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

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