649,647 research outputs found
Wavelet analysis of the formation of the cosmic web
According to the modern cosmological paradigm galaxies and galaxy systems
form from tiny density perturbations generated during the very early phase of
the evolution of the Universe. Using numerical simulations we study the
evolution of phases of density perturbations of different scales to understand
the formation and evolution of the cosmic web. We apply the wavelet analysis to
follow the evolution of high-density regions (clusters and superclusters) of
the cosmic web. We show that the positions of maxima and minima of density
waves (their spatial phases) almost do not change during the evolution of the
structure. Positions of extrema of density perturbations are the more stable,
the larger is the wavelength of perturbations. Combining observational and
simulation data we conclude that the skeleton of the cosmic web was present
already in an early stage of structure evolution.Comment: 12 pages, 8 figures, revised versio
Evolution of community structure in the world trade web
In this note we study the bilateral merchandise trade flows between 186
countries over the 1948-2005 period using data from the International Monetary
Fund. We use Pajek to identify network structure and behavior across thresholds
and over time. In particular, we focus on the evolution of trade "islands" in
the a world trade network in which countries are linked with directed edges
weighted according to fraction of total dollars sent from one country to
another. We find mixed evidence for globalization.Comment: To be submitted to APFA 6 Proceedings, 8 pages, 3 Figure
The cosmic web for density perturbations of various scales
We follow the evolution of galaxy systems in numerical simulation. Our goal
is to understand the role of density perturbations of various scales in the
formation and evolution of the cosmic web. We perform numerical simulations
with the full power spectrum of perturbations, and with spectrum cut at long
wavelengths. Additionally, we have one model, where we cut the intermediate
waves. We analyze the density field and study the void sizes and density field
clusters in different models. Our analysis shows that the fine structure
(groups and clusters of galaxies) is created by small-scale density
perturbations of scale \Mpc. Filaments of galaxies and clusters are
created by perturbations of intermediate scale from to \Mpc,
superclusters of galaxies by larger perturbations. We conclude that the scale
of the pattern of the cosmic web is determined by density perturbations of
scale up to \Mpc. Larger perturbations do not change the pattern of
the web, but modulate the richness of galaxy systems, and make voids emptier.
The stop of the increase of the scale of the pattern of the cosmic web with
increasing scale of density perturbations can probably be explained as the
freezing of the web at redshift .Comment: 12 pages, 7 figures, accepted for publication in Astronomy and
Astrophysic
Tracing early stellar evolution with asteroseismology: pre-main sequence stars in NGC 2264
Asteroseismology has been proven to be a successful tool to unravel details
of the internal structure for different types of stars in various stages of
their main sequence and post-main sequence evolution. Recently, we found a
relation between the detected pulsation properties in a sample of 34 pre-main
sequence (pre-MS) delta Scuti stars and the relative phase in their pre-MS
evolution. With this we are able to demonstrate that asteroseismology is
similarly powerful if applied to stars in the earliest stages of evolution
before the onset of hydrogen core burning.Comment: CoRoT Symposium 3 / Kepler KASC-7 joint meeting, Toulouse, July 2014.
To be published by EPJ Web of Conference
Photometric redshift galaxies as tracers of the filamentary network
Galaxy filaments are the dominant feature in the overall structure of the
cosmic web. The study of the filamentary web is an important aspect in
understanding galaxy evolution and the evolution of matter in the Universe. A
map of the filamentary structure is an adequate probe of the web. We propose
that photometric redshift galaxies are significantly positively associated with
the filamentary structure detected from the spatial distribution of
spectroscopic redshift galaxies. The catalogues of spectroscopic and
photometric galaxies are seen as point-process realisations in a sphere, and
the catalogue of filamentary spines is proposed to be a realisation of a random
set in a sphere. The positive association between these sets was studied using
a bivariate function, which is a summary statistics studying clustering. A
quotient was built to estimate the distance distribution of the filamentary
spine to galaxies in comparison to the distance distribution of the filamentary
spine to random points in dimensional Euclidean space. This measure gives a
physical distance scale to the distances between filamentary spines and the
studied sets of galaxies. The bivariate function shows a statistically
significant clustering effect in between filamentary spines and photometric
redshift galaxies. The quotient confirms the previous result that smaller
distances exist with higher probability between the photometric galaxies and
filaments. The trend of smaller distances between the objects grows stronger at
higher redshift. Additionally, the quotient for photometric galaxies gives
a rough estimate for the filamentary spine width of about ~Mpc. Photometric
redshift galaxies are positively associated with filamentary spines detected
from the spatial distribution of spectroscopic galaxies.Comment: Accepted to Astronomy & Astrophysics. 13 pages and 9 figure
The Influence of Predator-Prey Population Dynamics on the Long-term Evolution of Food Web Structure
We develop a set of equations to describe the population dynamics of many
interacting species in food webs. Predator-prey interactions are non-linear,
and are based on ratio-dependent functional responses. The equations account
for competition for resources between members of the same species, and between
members of different species. Predators divide their total hunting/foraging
effort between the available prey species according to an evolutionarily stable
strategy (ESS). The ESS foraging behaviour does not correspond to the
predictions of optimal foraging theory. We use the population dynamics
equations in simulations of the Webworld model of evolving ecosystems. New
species are added to an existing food web due to speciation events, whilst
species become extinct due to coevolution and competition. We study the
dynamics of species-diversity in Webworld on a macro-evolutionary timescale.
Coevolutionary interactions are strong enough to cause continuous overturn of
species, in contrast to our previous Webworld simulations with simpler
population dynamics. Although there are significant fluctuations in species
diversity because of speciation and extinction, very large scale extinction
avalanches appear to be absent from the dynamics, and we find no evidence for
self-organised criticality.Comment: 40 pages, preprint forma
The Zeldovich approximation: key to understanding Cosmic Web complexity
We describe how the dynamics of cosmic structure formation defines the
intricate geometric structure of the spine of the cosmic web. The Zeldovich
approximation is used to model the backbone of the cosmic web in terms of its
singularity structure. The description by Arnold et al. (1982) in terms of
catastrophe theory forms the basis of our analysis.
This two-dimensional analysis involves a profound assessment of the
Lagrangian and Eulerian projections of the gravitationally evolving
four-dimensional phase-space manifold. It involves the identification of the
complete family of singularity classes, and the corresponding caustics that we
see emerging as structure in Eulerian space evolves. In particular, as it is
instrumental in outlining the spatial network of the cosmic web, we investigate
the nature of spatial connections between these singularities.
The major finding of our study is that all singularities are located on a set
of lines in Lagrangian space. All dynamical processes related to the caustics
are concentrated near these lines. We demonstrate and discuss extensively how
all 2D singularities are to be found on these lines. When mapping this spatial
pattern of lines to Eulerian space, we find a growing connectedness between
initially disjoint lines, resulting in a percolating network. In other words,
the lines form the blueprint for the global geometric evolution of the cosmic
web.Comment: 37 pages, 21 figures, accepted for publication in MNRA
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