158 research outputs found

### Characterizing the large scale inhomogeneity of the galaxy distribution

In order to investigate whether galaxy structures are compatible with the
predictions of the standard LCDM cosmology, we focus here on the analysis of
several simple and basic statistical properties of the galaxy density field.
Namely, we test whether, on large enough scales (i.e., r>10 Mpc/h), this is
self-averaging, uniform and characterized by a Gaussian probability density
function of fluctuations. These are three different and clear predictions of
the LCDM cosmology which are fulfilled in mock galaxy catalogs generated from
cosmological N-body simulations representing this model. We consider some
simple statistical measurements able to tests these properties in a finite
sample. We discuss that the analysis of several samples of the Two Degree Field
Galaxy Redshift Survey and of the Sloan Digital Sky Survey show that galaxy
structures are non self-averaging and inhomogeneous on scales of ~100 Mpc/h,
and are thus intrinsically different from LCDM model predictions.
Correspondingly the probability density function of fluctuations shows a "fat
tail" and it is thus different from the Gaussian prediction. Finally we discuss
other recent observations which are odds with LCDM predictions and which are,
at least theoretically, compatible with the highly inhomogeneous nature of
galaxy distribution. We point out that inhomogeneous structures can be fully
compatible with statistical isotropy and homogeneity, and thus with a relaxed
version of the Cosmological Principle.Comment: 10 pages, 7 figures, to appear in the proceedings of the "Invisible
Universe International Conference", AIP proceedings serie

### Gravitational fluctuations of the galaxy distribution

We study the statistical properties of the gravitational field generated by
galaxy distribution observed bythe Sloan Digital Sky Survey (DR7). We
characterize the probability density function of gravitational force
fluctuations and 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. We consider several
volume-limited samples of the Sloan Digital Sky Survey and we compute the
gravitational force probability density function (PDF). The gravitational force
is computed in spheres of varying radius as is its PDF. We find that (i) the
PDF of the force displays features that can be understood in terms of galaxy
two-point correlations and (ii) density fluctuations on the largest scales
probed, i.e. r~100 Mpc/h, still contribute significantly to the amplitude of
the gravitational force. Our main conclusion is that fluctuations in the
gravitational force field generated by galaxy structures are also relevant on
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 large-scale fluctuations in the galaxy density field may be the source of
the large-scale flows recently observed.Comment: 6 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysics. Version v2 matches the published pape

### Fractal structures and the large scale distribution of galaxies

Galaxy structures are certainly fractal up to a certain crossover scale
\lambda_0. A clear determination of such a scale is still missing. Usually, the
conceptual and practical implications of this property are neglected and the
structures are only discussed in terms of their global amplitude. Here we
present a compact summary of these implications. First, we discuss the problem
of the identification of the crossover scale \lambda_0 and the proper
characterization of the scaling. We then consider the implications of these
properties with respect to various physical phenomena and to the corresponding
characteristic values, i.e. r_0, \sigma_8, \Omega, etc. These implications
crucially depend on the value of \lambda_0, but they are still important for a
relatively small value, say \lambda_0 \approx 50 \hmp. Finally we consider the
main theoretical consequences of these results.Comment: 27 pages, 3 figures. To appear in the proceedings of the 7th Course
in astrofundamental physics, Nato Advanced Study Institute, International
Euroconference Erice, 5-16 December 199

### Growth of correlations in gravitational N-body simulations

In the gravitational evolution of a cold infinite particle distribution,
two-body interactions can be predominant at early times: we show that, by
treating the simple case of a Poisson particle distribution in a static
universe as an ensemble of isolated two-body systems, one may capture the
origin of the first non-linear correlated structures. The developed power-law
like behavior of the two-point correlation function is then simply related to
the functional form of the time evolved nearest-neighbor probability
distribution, whose time dependence can be computed by using Liouville theorem
for the gravitational two-body problem. We then show that a similar dynamical
evolution is also found in a large-scale ordered distribution, which has
striking similarities to the case of a cosmological CDM simulation which we
also consider.Comment: 15 pages, 28 figure

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