The large difference between the statistical properties of galaxies in observations and simulations has previously been analysed using the fractal dimension, see http://cosmocoffee.info/viewtopic.php?t=1543. Simply put, [tex]\Lambda[/tex]CDM simulations are much more homogeneous than the observations.
This paper compares the probability distribution function of the (regularised) Newtonian force in SDSS DR7 and the Millennium simulation semianalytic galaxy catalog. The difference in the matter distribution is of course reflected in the force. In simulations, the force stabilises beyond few tens of Mpc, whereas for the observations, the force does not converge up to scales of 80100 Mpc/h, in agreement with previous analyses using different methods (astroph/0510106, 0810.3658).
The author notes that, assuming that peculiar velocity is parallel to the gravitational acceleration, the forces calculated from observations are compatible with the claimed large bulk flows (see e.g. 0809.3734, 0910.4958, 0911.5516, 1001.1261). Such large flows are (like the observed statistical properties of the galaxy distribution) unexpected in a vanilla [tex]\Lambda[/tex]CDM universe. (I should say, though, that the role of systematics in the bulk flow analyses is not clear to me: in one method, you have to put together different catalogs, and in the other, remove the cosmological CMB signal from the maps to see the kinetic SZ effect.)
[1007.1860] Gravitational fluctuations of the galaxy distribution
Authors:  Francesco Sylos Labini 
Abstract:  Aims. We study the statistical properties of the gravitational field generated by galaxy distribution from the Sloan Digital Sky Survey (DR7). We characterize the probability density function (PDF) of gravitational force fluctuations and we relate its limiting behaviors to the correlation properties of the underlying density field. In addition, we study whether the PDF converges to an asymptotic shape within sample volumes. Methods. We consider several volume limited samples of the Sloan Digital Sky Survey and we compute the gravitational force PDF. The gravitational force is computed in spheres of varying radius, and so its PDF. Results. We find that (i) the PDF of the force displays features that can be understood in terms of galaxy twopoint correlations and that (ii) density fluctuations at the largest scales probed, i.e. r \approx 100 Mpc/h, still significantly contribute to the amplitude of the gravitational force. Conclusions. Our main conclusion is that fluctuations in the gravitational force field generated by galaxy structures are relevant also at scales ~ 100 Mpc/h. By assuming that the gravitational fluctuations in the galaxy distribution reflect those in the whole matter distribution, and that peculiar velocities and accelerations are simply correlated, we may conclude that largescale fluctuations in the galaxy density field can be the source of the large scale flows recently observed. 
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 Joined: March 02 2005
 Affiliation: University of Helsinki