1,720,974 research outputs found
Glass-like universe: Real-space correlation properties of standard cosmological models
No full textAfter reviewing the basic relevant properties of stationary stochastic processes (SSP), defining basic terms and quantities, we discuss the properties of the so-called Harrison-Zeldovich like spectra. These correlations, usually characterized exclusively in k space [i.e., in terms of power spectra P(k)], are a fundamental feature of all current standard cosmological models. Examining them in real space we note their characteristics to be a negative power law tail xi(r)similar to-r(-4), and a sub-Poissonian normalized variance in spheres sigma(2)(R)similar toR(-4)ln R. We note in particular that this latter behavior is at the limit of the most rapid decay (similar toR(-4)) of this quantity possible for any stochastic distribution (continuous or discrete). This very particular characteristic is usually obscured in cosmology by the use of Gaussian spheres. In a simple classification of all SSP into three categories, we highlight with the name "superhomogeneous" the properties of the class to which models such as this, with P(0)=0, belong. In statistical physics language they are well described as glass-like. They have neither "scale-invariant" features, in the sense of critical phenomena, nor fractal properties. We illustrate their properties with some simple examples, in particular that of a "shuffled" lattice
Biasing in Gaussian Random Fields and Galaxy Correlations
No full textIn 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 as being caused by the increasing sparseness of peaks over threshold. This clarifies a 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
Fractals in cosmology
No full textCosmology is based more on observational and testable grounds than ever in the past. As in many other scientific areas. a statistical-mathematical filter is necessary to process data and obtain the correct information to be compared with theories and hypothesis. The new approach we have proposed here can been see as filter to interpret the new many data which are appearing in Cosmology. The methods of modern Statistical Physics, which have been successfully applied in many different fields, represent a new general framework for the study of cosmic structures. both from a phenomenological and theoretical points of view. This new approach includes as a particular case the old analytical liquid~ like approach (homogeneous mass density fields with small fluctuations), but it is able to shed light in more complex cases as matter distribution in the universe seems to be
Zipf’s law for cosmic structures: How large are the greatest structures in the universe?
No full textThe statistical characterization of the distribution of visible matter in the universe is a central problem in modern cosmology. In this respect, a crucial question still lacking a definitive answer concerns how large the greatest structures in the universe are. This point is closely related to whether or not such a distribution can be approximated as being homogeneous on large enough scales. Here we assess this problem by considering the size distribution of superclusters of galaxies and by leveraging the properties of Zipf–Mandelbrot law, providing a novel approach which complements the standard analysis based on the correlation functions. We find that galaxy superclusters are well described by a pure Zipf’s law with no deviations and this implies that all the catalogs currently available are not sufficiently large to spot a truncation in the power-law behavior. This finding provides evidence that structures larger than the greatest superclusters already observed are expected to be found when deeper redshift surveys will be completed. As a consequence, the scale beyond which galaxy distribution crossovers toward homogeneity, if any, should increase accordingly
La struttura frattale dell'universo osservabile
No full textDottorato di ricerca in fisica. 8. ciclo. Tutore G. Venturi. Coordinatore G. VeloConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome; Biblioteca Nazionale Centrale - P.za Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal
Science and the economic crisis: impact on science, lessons from science
No full textThis book not only explores the ways in which the economic crisis and associated austerity policies have adversely impacted the physical and human infrastructure and conduct of scientific research, but also considers how science can help us to understand the crisis and provide original solutions. Starting with a detailed but accessible analysis of the scientific method and the nature of scientific prediction, the book proceeds to address the failure to forecast the economic crisis and the origins of the continuing inertia in economic policy and theory. Attention is drawn in particular to the shortcomings of neoclassical economics in terms of its description of the economic system as being mechanical in nature and characterized by equilibrium. This perspective mirrors the limitations and outdated ideas of nineteenth century physics, which the book contrasts with the insights offered by modern physics. The impact of neoliberal ideologies on scientific research is also discussed in detail, highlighting their stifling effect on innovation and diversification. In closing, the book emphasizes the need for state intervention to guide and support scientific research as the core engine of economic development that will deliver a sustainable future
A fractal galaxy distribution in a homogeneous universe ? : letter to the editor
No full textIn this letter we present an idea which reconciles a homogeneous and isotropic Friedmann universe with a fractal distribution of galaxies. We use two observational facts: The flat rotation curves of galaxies and the (still debated) fractal distribution of galaxies with fractal dimension [FORMULA]. Our idea can also be interpreted as a redefinition of the notion of bias
Real Space Statistical Properties of Standard Cosmological Models
No full text7 pages, 1 figure, proceedings of the 7th Granada Seminar of Computational and Statistical Physics (2002, Spain)International audienceAfter 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 normalized variance in spheres (R) ~ . In particular this latter behavior 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 P(0) = 0, 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
Inhomogeneities in the universe
No full textInternational audienceStandard models of galaxy formation predict that matter distribution is statistically homogeneous and isotropic and characterized by (i) spatial homogeneity for r r c ≈ 150 Mpc/h (i.e., no structures of size larger than r c). Whether or not the observed galaxy distribution is interpreted to be compatible with these predictions depend on the a-priori assumptions encoded in the statistical methods employed to characterize the data and on the aposteriori hypotheses made to interpret the results. We present strategies to test the most common assumptions and we find evidences that, in the available samples, galaxy distribution is spatially inhomogeneous for r < 100 Mpc/h but statistically homogeneous and isotropic. We conclude that the observed inhomogeneities pose a fundamental challenge to the standard picture of cosmology but they also represent an important opportunity which may open new directions for many cosmological puzzles
Long-lived transient structure in collisionless self-gravitating systems
No full textInternational audienceThe evolution of self-gravitating systems, and long-range interacting systems more generally, from initial configurations far from dynamical equilibrium is often described as a simple two-phase process: a first phase of violent relaxation bringing it to a quasistationary state in a few dynamical times, followed by a slow adiabatic evolution driven by collisional processes. In this context the complex spatial structure evident, for example, in spiral galaxies is understood either in terms of instabilities of quasistationary states or as a result of dissipative nongravitational interactions. We illustrate here, using numerical simulations, that purely self-gravitating systems evolving from quite simple initial configurations can in fact give rise easily to structures of this kind, of which the lifetime can be large compared to the dynamical characteristic time but short compared to the collisional relaxation timescale. More specifically, for a broad range of nonspherical and nonuniform rotating initial conditions, gravitational relaxation gives rise quite generically to long-lived nonstationary structures of a rich variety, characterized by spiral-like arms, bars, and even ringlike structures in special cases. These structures are a feature of the intrinsically out-of-equilibrium nature of the system's collapse, associated with a part of the system's mass while the bulk is well virialized. They are characterized by predominantly radial motions in their outermost parts, but also incorporate an extended flattened region which rotates coherently about a well-virialized core of triaxial shape with an approximately isotropic velocity dispersion. We characterize the kinematical and dynamical properties of these complex velocity fields and we briefly discuss the possible relevance of these simple toy models to the observed structure of real galaxies, emphasizing the difference between dissipative and dissipationless disk formation
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