolute magnitude distribution. In Pan et al.
(2011), we found a total of 1054 dynamically distinct voids in the SDSS with
radius larger than 10h^-1 Mpc. The voids are underdense, with delta rho/rho <
-0.9 in their centers. Here we study the photometric properties of these void
galaxies. We look at the u - r colours as an indication of star formation
activity and the inverse concentration index as an indication of galaxy type.
We find that void galaxies are statistically bluer than galaxies found in
higher density environments with the same magnitude distribution. We examine
the colours of the galaxies as a function of magnitude, and we fit each colour
distribution with a double-Gaussian model for the red and blue subpopulations.
As we move from bright to dwarf galaxies, the population of red galaxies
steadily decreases and the fraction of blue galaxies increases in both voids
and walls, however the fraction of blue galaxies in the voids is always higher
and bluer than in the walls. We also split the void and wall galaxies into
samples depending on galaxy type. We find that late type void galaxies are
bluer than late type wall galaxies and the same holds for early galaxies. We
also find that early type, dwarf void galaxies are blue in colour. We also
study the properties of void galaxies as a function of their distance from the
center of the void. We find very little variation in the properties, such as
magnitude, colour and type, of void galaxies as a function of their location in
the void. The only exception is that the dwarf void galaxies may live closer to
the center. The centers of voids have very similar density contrast and hence
all void galaxies live in very similar density environments (ABRIDGED)Comment: 10 pages, 25 figure
Resummation scheme for 3d Yang-Mills and the two-loop magnetic mass for hot gauge theories
Perturbation theory for non-Abelian gauge theories at finite temperature is
plagued by infrared divergences caused by magnetic soft modes ,
which correspond to the fields of a 3d Yang-Mills theory. We revisit a gauge
invariant resummation scheme to solve this problem by self-consistent mass
generation using an auxiliary scalar field, improving over previous attempts in
two respects. First, we generalise earlier SU(2) treatments to SU(N). Second,
we obtain a gauge independent two-loop gap equation, correcting an error in the
literature. The resulting two-loop approximation to the magnetic mass
represents a correction to the leading one-loop value, indicating a
reasonable convergence of the resummation.Comment: 16 pages, 3 figure
Pulsed electromagnetic energy treatment offers no clinical benefit in reducing the pain of knee osteoarthritis: a systematic review
Background
The rehabilitation of knee osteoarthritis often includes electrotherapeutic modalities as well as advice and exercise. One commonly used modality is pulsed electromagnetic field therapy (PEMF). PEMF uses electro magnetically generated fields to promote tissue repair and healing rates. Its equivocal benefit over placebo treatment has been previously suggested however recently a number of randomised controlled trials have been published that have allowed a systematic review to be conducted.
Methods
A systematic review of the literature from 1966 to 2005 was undertaken. Relevant computerised bibliographic databases were searched and papers reviewed independently by two reviewers for quality using validated criteria for assessment. The key outcomes of pain and functional disability were analysed with weighted and standardised mean differences being calculated.
Results
Five randomised controlled trials comparing PEMF with placebo were identified. The weighted mean differences of the five papers for improvement in pain and function, were small and their 95% confidence intervals included the null.
Conclusion
This systematic review provides further evidence that PEMF has little value in the management of knee osteoarthritis. There appears to be clear evidence for the recommendation that PEMF does not significantly reduce the pain of knee osteoarthritis
Theorems on existence and global dynamics for the Einstein equations
This article is a guide to theorems on existence and global dynamics of
solutions of the Einstein equations. It draws attention to open questions in
the field. The local-in-time Cauchy problem, which is relatively well
understood, is surveyed. Global results for solutions with various types of
symmetry are discussed. A selection of results from Newtonian theory and
special relativity that offer useful comparisons is presented. Treatments of
global results in the case of small data and results on constructing spacetimes
with prescribed singularity structure or late-time asymptotics are given. A
conjectural picture of the asymptotic behaviour of general cosmological
solutions of the Einstein equations is built up. Some miscellaneous topics
connected with the main theme are collected in a separate section.Comment: Submitted to Living Reviews in Relativity, major update of Living
Rev. Rel. 5 (2002)
The Luminosity, Colour and Morphology dependence of galaxy filaments in the Sloan Digital Sky Survey Data Release Four
We have tested for luminosity, colour and morphology dependence of the degree
of filamentarity in seven nearly two dimensional strips from the Sloan Digital
Sky Survey Data Release Four (SDSS DR4). The analysis is carried out at various
levels of coarse graining allowing us to address different length-scales. We
find that the brighter galaxies have a less filamentary distribution than the
fainter ones at all levels of coarse graining. The distribution of red galaxies
and ellipticals shows a higher degree of filamentarity compared to blue
galaxies and spirals respectively at low levels of coarse graining. The
behaviour is reversed at higher levels of coarse graining. We propose a picture
where the ellipticals are densely distributed in the vicinity of the nodes
where the filaments intersect while the spirals are sparsely distributed along
the entire extent of the filaments. Our findings indicate that the regions with
an excess of ellipticals are larger than galaxy clusters, protruding into the
filaments. We have also compared the predictions of a semi-analytic model of
galaxy formation (the Millennium Run galaxy catalogue) against our results for
the SDSS. We find the two to be in agreement for the galaxies and for
the red galaxies, while the model fails to correctly predict the filamentarity
of the brighter galaxies and the blue galaxies.Comment: 14 Pages, 2 tables, 11 figures, Accepted for publication in MNRAS,
new section added for a comparison with semi analytical models of galaxy
formation, substantial revisio
The Hubble Constant
I review the current state of determinations of the Hubble constant, which
gives the length scale of the Universe by relating the expansion velocity of
objects to their distance. There are two broad categories of measurements. The
first uses individual astrophysical objects which have some property that
allows their intrinsic luminosity or size to be determined, or allows the
determination of their distance by geometric means. The second category
comprises the use of all-sky cosmic microwave background, or correlations
between large samples of galaxies, to determine information about the geometry
of the Universe and hence the Hubble constant, typically in a combination with
other cosmological parameters. Many, but not all, object-based measurements
give values of around 72-74km/s/Mpc , with typical errors of 2-3km/s/Mpc.
