173 research outputs found
Bayesian cosmic density field inference from redshift space dark matter maps
We present a self-consistent Bayesian formalism to sample the primordial
density fields compatible with a set of dark matter density tracers after
cosmic evolution observed in redshift space. Previous works on density
reconstruction did not self-consistently consider redshift space distortions or
included an additional iterative distortion correction step. We present here
the analytic solution of coherent flows within a Hamiltonian Monte Carlo
posterior sampling of the primordial density field. We test our method within
the Zel'dovich approximation, presenting also an analytic solution including
tidal fields and spherical collapse on small scales using augmented Lagrangian
perturbation theory. Our resulting reconstructed fields are isotropic and their
power spectra are unbiased compared to the true one defined by our mock
observations. Novel algorithmic implementations are introduced regarding the
mass assignment kernels when defining the dark matter density field and
optimization of the time step in the Hamiltonian equations of motions. Our
algorithm, dubbed barcode, promises to be specially suited for analysis of the
dark matter cosmic web down to scales of a few Megaparsecs. This large scale
structure is implied by the observed spatial distribution of galaxy clusters
--- such as obtained from X-ray, SZ or weak lensing surveys --- as well as that
of the intergalactic medium sampled by the Lyman alpha forest or perhaps even
by deep hydrogen intensity mapping. In these cases, virialized motions are
negligible, and the tracers cannot be modeled as point-like objects. It could
be used in all of these contexts as a baryon acoustic oscillation
reconstruction algorithm.Comment: 34 pages, 25 figures, 1 table. Submitted to MNRAS. Accompanying code
at https://github.com/egpbos/barcod
Bayesian Cosmic Web Reconstruction: BARCODE for Clusters
We describe the Bayesian BARCODE formalism that has been designed towards the
reconstruction of the Cosmic Web in a given volume on the basis of the sampled
galaxy cluster distribution. Based on the realization that the massive compact
clusters are responsible for the major share of the large scale tidal force
field shaping the anisotropic and in particular filamentary features in the
Cosmic Web. Given the nonlinearity of the constraints imposed by the cluster
configurations, we resort to a state-of-the-art constrained reconstruction
technique to find a proper statistically sampled realization of the original
initial density and velocity field in the same cosmic region. Ultimately, the
subsequent gravitational evolution of these initial conditions towards the
implied Cosmic Web configuration can be followed on the basis of a proper
analytical model or an N-body computer simulation. The BARCODE formalism
includes an implicit treatment for redshift space distortions. This enables a
direct reconstruction on the basis of observational data, without the need for
a correction of redshift space artifacts. In this contribution we provide a
general overview of the the Cosmic Web connection with clusters and a
description of the Bayesian BARCODE formalism. We conclude with a presentation
of its successful workings with respect to test runs based on a simulated large
scale matter distribution, in physical space as well as in redshift space.Comment: 18 pages, 8 figures, Proceedings of IAU Symposium 308 "The Zeldovich
Universe: Genesis and Growth of the Cosmic Web", 23-28 June 2014, Tallinn,
Estoni
The darkness that shaped the void: dark energy and cosmic voids
Aims: We assess the sensitivity of void shapes to the nature of dark energy
that was pointed out in recent studies. We investigate whether or not void
shapes are useable as an observational probe in galaxy redshift surveys. We
focus on the evolution of the mean void ellipticity and its underlying physical
cause. Methods: We analyse the morphological properties of voids in five sets
of cosmological N-body simulations, each with a different nature of dark
energy. Comparing voids in the dark matter distribution to those in the halo
population, we address the question of whether galaxy redshift surveys yield
sufficiently accurate void morphologies. Voids are identified using the
parameter free Watershed Void Finder. The effect of redshift distortions is
investigated as well. Results: We confirm the statistically significant
sensitivity of voids in the dark matter distribution. We identify the level of
clustering as measured by \sigma_8(z) as the main cause of differences in mean
void shape . We find that in the halo and/or galaxy distribution it
is practically unfeasible to distinguish at a statistically significant level
between the various cosmologies due to the sparsity and spatial bias of the
sample.Comment: 22 pages, 23 figures, 3 tables; v2: added references and short
comparison of void size results; accepted for publication by MNRA
Felix:A Topology Based Framework for Visual Exploration of Cosmic Filaments
