332 research outputs found
Fractal correlations in the CfA2-South redshift survey
We report our analysis of the properties of galaxy clustering for a new
redshift sample of galaxies, the CfA2-South catalog, using statistical methods
which do not rely on the assumption of homogeneity. We find that, up to ~ 20
Mpc/h, which is the largest scale to which correlation properties can be
reliably inferred, the galaxy clustering is scale-invariant and characterized
by a fractal dimension D=1.9 \pm 0.1. Further there is no statistical evidence
for homogeneity at any of the larger scales (up to ~150 Mpc/h) probed more
weakly by the catalog. These results means that characteristic ``correlation
lengths'' for the clustering of galaxies derived using standards methods of
analysis are not meaningful. Further the results are very consistent with those
obtained from many other catalogs using the methods adopted here, which show
the D =2 fractal continuing to beyond 100 Mpc/h. The incompleteness of the
relevant data conjectured by various authors to give rise to such behaviour is
therefore proved to have no significant effect (up to 20 Mpc/h) on the measured
correlations.Comment: 18 pages, latex, 1 postscript figure, also available at
http://www.phys.uniroma1.it/DOCS/PIL/pil.html Accepted for publication in
Astrophysical Journal Letter
Cold uniform spherical collapse revisited
We report results of a study of the Newtonian dynamics of N self-gravitating
particles which start in a quasi-uniform spherical configuration, without
initial velocities. These initial conditions would lead to a density
singularity at the origin at a finite time when N \rightarrow \infty, but this
singularity is regulated at any finite N (by the associated density
fluctuations). While previous studies have focussed on the behaviour as a
function of N of the minimal size reached during the contracting phase, we
examine in particular the size and energy of the virialized halo which results.
We find the unexpected result that the structure decreases in size as N
increases, scaling in proportion to N^{-1/3}, a behaviour which is associated
with an ejection of kinetic energy during violent relaxation which grows in
proportion to N^{1/3}. This latter scaling may be qualitatively understood, and
if it represents the asymptotic behaviour in N implies that this ejected energy
is unbounded above. We discuss also tests we have performed which indicate that
this ejection is a mean-field phenomenon (i.e. a result of collisionless
dynamics).Comment: 10 pages, 9 figures; proceedings of "Invisible Universe" conference,
Paris, July 200
Scaling in cosmic structures
The study of the properties of cosmic structures in the universe is one of
the most fascinating subject of the modern cosmology research. Far from being
predicted, the large scale structure of the matter distribution is a very
recent discovery, which continuosly exhibits new features and issues. We have
faced such topic along two directions; from one side we have studied the
correlation properties of the cosmic structures, that we have found
substantially different from the commonly accepted ones. From the other side,
we have studied the statistical properties of the very simplified system, in
the attempt to capture the essential ingredients of the formation of the
observed structures.Comment: 10 pages, 3 figures. Accepted for pubblication in Fractals
Fractal Cosmology in an Open Universe
The clustering of galaxies is well characterized by fractal properties, with
the presence of an eventual cross-over to homogeneity still a matter of
considerable debate. In this letter we discuss the cosmological implications of
a fractal distribution of matter, with a possible cross-over to homogeneity at
an undetermined scale R_{homo}. Contrary to what is generally assumed, we show
that, even when R_{homo} -> \infty, this possibility can be treated
consistently within the framework of the expanding universe solutions of
Friedmann. The fractal is a perturbation to an open cosmology in which the
leading homogeneous component is the cosmic background radiation (CBR). This
cosmology, inspired by the observed galaxy distributions, provides a simple
explanation for the recent data which indicate the absence of deceleration in
the expansion (q_o \approx 0). Correspondingly the `age problem' is also
resolved. Further we show that the model can be extended back from the
curvature dominated arbitrarily deep into the radiation dominated era, and we
discuss qualitatively the modifications to the physics of the anisotropy of the
CBR, nucleosynthesis and structure formation.Comment: 7 pages, no figures, to appear in Europhysics Letter
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