Journal Article

Fractal dimensions of a weakly clustered distribution and the scale of homogeneity

J. S. Bagla, Jaswant Yadav and T. R. Seshadri

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 390, issue 2, pages 829-838
Published in print October 2008 | ISSN: 0035-8711
Published online October 2008 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2008.13793.x
Fractal dimensions of a weakly clustered distribution and the scale of homogeneity

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Homogeneity and isotropy of the universe at sufficiently large scales is a fundamental premise on which modern cosmology is based. Fractal dimensions of matter distribution are a parameter that can be used to test the hypothesis of homogeneity. In this method, galaxies are used as tracers of the distribution of matter and samples derived from various galaxy redshift surveys have been used to determine the scale of homogeneity in the Universe. Ideally, for homogeneity, the distribution should be a monofractal with the fractal dimension equal to the ambient dimension. While this ideal definition is true for infinitely large point sets, this may not be realized as in practice, we have only a finite point set. The correct benchmark for realistic data sets is a homogeneous distribution of a finite number of points and this should be used in place of the mathematically defined fractal dimension for infinite number of points (D) as a requirement for approach towards homogeneity. We derive the expected fractal dimension for a homogeneous distribution of a finite number of points. We show that for sufficiently large data sets the expected fractal dimension approaches D in absence of clustering. It is also important to take the weak, but non-zero amplitude of clustering at very large scales into account. In this paper we also compute the expected fractal dimension for a finite point set that is weakly clustered. Clustering introduces departures in the fractal dimensions from D and in most situations the departures are small if the amplitude of clustering is small. Features in the two-point correlation function, like those introduced by baryon acoustic oscillations can lead to non-trivial variations in the fractal dimensions where the amplitude of clustering and deviations from D are no longer related in a monotonic manner. We show that in the concordance model, the fractal dimension makes a rapid transition to values close to 3 at scales between 40 and 100 Mpc.

Keywords: methods: statistical; cosmology: theory; large-scale structure of Universe

Journal Article.  7555 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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