The large-scale structure of the universe is comprised of virialized blob-like clusters, linear filaments, sheet-like walls and huge near empty three-dimensional voids. Characterizing the large scale universe is essential to our understanding of the formation and evolution of galaxies. The density range of clusters, walls and voids are relatively well separated, when compared to filaments, which span a relatively larger range. The large scale filamentary network thus forms an intricate part of the cosmic web. In this paper, we describe Felix, a topology based framework for visual exploration of filaments in the cosmic web. The filamentary structure is represented by the ascending manifold geometry of the 2-saddles in the Morse-Smale complex of the density field. We generate a hierarchy of Morse-Smale complexes and query for filaments based on the density ranges at the end points of the filaments. The query is processed efficiently over the entire hierarchical Morse-Smale complex, allowing for interactive visualization. We apply Felix to computer simulations based on the heuristic Voronoi kinematic model and the standard LCDM cosmology, and demonstrate its usefulness through two case studies. First, we extract cosmic filaments within and across cluster like regions in Voronoi kinematic simulation datasets. We demonstrate that we produce similar results to existing structure finders. Second, we extract different classes of filaments based on their density characteristics from the LCDM simulation datasets. Filaments that form the spine of the cosmic web, which exist in high density regions in the current epoch, are isolated using Felix. Also, filaments present in void-like regions are isolated and visualized. These filamentary structures are often over shadowed by higher density range filaments and are not easily characterizable and extractable using other filament extraction methodologies
Alpha, Betti and the Megaparsec Universe: on the Topology of the Cosmic Web
We study the topology of the Megaparsec Cosmic Web in terms of the
scale-dependent Betti numbers, which formalize the topological information
content of the cosmic mass distribution. While the Betti numbers do not fully
quantify topology, they extend the information beyond conventional cosmological
studies of topology in terms of genus and Euler characteristic. The richer
information content of Betti numbers goes along the availability of fast
algorithms to compute them.
For continuous density fields, we determine the scale-dependence of Betti
numbers by invoking the cosmologically familiar filtration of sublevel or
superlevel sets defined by density thresholds. For the discrete galaxy
distribution, however, the analysis is based on the alpha shapes of the
particles. These simplicial complexes constitute an ordered sequence of nested
subsets of the Delaunay tessellation, a filtration defined by the scale
parameter, . As they are homotopy equivalent to the sublevel sets of
the distance field, they are an excellent tool for assessing the topological
structure of a discrete point distribution. In order to develop an intuitive
understanding for the behavior of Betti numbers as a function of , and
their relation to the morphological patterns in the Cosmic Web, we first study
them within the context of simple heuristic Voronoi clustering models.
Subsequently, we address the topology of structures emerging in the standard
LCDM scenario and in cosmological scenarios with alternative dark energy
content. The evolution and scale-dependence of the Betti numbers is shown to
reflect the hierarchical evolution of the Cosmic Web and yields a promising
measure of cosmological parameters. We also discuss the expected Betti numbers
as a function of the density threshold for superlevel sets of a Gaussian random
field.Comment: 42 pages, 14 figure
First observation of Bs -> D_{s2}^{*+} X mu nu decays
Using data collected with the LHCb detector in proton-proton collisions at a
centre-of-mass energy of 7 TeV, the semileptonic decays Bs -> Ds+ X mu nu and
Bs -> D0 K+ X mu nu are detected. Two structures are observed in the D0 K+ mass
spectrum at masses consistent with the known D^+_{s1}(2536) and
$D^{*+}_{s2}(2573) mesons. The measured branching fractions relative to the
total Bs semileptonic rate are B(Bs -> D_{s2}^{*+} X mu nu)/B(Bs -> X mu nu)=
(3.3\pm 1.0\pm 0.4)%, and B(Bs -> D_{s1}^+ X munu)/B(Bs -> X mu nu)= (5.4\pm
1.2\pm 0.5)%, where the first uncertainty is statistical and the second is
systematic. This is the first observation of the D_{s2}^{*+} state in Bs
decays; we also measure its mass and width.Comment: 8 pages 2 figures. Published in Physics Letters
Multiple sclerosis genomic map implicates peripheral immune cells and microglia in susceptibility
We analyzed genetic data of 47,429 multiple sclerosis (MS) and 68,374 control subjects and established a reference map of the genetic architecture of MS that includes 200 autosomal susceptibility variants outside the major histocompatibility complex (MHC), one chromosome X variant, and 32 variants within the extended MHC. We used an ensemble of methods to prioritize 551 putative susceptibility genes that implicate multiple innate and adaptive pathways distributed across the cellular components of the immune system. Using expression profiles from purified human microglia, we observed enrichment for MS genes in these brain-resident immune cells, suggesting that these may have a role in targeting an autoimmune process to the central nervous system, although MS is most likely initially triggered by perturbation of peripheral immune responses
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