3,984 research outputs found
An algorithm for the direct reconstruction of the dark matter correlation function from weak lensing and galaxy clustering
The clustering of matter on cosmological scales is an essential probe for
studying the physical origin and composition of our Universe. To date, most of
the direct studies have focused on shear-shear weak lensing correlations, but
it is also possible to extract the dark matter clustering by combining
galaxy-clustering and galaxy-galaxy-lensing measurements. In this study we
develop a method that can constrain the dark matter correlation function from
galaxy clustering and galaxy-galaxy-lensing measurements, by focusing on the
correlation coefficient between the galaxy and matter overdensity fields. To
generate a mock galaxy catalogue for testing purposes, we use the Halo
Occupation Distribution approach applied to a large ensemble of N-body
simulations to model pre-existing SDSS Luminous Red Galaxy sample observations.
Using this mock catalogue, we show that a direct comparison between the excess
surface mass density measured by lensing and its corresponding galaxy
clustering quantity is not optimal. We develop a new statistic that suppresses
the small-scale contributions to these observations and show that this new
statistic leads to a cross-correlation coefficient that is within a few percent
of unity down to 5 Mpc/h. Furthermore, the residual incoherence between the
galaxy and matter fields can be explained using a theoretical model for
scale-dependent bias, giving us a final estimator that is unbiased to within
1%. We also perform a comprehensive study of other physical effects that can
affect the analysis, such as redshift space distortions and differences in
radial windows between galaxy clustering and weak lensing observations. We
apply the method to a range of cosmological models and show the viability of
our new statistic to distinguish between cosmological models.Comment: 23 pages, 14 figures, accepted by PRD; minor changes to V1, 1 new
figure, more detailed discussion of the covariance of the new ADSD statisti
Minimizing the stochasticity of halos in large-scale structure surveys
In recent work (Seljak, Hamaus and Desjacques 2009) it was found that
weighting central halo galaxies by halo mass can significantly suppress their
stochasticity relative to the dark matter, well below the Poisson model
expectation. In this paper we extend this study with the goal of finding the
optimal mass-dependent halo weighting and use -body simulations to perform a
general analysis of halo stochasticity and its dependence on halo mass. We
investigate the stochasticity matrix, defined as , where is the dark matter
overdensity in Fourier space, the halo overdensity of the -th
halo mass bin and the halo bias. In contrast to the Poisson model
predictions we detect nonvanishing correlations between different mass bins. We
also find the diagonal terms to be sub-Poissonian for the highest-mass halos.
The diagonalization of this matrix results in one large and one low eigenvalue,
with the remaining eigenvalues close to the Poisson prediction ,
where is the mean halo number density. The eigenmode with the lowest
eigenvalue contains most of the information and the corresponding eigenvector
provides an optimal weighting function to minimize the stochasticity between
halos and dark matter. We find this optimal weighting function to match linear
mass weighting at high masses, while at the low-mass end the weights approach a
constant whose value depends on the low-mass cut in the halo mass function.
Finally, we employ the halo model to derive the stochasticity matrix and the
scale-dependent bias from an analytical perspective. It is remarkably
successful in reproducing our numerical results and predicts that the
stochasticity between halos and the dark matter can be reduced further when
going to halo masses lower than we can resolve in current simulations.Comment: 17 pages, 14 figures, matched the published version in Phys. Rev. D
including one new figur
Recommended from our members
Prospective Cohort Study of Type 2 Diabetes and the Risk of Parkinson's Disease
OBJECTIVE—To evaluate the association between type 2 diabetes and newly reported Parkinson's disease
Outstanding Issues in Solar Dynamo Theory
The magnetic activity of the Sun, as manifested in the sunspot cycle,
originates deep within its convection zone through a dynamo mechanism which
involves non-trivial interactions between the plasma and magnetic field in the
solar interior. Recent advances in magnetohydrodynamic dynamo theory have led
us closer towards a better understanding of the physics of the solar magnetic
cycle. In conjunction, helioseismic observations of large-scale flows in the
solar interior has now made it possible to constrain some of the parameters
used in models of the solar cycle. In the first part of this review, I briefly
describe this current state of understanding of the solar cycle. In the second
part, I highlight some of the outstanding issues in solar dynamo theory related
to the the nature of the dynamo -effect, magnetic buoyancy and the
origin of Maunder-like minima in activity. I also discuss how poor constraints
on key physical processes such as turbulent diffusion, meridional circulation
and turbulent flux pumping confuse the relative roles of these vis-a-vis
magnetic flux transport. I argue that unless some of these issues are
addressed, no model of the solar cycle can claim to be ``the standard model'',
nor can any predictions from such models be trusted; in other words, we are
still not there yet.Comment: To appear in "Magnetic Coupling between the Interior and the
Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten, Astrophysics and
Space Science Proceedings, Springer-Verlag, Heidelberg, Berlin, 200
Strength in diversity: enhancing learning in vocationally-orientated, master's level courses
Postgraduate education in geography, especially at the Master’s level, is undergoing significant changes in the developed world. There is an expansion of vocationally-oriented degree programmes, increasing recruitment of international students, integration of work place skills, and the engagement of non-traditional postgraduate students as departments respond to policies for a more ‘inclusive’ higher education. This paper sets the context by outlining some programmatic changes in selected countries (Australia, the UK, and the USA). We briefly reflect on how postgraduate ‘bars’ or ‘levels’ are defined and explore in detail what ‘diversity’ or ‘heterogeneity’ means in these new postgraduate settings. The paper then explores some examples of practice drawn from our own experiences, whilst recognising that relevance will vary in other contexts. Finally we consider how diversity can be harnessed as a strength that has potential to enhance taught elements of contemporary postgraduate education in and beyond the discipline