This is in mild discrepancy with CMB-based measurements, in particular those
from the Planck satellite, which give values of 67-68km/s/Mpc and typical
errors of 1-2km/s/Mpc. The size of the remaining systematics indicate that
accuracy rather than precision is the remaining problem in a good determination
of the Hubble constant. Whether a discrepancy exists, and whether new physics
is needed to resolve it, depends on details of the systematics of the
object-based methods, and also on the assumptions about other cosmological
parameters and which datasets are combined in the case of the all-sky methods.Comment: Extensively revised and updated since the 2007 version: accepted by
Living Reviews in Relativity as a major (2014) update of LRR 10, 4, 200
Exploring the Cosmic Web in the Sloan Digital Sky Survey Data Release Seven using the Local Dimension
It is possible to visualize the Cosmic Web as an interconnected network of
one-dimensional filaments, two-dimensional sheets and three-dimensional
volume-filling structures which we refer to as clusters. We have used the Local
Dimension D, which takes values D = 1, 2 and 3 for filaments, sheets and
clusters, respectively, to analyse the Cosmic Web in a three-dimensional
volume-limited galaxy sample from the Sloan Digital Sky Survey Data Release 7.
The analysis was carried out separately using three different ranges of
length-scales: 0.5-5, 1-10 and 5-50 . We find that there is a
progressive increase in the D values as we move to larger length-scales. At the
smallest length-scale, the galaxies predominantly reside in filaments and
sheets. There is a shift from filaments to sheets and clusters at larger
scales. Filaments are completely absent at the largest length-scale (5-50
). Considering the effect of the density environment on the
Cosmic Web, we find that the filaments preferentially inhabit regions with a
lower density environment as compared to sheets and clusters which prefer
relatively higher density environments. A similar length-scale dependence and
environment dependence was also found in a galaxy sample drawn from the
Millennium Simulation which was analysed in exactly the same way as the actual
data.Comment: 17 pages, 10 figures, published in MNRA
Stochastic Gravity: Theory and Applications
Whereas semiclassical gravity is based on the semiclassical Einstein equation
with sources given by the expectation value of the stress-energy tensor of
quantum fields, stochastic semiclassical gravity is based on the
Einstein-Langevin equation, which has in addition sources due to the noise
kernel.In the first part, we describe the fundamentals of this new theory via
two approaches: the axiomatic and the functional. In the second part, we
describe three applications of stochastic gravity theory. First, we consider
metric perturbations in a Minkowski spacetime: we compute the two-point
correlation functions for the linearized Einstein tensor and for the metric
perturbations. Second, we discuss structure formation from the stochastic
gravity viewpoint. Third, we discuss the backreaction of Hawking radiation in
the gravitational background of a quasi-static black hole.Comment: 75 pages, no figures, submitted to Living Reviews in Relativit
Large Scale Structure of the Universe
Galaxies are not uniformly distributed in space. On large scales the Universe
displays coherent structure, with galaxies residing in groups and clusters on
scales of ~1-3 Mpc/h, which lie at the intersections of long filaments of
galaxies that are >10 Mpc/h in length. Vast regions of relatively empty space,
known as voids, contain very few galaxies and span the volume in between these
structures. This observed large scale structure depends both on cosmological
parameters and on the formation and evolution of galaxies. Using the two-point
correlation function, one can trace the dependence of large scale structure on
galaxy properties such as luminosity, color, stellar mass, and track its
evolution with redshift. Comparison of the observed galaxy clustering
signatures with dark matter simulations allows one to model and understand the
clustering of galaxies and their formation and evolution within their parent
dark matter halos. Clustering measurements can determine the parent dark matter
halo mass of a given galaxy population, connect observed galaxy populations at
different epochs, and constrain cosmological parameters and galaxy evolution
models. This chapter describes the methods used to measure the two-point
correlation function in both redshift and real space, presents the current
results of how the clustering amplitude depends on various galaxy properties,
and discusses quantitative measurements of the structures of voids and
filaments. The interpretation of these results with current theoretical models
is also presented.Comment: Invited contribution to be published in Vol. 8 of book "Planets,
Stars, and Stellar Systems", Springer, series editor T. D. Oswalt, volume
editor W. C. Keel, v2 includes additional references, updated to match
published versio
Theorems on existence and global dynamics for the Einstein equations
This article is a guide to theorems on existence and global dynamics of
solutions of the Einstein equations. It draws attention to open questions in
the field. The local in time Cauchy problem, which is relatively well
understood, is surveyed. Global results for solutions with various types of
symmetry are discussed. A selection of results from Newtonian theory and
special relativity which offer useful comparisons is presented. Treatments of
global results in the case of small data and results on constructing spacetimes
with prescribed singularity structure are given. A conjectural picture of the
asymptotic behaviour of general cosmological solutions of the Einstein
equations is built up. Some miscellaneous topics connected with the main theme
are collected in a separate section.Comment: 54 pages, submitted to Living Reviews in Relativit
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