Characterizing cosmic inhomogeneity with anomalous diffusion

Abstract

Dark matter (DM) clustering at the present epoch is investigated from a fractal viewpoint in order to determine the scale where the self-similar scaling property of the DMhalo distribution transits to homogeneity. Methods based on well-established 'counts-in-cells' as well as new methods based on anomalous diffusion and random walks are investigated. Both are applied to DMhaloes of the biggest N-body simulation in the 'Dark Sky Simulations' (DS) catalogue and an equivalent randomly distributed catalogue. Results based on the smaller 'Millennium Run' (MR) simulation are revisited and improved. It is found that the MR simulation volume is too small and prone to bias to reliably identify the onset of homogeneity. Transition to homogeneity is defined when the fractal dimension of the clustered and random distributions cannot be distinguished within the associated uncertainties. The 'counts-in-cells' method applied to the DS then yields a homogeneity scale roughly consistent with previous work (~150 h-1 Mpc). The characteristic length-scale for anomalous diffusion to behave homogeneously is found to be at about 250 h-1 Mpc. The behaviour of the fractal dimensions for a halo catalogue with the same two-point function as the original but with shuffled Fourier phases is investigated. The methods based on anomalous diffusion are shown to be sensitive to the phase information, whereas the 'counts-in-cells' methods are not.