To be published in Reviews of Modern Physics

For a PDF version of the article, click here.


Bernard J.T. Jones

Kapteyn Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands

Vincent J. Martínez

Observatori Astronòmic de la Universitat de València, Edifici d'Instituts de Paterna, Apartat de Correus 22085, 46071 València, Spain

Enn Saar

Tartu Observatory, Tõravere, 61602 Estonia

Virginia Trimble

Astronomy Department, University of Maryland, College Park MD 20742, USA
Physics Department, University of California, Irvine CA 92697 USA

Abstract. Research done during the previous century established our Standard Cosmological Model. There are many details still to be filled in, but few would seriously doubt the basic premise. Past surveys have revealed that the large-scale distribution of galaxies in the Universe is far from random: it is highly structured over a vast range of scales. Surveys being currently undertaken and being planned for the next decades will provide a wealth of information about this structure. The ultimate goal must be not only to describe galaxy clustering as it is now, but also to explain how this arose as a consequence of evolutionary processes acting on the initial conditions that we see in the Cosmic Microwave Background anisotropy data.

In order to achieve this we will want to describe cosmic structure quantitatively: we need to build mathematically quantifiable descriptions of structure. Identifying where scaling laws apply and the nature of those scaling laws is an important part of understanding which physical mechanisms have been responsible for the organization of clusters, superclusters of galaxies and the voids between them. Finding where these scaling laws are broken is equally important since this indicates the transition to different underlying physics.

In describing scaling laws we are helped by making analogies with fractals: mathematical constructs that can possess a wide variety of scaling properties. We must beware, however, of saying that the Universe is a fractal on some range of scales: it merely exhibits a specific kind of fractal-like behavior on those scales. We exploit the richness of fractal scaling behavior merely as an important supplement to the usual battery of statistical descriptors.

We review the history of how we have learned about the structure of the Universe and present the data and methodologies that are relevant to the question of discovering and understanding any scaling properties that structure may have. The ultimate goal is to have a complete understanding of how that structure emerged. We are getting close!

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