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[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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Syksy Rasanen
Joined: 02 Mar 2005 Posts: 128 Affiliation: University of Helsinki

Posted: July 29 2010 


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, ΛCDM 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 80−100 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 ΛCDM 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.) 

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