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The complex universe: Recent observations and theoretical challenges
No full textThe large-scale distribution of galaxies in the universe displays a complex pattern of clusters, super-clusters, filaments and voids with sizes limited only by the boundaries of the available samples. A quantitative statistical characterization of these structures shows that galaxy distribution is inhomogeneous in these samples, being characterized by large amplitude fluctuations of large spatial extension. Over a large range of scales, both the average conditional density and its variance show a non-trivial scaling behavior: at small scales, r < 20 Mpc/h, the average (conditional) density scales as r-1. At larger scales, the density depends only weakly (logarithmically) on the system size and density fluctuations follow the Gumbel distribution of extreme value statistics. These complex behaviors are different from what is expected in a homogeneous distribution with Gaussian fluctuations. The observed density inhomogeneities pose a fundamental challenge to the standard picture of cosmology but they also represent an important opportunity which points to new directions with respect to many cosmological puzzles. Indeed, the fact that matter distribution is not uniform, in the limited range of scales sampled by observations, raises the question of understanding how inhomogeneities affect the large-scale dynamics of the universe. We discuss several attempts which try to model inhomogeneities in cosmology, considering their effects with respect to the role and abundance of dark energy and dark matter. © 2010 IOP Publishing Ltd and SISSA
UNIVERSITÀ E RICERCA: ALCUNE PROPOSTE
No full textL’Università italiana ha subito negli ultimi anni un lungo e travagliato processo riformatore. Proponiamo di seguito alcune misure di riforma a breve termine, più leggere e di più facile attuazione e misure di più lungo termine, che andranno considerate dopo un attento monitoraggio del funzionamento del sistema dell’Università e della ricerca
UNIVERSITÀ E RICERCA: ALCUNE PROPOSTE
No full textL’Università italiana ha subito negli ultimi anni un lungo e travagliato processo riformatore. Proponiamo di seguito alcune misure di riforma a breve termine, più leggere e di più facile attuazione e misure di più lungo termine, che andranno considerate dopo un attento monitoraggio del funzionamento del sistema dell’Università e della ricerca
Round table discussion at the workshop "New directions in modern cosmology"
No full textThe workshop "New directions in modern cosmology", organized by Theo Nieuwenhuizen, Rudy Schild, Francesco Sylos Labini and Ruth Durrer, was held from September 27 until October 1, 2010, in the Lorentz Center in Leiden, the Netherlands. A transcript of the final round table discussion, chaired by Theo Nieuwenhuizen and Rudy Schild, is presented. The subjects are: 0) spread in data; 1) back reaction; 2) N-body simulations; 3) neutrinos as the dark matter; 4) gravitational hydrodynamics, 5) missing baryons and lensing in an inhomogeneous universe, and 6) final points
Generalized Rotation Curves of the Milky Way from the GAIA DR3 Data Set: Constraints on Mass Models
No full textThe circular velocity curve traced by stars provides a direct means of investigating the potential and mass distribution of the Milky Way. Recent measurements of the Galaxy’s rotation curve have revealed a significant decrease in velocity for Galactic radii larger than approximately 15 kpc. While these determinations have primarily focused on the Galactic plane, the Gaia DR3 data also offer information about off-plane velocity components. By assuming the Milky Way is in a state of Jeans equilibrium, we derived the generalized rotation curve for radial distances spanning from 8.5 kpc to 25 kpc and vertical heights ranging from −2 kpc to 2 kpc. These measurements were employed to constrain the matter distribution using two distinct mass models. The first is the canonical Navarro–Frenk–White (NFW) halo model, while the second, the dark matter disk (DMD) model, posits that dark matter is confined to the Galactic plane and follows the distribution of neutral hydrogen. The best-fitting NFW model yields a virial mass of M _vir = (6.5 ± 0.5) × 10 ^11 M _⊙ , whereas the DMD model indicates a total mass of M _DMD = (1.7 ± 0.2) × 10 ^11 M _⊙ . Our findings indicate that the DMD model generally shows a better fit to both the on-plane and off-plane behaviors at large radial distances of the generalized rotation curves than the NFW model. We emphasize that studying the generalized rotation curves at different vertical heights has the potential to provide better constraints on the geometrical properties of the dark matter distribution
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