The edge of neutral evolution in social dilemmas
The functioning of animal as well as human societies fundamentally relies on
cooperation. Yet, defection is often favorable for the selfish individual, and
social dilemmas arise. Selection by individuals' fitness, usually the basic
driving force of evolution, quickly eliminates cooperators. However, evolution
is also governed by fluctuations that can be of greater importance than fitness
differences, and can render evolution effectively neutral. Here, we investigate
the effects of selection versus fluctuations in social dilemmas. By studying
the mean extinction times of cooperators and defectors, a variable sensitive to
fluctuations, we are able to identify and quantify an emerging 'edge of neutral
evolution' that delineates regimes of neutral and Darwinian evolution. Our
results reveal that cooperation is significantly maintained in the neutral
regimes. In contrast, the classical predictions of evolutionary game theory,
where defectors beat cooperators, are recovered in the Darwinian regimes. Our
studies demonstrate that fluctuations can provide a surprisingly simple way to
partly resolve social dilemmas. Our methods are generally applicable to
estimate the role of random drift in evolutionary dynamics.Comment: 17 pages, 4 figure
A social inference model of idealization and devaluation
People often form polarized beliefs, imbuing objects (e.g., themselves or others) with unambiguously positive or negative qualities. In clinical settings, this is referred to as dichotomous thinking or "splitting" and is a feature of several psychiatric disorders. Here, we introduce a Bayesian model of splitting that parameterizes a tendency to rigidly categorize objects as either entirely "Bad" or "Good," rather than to flexibly learn dispositions along a continuous scale. Distinct from the previous descriptive theories, the model makes quantitative predictions about how dichotomous beliefs emerge and are updated in light of new information. Specifically, the model addresses how splitting is context-dependent, yet exhibits stability across time. A key model feature is that phases of devaluation and/or idealization are consolidated by rationally attributing counter-evidence to external factors. For example, when another person is idealized, their less-than-perfect behavior is attributed to unfavorable external circumstances. However, sufficient counter-evidence can trigger switches of polarity, producing bistable dynamics. We show that the model can be fitted to empirical data, to measure individual susceptibility to relational instability. For example, we find that a latent categorical belief that others are "Good" accounts for less changeable, and more certain, character impressions of benevolent as opposed to malevolent others among healthy participants. By comparison, character impressions made by participants with borderline personality disorder reveal significantly higher and more symmetric splitting. The generative framework proposed invites applications for modeling oscillatory relational and affective dynamics in psychotherapeutic contexts. (PsycInfo Database Record (c) 2023 APA, all rights reserved)
Comparative Analysis of Tandem Repeats from Hundreds of Species Reveals Unique Insights into Centromere Evolution
Centromeres are essential for chromosome segregation, yet their DNA sequences
evolve rapidly. In most animals and plants that have been studied, centromeres
contain megabase-scale arrays of tandem repeats. Despite their importance, very
little is known about the degree to which centromere tandem repeats share
common properties between different species across different phyla. We used
bioinformatic methods to identify high-copy tandem repeats from 282 species
using publicly available genomic sequence and our own data. The assumption that
the most abundant tandem repeat is the centromere DNA was true for most species
whose centromeres have been previously characterized, suggesting this is a
general property of genomes. Our methods are compatible with all current
sequencing technologies. Long Pacific Biosciences sequence reads allowed us to
find tandem repeat monomers up to 1,419 bp. High-copy centromere tandem repeats
were found in almost all animal and plant genomes, but repeat monomers were
highly variable in sequence composition and in length. Furthermore,
phylogenetic analysis of sequence homology showed little evidence of sequence
conservation beyond ~50 million years of divergence. We find that despite an
overall lack of sequence conservation, centromere tandem repeats from diverse
species showed similar modes of evolution, including the appearance of higher
order repeat structures in which several polymorphic monomers make up a larger
repeating unit. While centromere position in most eukaryotes is epigenetically
determined, our results indicate that tandem repeats are highly prevalent at
centromeres of both animals and plants. This suggests a functional role for
such repeats, perhaps in promoting concerted evolution of centromere DNA across
chromosomes
Land sparing versus land sharing:Moving forward
To address the challenges of biodiversity conservation and commodity production, a framework has been proposed that distinguishes between the integration (land sharing) and separation (land sparing) of conservation and production. Controversy has arisen around this framework partly because many scholars have focused specifically on food production rather than more encompassing notions such as land scarcity or food security. Controversy further surrounds the practical value of partial trade-off analyses, the ways in which biodiversity should be quantified, and a series of scale effects that are not readily accounted for. We see key priorities for the future in (1) addressing these issues when using the existing framework, and (2) developing alternative, holistic ways to conceptualise challenges related to food, biodiversity, and land scarcity
- …