202 research outputs found
Constraint of Void Bias on Primordial non-Gaussianity
We study the large-scale bias parameter of cosmic voids with primordial
non-Gaussian (PNG) initial conditions of the local type. In this scenario, the
dark matter halo bias exhibits a characteristic scale dependence on large
scales, which has been recognized as one of the most promising probes of the
local PNG. Using a suite of -body simulations with Gaussian and non-Gaussian
initial conditions, we find that the void bias features scale-dependent
corrections on large scales, similar to its halo counterpart. We find excellent
agreement between the numerical measurement of the PNG void bias and the
general peak-background split prediction. Contrary to halos, large voids
anti-correlate with the dark matter density field, and the large-scale Gaussian
void bias ranges from positive to negative values depending on void size and
redshift. Thus, the information in the clustering of voids can be complementary
to that of the halos. Using the Fisher matrix formalism for multiple tracers,
we demonstrate that including the scale-dependent bias information from voids,
constraints on the PNG parameter can be tightened by a factor of
two compared to the accessible information from halos alone, when the sampling
density of tracers reaches .Comment: 7 pages, 4 figures; dn/dlnsigma_8 prediction implemented and
excellent agreement with simulation results obtained. Matched to published
versio
Large-Scale Clustering of Cosmic Voids
We study the clustering of voids using -body simulations and simple
theoretical models. The excursion-set formalism describes fairly well the
abundance of voids identified with the watershed algorithm, although the void
formation threshold required is quite different from the spherical collapse
value. The void cross bias is measured and its large-scale value
is found to be consistent with the peak background split results. A simple
fitting formula for is found. We model the void auto-power
spectrum taking into account the void biasing and exclusion effect. A good fit
to the simulation data is obtained for voids with radii 30 Mpc/,
especially when the void biasing model is extended to 1-loop order. However,
the best-fit bias parameters do not agree well with the peak-background split
results. Being able to fit the void auto-power spectrum is particularly
important not only because it is the direct observable in galaxy surveys, but
also our method enables us to treat the bias parameters as nuisance parameters,
which are sensitive to the techniques used to identify voids.Comment: 20 pages, 14 figures, minor changes to match published versio
Probing cosmology and gravity with redshift-space distortions around voids
Cosmic voids in the large-scale structure of the Universe affect the peculiar
motions of objects in their vicinity. Although these motions are difficult to
observe directly, the clustering pattern of their surrounding tracers in
redshift space is influenced in a unique way. This allows to investigate the
interplay between densities and velocities around voids, which is solely
dictated by the laws of gravity. With the help of -body simulations and
derived mock-galaxy catalogs we calculate the average density fluctuations
around voids identified with a watershed algorithm in redshift space and
compare the results with the expectation from general relativity and the
CDM model. We find linear theory to work remarkably well in describing
the dynamics of voids. Adopting a Bayesian inference framework, we explore the
full posterior of our model parameters and forecast the achievable accuracy on
measurements of the growth rate of structure and the geometric distortion
through the Alcock-Paczynski effect. Systematic errors in the latter are
reduced from to when peculiar velocities are taken into
account. The relative parameter uncertainties in galaxy surveys with number
densities comparable to the SDSS MAIN (CMASS) sample probing a volume of
yield () and
(), respectively. At this level of precision
the linear-theory model becomes systematics dominated, with parameter biases
that fall beyond these values. Nevertheless, the presented method is highly
model independent; its viability lies in the underlying assumption of
statistical isotropy of the Universe.Comment: 38 pages, 14 figures. Published in JCAP. Referee comments
incorporated, typos corrected, references added. Considerably improved
results thanks to consideration of full covariance matrix in the MCMC
analysi
Universal Density Profile for Cosmic Voids
We present a simple empirical function for the average density profile of
cosmic voids, identified via the watershed technique in CDM N-body
simulations. This function is universal across void size and redshift,
accurately describing a large radial range of scales around void centers with
only two free parameters. In analogy to halo density profiles, these parameters
describe the scale radius and the central density of voids. While we initially
start with a more general four-parameter model, we find two of its parameters
to be redundant, as they follow linear trends with the scale radius in two
distinct regimes of the void sample, separated by its compensation scale.
Assuming linear theory, we derive an analytic formula for the velocity profile
of voids and find an excellent agreement with the numerical data as well. In
our companion paper [Sutter et al., Mon. Not. R. Astron. Soc. 442, 462 (2014)]
the presented density profile is shown to be universal even across tracer type,
properly describing voids defined in halo and galaxy distributions of varying
sparsity, allowing us to relate various void populations by simple rescalings.
This provides a powerful framework to match theory and simulations with
observational data, opening up promising perspectives to constrain competing
models of cosmology and gravity.Comment: 5 pages, 3 figures. Matches PRL published version after minor
correction
Bridge Gender and Social Movements Cutting Edge Programme: Stories of Influence
This story of influence provides an evaluation of the Bridge Gender and Social Movements Cutting Edge Programme carried out under the Sida sponsored Gender, Power and Sexuality programme. The aim of the Bridge Gender and Social Movements programme is to work towards more inclusive and effective social justice movements, better able to generate deep and lasting positive change, and better equipped to shape inequitable structures and processes.
Through interviews with recipients of the Cutting Edge pack, members from communities of practice and programme advisers, this report explores how the process of developing the pack and the pack itself has impacted on them in terms of their perception and knowledge of the issue and their ability to influence policy and practice to support building gender-just movements.
The first chapters provide a background to the purpose and methodology behind the programme development and the collaborative process employed. The last chapters reflect on the programme's effects, lessons learnt and the next steps in the process.Swedish International Development Agency (Sida
Dark matter voids in the SDSS galaxy survey
What do we know about voids in the dark matter distribution given the Sloan
Digital Sky Survey (SDSS) and assuming the model? Recent
application of the Bayesian inference algorithm BORG to the SDSS Data Release 7
main galaxy sample has generated detailed Eulerian and Lagrangian
representations of the large-scale structure as well as the possibility to
accurately quantify corresponding uncertainties. Building upon these results,
we present constrained catalogs of voids in the Sloan volume, aiming at a
physical representation of dark matter underdensities and at the alleviation of
the problems due to sparsity and biasing on galaxy void catalogs. To do so, we
generate data-constrained reconstructions of the presently observed large-scale
structure using a fully non-linear gravitational model. We then find and
analyze void candidates using the VIDE toolkit. Our methodology therefore
predicts the properties of voids based on fusing prior information from
simulations and data constraints. For usual void statistics (number function,
ellipticity distribution and radial density profile), all the results obtained
are in agreement with dark matter simulations. Our dark matter void candidates
probe a deeper void hierarchy than voids directly based on the observed
galaxies alone. The use of our catalogs therefore opens the way to
high-precision void cosmology at the level of the dark matter field. We will
make the void catalogs used in this work available at
http://www.cosmicvoids.net.Comment: 15 pages, 6 figures, matches JCAP published version, void catalogs
publicly available at http://www.cosmicvoids.ne
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