17,192 research outputs found
The Fractal Geometry of the Cosmic Web and its Formation
The cosmic web structure is studied with the concepts and methods of fractal
geometry, employing the adhesion model of cosmological dynamics as a basic
reference. The structures of matter clusters and cosmic voids in cosmological
N-body simulations or the Sloan Digital Sky Survey are elucidated by means of
multifractal geometry. A non-lacunar multifractal geometry can encompass three
fundamental descriptions of the cosmic structure, namely, the web structure,
hierarchical clustering, and halo distributions. Furthermore, it explains our
present knowledge of cosmic voids. In this way, a unified theory of the
large-scale structure of the universe seems to emerge. The multifractal
spectrum that we obtain significantly differs from the one of the adhesion
model and conforms better to the laws of gravity. The formation of the cosmic
web is best modeled as a type of turbulent dynamics, generalizing the known
methods of Burgers turbulence.Comment: 35 pages, 8 figures; corrected typos, added references; further
discussion of cosmic voids; accepted by Advances in Astronom
Constraining gravity by the Large Scale Structure
Over the past decades, General Relativity and the concordance CDM
model have been successfully tested using several different astrophysical and
cosmological probes based on large datasets ({\it precision cosmology}).
Despite their successes, some shortcomings emerge due to the fact that General
Relativity should be revised at infrared and ultraviolet limits and to the fact
that the fundamental nature of Dark Matter and Dark Energy is still a puzzle to
be solved. In this perspective, gravity have been extensively
investigated being the most straightforward way to modify General Relativity
and to overcame some of the above shortcomings. In this paper, we review
various aspects of gravity at extragalactic and cosmological levels. In
particular, we consider cluster of galaxies, cosmological perturbations, and
N-Body simulations, focusing on those models that satisfy both cosmological and
local gravity constraints. The perspective is that some classes of
models can be consistently constrained by Large Scale Structure.Comment: 37 pages, 3 Tables, 6 Figures. Invited Review belonging "Special
Issue Modified Gravity Cosmology: From Inflation to Dark Energy". The
manuscript matches the accepted version. References update
Dark Energy vs. Modified Gravity
Understanding the reason for the observed accelerated expansion of the
Universe represents one of the fundamental open questions in physics. In
cosmology, a classification has emerged among physical models for the
acceleration, distinguishing between Dark Energy and Modified Gravity. In this
review, we give a brief overview of models in both categories as well as their
phenomenology and characteristic observable signatures in cosmology. We also
introduce a rigorous distinction between Dark Energy and Modified Gravity based
on the strong and weak equivalence principles.Comment: 29 pages, 4 figures; invited review submitted to Annual Reviews of
Nuclear and Particle Science; v2: some pertinent references added; v3: table
with constraints added, reflects published version; v4 [trivial]: fixed
missing references in arxiv versio
Modeling and Optimal Design of Machining-Induced Residual Stresses in Aluminium Alloys Using a Fast Hierarchical Multiobjective Optimization Algorithm
The residual stresses induced during shaping and machining play an important role in determining the integrity and durability of metal components. An important issue of producing safety critical components is to find the machining parameters that create compressive surface stresses or minimise tensile surface stresses. In this paper, a systematic data-driven fuzzy modelling methodology is proposed, which allows constructing transparent fuzzy models considering both accuracy and interpretability attributes of fuzzy systems. The new method employs a hierarchical optimisation structure to improve the modelling efficiency, where two learning mechanisms cooperate together: NSGA-II is used to improve the model’s structure while the gradient descent method is used to optimise the numerical parameters. This hybrid approach is then successfully applied to the problem that concerns the prediction of machining induced residual stresses in aerospace aluminium alloys. Based on the developed reliable prediction models, NSGA-II is further applied to the multi-objective optimal design of aluminium alloys in a ‘reverse-engineering’ fashion. It is revealed that the optimal machining regimes to minimise the residual stress and the machining cost simultaneously can be successfully located
Primordial non-Gaussianity in the Bispectrum of the Halo Density Field
The bispectrum vanishes for linear Gaussian fields and is thus a sensitive
probe of non-linearities and non-Gaussianities in the cosmic density field.
Hence, a detection of the bispectrum in the halo density field would enable
tight constraints on non-Gaussian processes in the early Universe and allow
inference of the dynamics driving inflation. We present a tree level derivation
of the halo bispectrum arising from non-linear clustering, non-linear biasing
and primordial non-Gaussianity. A diagrammatic description is developed to
provide an intuitive understanding of the contributing terms and their
dependence on scale, shape and the non-Gaussianity parameter fNL. We compute
the terms based on a multivariate bias expansion and the peak-background split
method and show that non-Gaussian modifications to the bias parameters lead to
amplifications of the tree level bispectrum that were ignored in previous
studies. Our results are in a good agreement with published simulation
measurements of the halo bispectrum. Finally, we estimate the expected signal
to noise on fNL and show that the constraint obtainable from the bispectrum
analysis significantly exceeds the one obtainable from the power spectrum
analysis.Comment: 34 pages, 15 figures, (v3): matches JCAP published versio
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