1,096 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
Social Networks and Wages: It's All About Connections!
Empirical evidence suggests that networks of personal relations are very important in the micro dynamics of labor markets: irrespectively of the country and the occupation considered a high share of jobs are filled by social referrals. In this paper we add theoretical speculation and empirical evidence to this stylized fact shedding light on an apparent puzzle: the effect of informal contacts on wages. We, first, review the literature arguing that economic perspectives on the effect of social networks use on wages can benefit from considering differences in the nature of social ties. Second, we propose a formal model which considers two different informal contacts which we call "family" and "professional". The model predicts that while the use of the former type is likely to have a negative impact on wages, the opposite is true for the latter. Third, we use a relatively unexploited Italian data set to show how different ties have different properties and are likely to be used for different purposes. Finally, we concentrate on the relation between informal contacts and wages, obtaining results which are consistent with our theoretical insights.Labor Markets, Job Search, Social Networks
Breaking the self-averaging properties of spatial galaxy fluctuations in the Sloan Digital Sky Survey - Data Release Six
Statistical analyses of finite sample distributions usually assume that
fluctuations are self-averaging, i.e. that they are statistically similar in
different regions of the given sample volume. By using the scale-length method,
we test whether this assumption is satisfied in several samples of the Sloan
Digital Sky Survey Data Release Six. We find that the probability density
function (PDF) of conditional fluctuations, filtered on large enough spatial
scales (i.e., r>30 Mpc/h), shows relevant systematic variations in different
sub-volumes of the survey. Instead for scales r<30 Mpc/h the PDF is
statistically stable, and its first moment presents scaling behavior with a
negative exponent around one. Thus while up to 30 Mpc/h galaxy structures have
well-defined power-law correlations, on larger scales it is not possible to
consider whole sample average quantities as meaningful and useful statistical
descriptors. This situation is due to the fact that galaxy structures
correspond to density fluctuations which are too large in amplitude and too
extended in space to be self-averaging on such large scales inside the sample
volumes: galaxy distribution is inhomogeneous up to the largest scales, i.e. r
~ 100 Mpc/h, probed by the SDSS samples. We show that cosmological corrections,
as K-corrections and standard evolutionary corrections, do not qualitatively
change the relevant behaviors. Finally we show that the large amplitude galaxy
fluctuations observed in the SDSS samples are at odds with the predictions of
the standard LCDM model of structure formation.(Abridged version).Comment: 32 pages, 28 figures, accepted for publication in Astronomy and
Astrophysics. A higher resolution version is available at
http://pil.phys.uniroma1.it/~sylos/fsl_highlights.html . Version v2 has been
corrected to match the published on
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
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