638 research outputs found
The Scientific Competitiveness of Nations
We use citation data of scientific articles produced by individual nations in
different scientific domains to determine the structure and efficiency of
national research systems. We characterize the scientific fitness of each
nation (that is, the competitiveness of its research system) and the complexity
of each scientific domain by means of a non-linear iterative algorithm able to
assess quantitatively the advantage of scientific diversification. We find that
technological leading nations, beyond having the largest production of
scientific papers and the largest number of citations, do not specialize in a
few scientific domains. Rather, they diversify as much as possible their
research system. On the other side, less developed nations are competitive only
in scientific domains where also many other nations are present.
Diversification thus represents the key element that correlates with scientific
and technological competitiveness. A remarkable implication of this structure
of the scientific competition is that the scientific domains playing the role
of "markers" of national scientific competitiveness are those not necessarily
of high technological requirements, but rather addressing the most
"sophisticated" needs of the society
Biasing in Gaussian random fields and galaxy correlations
In this letter we show that in a Gaussian random field the correlation
length, the typical size of correlated structures, does not change with
biasing. We interpret the amplification of the correlation functions of subsets
identified by different thresholds being due to the increasing sparseness of
peaks over threshold. This clarifies an long-standing misconception in the
literature. We also argue that this effect does not explain the observed
increase of the amplitude of the correlation function xi(r) when galaxies of
brighter luminosity or galaxy clusters of increasing richness are considered.Comment: 16 pages, 3 figures, minor changes and corrected some typos to match
the version in Astrophysical Journal Letters (2000
Complexity in cosmic structures
We discuss correlation properties of a general mass density field introducing
a classification of structures based on their complexity. Standard cosmological
models for primordial mass fluctuations are characterized by a sort of
large-scale stochastic order, that we call super-homogeneity to highlight the
fact that mass fluctuations increase as a function of scale in the slowest
possible way for any stochastic mass field. On the other hand the galaxy
spatial distribution show complex structures with a high degree of
inhomogeneity and fractal-like spatial correlations up to some relevant
cosmological scale. The theoretical problem of cosmological structure formation
should then explain the growth of strongly correlated and non-linear structures
from the very uniform field of density fluctuations given as standard initial
condition.Comment: 6 pages 5 postscript figures. To be published in the proceedings of
the Conference "A non-linear worlds: the real world" Second international
conference on Frontier science, (08-12.09.2003, Pavia Italy
Correlation and clustering
In this lecture we clarify the basic difference between the correlation
properties for systems characterized by small or large fluctuations. The
concepts of correlation length, homogeneity scale, scale invariance and
criticality are discussed as well. We relate these concepts to the
interpretation of galaxy clsutering.Comment: 10 pages, latex, no figures, to appear proceedings of the NATO
Advanced Study Institute ``Astrophysical Sources of High Energy Particles &
Radiation'' Erice (Italy) 11-21 November 200
Real space statistical properties of standard cosmological models
After reviewing some basic relevant properties of stationary stochastic
processes (SSP), we discuss the properties of the so-called Harrison-Zeldovich
like spectra of mass density perturbations. These correlations are a
fundamental feature of all current standard cosmological models. Examining them
in real space we note they imply a "sub-poissonian" normalised variance in
spheres . In particular this latter behaviour
is at the limit of the most rapid decay () of this quantity
possible for any stochastic distribution (continuous or discrete). In a simple
classification of all SSP into three categories, we highlight with the name
``super-homogeneous'' the properties of the class to which models like this,
with , belong. In statistical physics language they are well described
as lattice or glass-like. We illustrate their properties through two simple
examples: (i) the ``shuffled'' lattice and the One Component Plasma at thermal
equilibrium.Comment: 7 pages, 1 figure, proceedings of the 7th Granada Seminar of
Computational and Statistical Physics (2002, Spain
TOPSEM, TwO Parameters Semi Empirical Model: Galaxy Evolution and Bulge/Disk Dicothomy from Two-Stage Halo Accretion
In recent years, increasing attention has been devoted to semi empirical,
data-driven models to tackle some aspects of the complex and still largely
debated topic of galaxy formation and evolution. We here present a new semi
empirical model whose marking feature is simplicity: it relies on solely two
assumptions, one initial condition and two free parameters. Galaxies are
connected to evolving dark matter haloes through abundance matching between
specific halo accretion rate (sHAR) and specific star formation rate (sSFR).
Quenching is treated separately, in a fully empirical way, to marginalize over
quiescent galaxies and test our assumption on the sSFR evolution without
contaminations from passive objects. Our flexible and transparent model is able
to reproduce the observed stellar mass functions up to , giving
support to our hypothesis of a monotonic relation between sHAR and sSFR. We
then exploit the model to test a hypothesis on morphological evolution of
galaxies. We attempt to explain the bulge/disk bimodality in terms of the two
halo accretion modes: fast and slow accretion. Specifically, we speculate that
bulge/spheroidal components might form during the early phase of fast halo
growth, while disks form during the later phase of slow accretion. We find
excellent agreement with both the observational bulge and elliptical mass
functions.Comment: 22 pages, 13 Figure
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