Nakia Carlevaro

I.C.R.A.Net- International Center for Relativistic Astrophysics Network, University of Rome "La Sapienza"

Title:  Viscosity effects on gravitational collapse

Authors:  Nakia Carlevaro, Giovanni Montani

Speaker:  Nakia Carlevaro

Abstract:  We analyze the effects induced by the bulk viscosity on the dynamics associated to the extreme gravitational collapse. Aim of the

work is to investigate whether the presence of viscous corrections to  the evolution of a collapsing gas cloud influence the top-down fragmentation process. To this end, we generalize the approach presented  by C. Hunter to include in the dynamics of the (uniform and spherically symmetric) cloud the negative pressure contribution associated to the  bulk viscosity phenomenology. Within the framework of a Newtonian approach, we extend to the viscous case either the Lagrangian, either  the Eulerian motion of the system and we treat the asymptotic evolution in correspondence to a viscosity coefficient of the form  $\zeta=\zeta_0\rho^{5/6}$ ($\rho$ being the cloud density and $\zeta_0=const.$). We show how the adiabatic-like behavior of the gas is deeply influenced by viscous correction when its collapse reaches the extreme regime toward the singularity. In fact, for sufficiently large viscous contributions, density contrasts associated to a given scale of the fragmentation process acquire, asymptotically, a vanishing behavior which prevents the formation of sub-structures. Since in the non-dissipative case density contrasts diverge, we can conclude that in the adiabatic-like collapse the top down mechanism of structures formation is suppressed as soon as enough strong viscous effects are taken into account. Such a feature is not present in the isothermal-like collapse because the sub-structures formation is yet present and outlines the same behavior as in the non-viscous case. We emphasize that  in the adiabatic-like collapse the bulk viscosity is also responsible for the appearance of a threshold scale beyond which perturbations begin to increase; this issue, absent in the non-viscous case, is equivalent to deal with a Jeans length.

V. Belinski

I.C.R.A.Net- International Center for Relativistic Astrophysics Network, University of Rome "La Sapienza"

Title: Equilibrium confgurations of two charged masses in General Relativity".

Author: G. Alekseev and V. Belinski

Abstract: A family of static asymptotically flat solutions for Einstein-Maxwell equations which describe two

non-rotating electrically charged massive objects (the Reissner - Nordstrom sources) in equilibrium is presented. From the exact expressions it is shown that mutual gravitational interaction of these sources can be balanced by their electrostatic interaction (without any struts) if the masses, charges and distance between them satisfy the balance equation. This equation shows that the equilibrium is not possible for two black holes or for two naked singularity objects. However, in the case when one of the sources is a black hole and another one is a naked singularityE the equilibrium is possibleE for some appropriate distance.E The derived equilibrium condition for large distances asymptotes the Newtonian condition. It is interesting that for sufficiently short distances even an uncharged Schwarzshild black hole can "hang" freely in the field of the Reissner-Nordstrom naked singularity center.

Donato Bini

Istituto per le Applicazioni del Calcolo ``M. Picone'', CNR I-00161 Rome, Italy , and ICRA, University of Rome, I-00185 Rome, Italy

Title talk: Frenet-Serret formalism for null world lines

Abstract: The Frenet-Serret curve analysis is extended from nonnull to null trajectories in a generic spacetime using the Newman-Penrose formalism, recovering old results which are not well known and clarifying the associated Fermi-Walker transport which has been left largely unexplored in the literature.This machinery is then used to discuss null circular orbits in stationary axisymmetric spacetimes using the Kerr spacetime as a concrete example, and to integrate the equations of parallel transport along null geodesics in any spacetime.

Marco Valerio Battisti

Title: Quantum Cosmology: Evolutionary Quantum Dynamics and Generalized Uncertainty Principle

Speaker: Marco Valerio Battisti (battisti@icra.it)

Co-author: Giovanni Montani

ICRANet  - Physics Department University of Rome ``La Sapienza''

Abstract: The comparison of two different quantum cosmological approaches is investigated. We consider a Schrodinger dynamics for the gravitational field associated to a Bianchi IX cosmological model and then we solve the corresponding eigenvalue problem. We show that, form a phenomenological point of view, an Evolutionary Quantum Cosmology overlaps the Wheeler-De Witt approach.

This model is compared to a quantization of the same cosmological model using a generalized uncertainty relation. In particular we quantize the anisotropy variable of the Taub cosmological model, as well as Bianchi IX, using generalized commutation relations that imply that we have a minimal anisotropy uncertainty.

Riccardo Benini

Title: "Influcence of a Vector Field on the Mixmaster Dynamics"

Authors: Riccardo Benini, Giovanni Montani

Speaker: Riccardo Benini

Abstract: We show that in multidimensional gravity, vector fields completely determine the structure and properties of singularity. It turns out that in the presence of a vector field the oscillatory regime exists in all spatial dimensions and for all homogeneous models. By analysing the Hamiltonian equations we derive the Poincar´e return

map associated with the Kasner indexes and fix the rules according to which the Kasner vectors rotate.

In correspondence to a fourdimensional spacetime, the oscillatory regime here constructed overlaps the usual Belinski-Khalatnikov-Liftshitz one.

Gurzadyan Vahe

I.C.R.A.Net- International Center for Relativistic Astrophysics Network, University of Rome "La Sapienza"

Title: Relevant vacuum modes, dark energy and observations

Abstract: Certain observational consequencies of the dark energy density, as of a contribution of relevant vacuum fluctuations, will be discussed.

Orchidea Maria Lecian

*University of Rome "La Sapienza"

*I.C.R.A.- International Center for Relativistic Astrophysics

Title: Accelerated universe from an exponential Lagrangian of the metric field.

Authors: Orchidea Maria Lecian*, Giovanni Montani*

Speaker: Orchidea Maria Lecian

Abstract: Generalized Einstein equations are calculated for an exponential gravitational action, and the results are illustrated to be equivalent to a higher-order variational calculation in FRW metric. Such an action is explained to be in line with the features of the present accelerated universe. Semi-analytical de Sitter-like solutions are found for the considered dynamics, and compared to numerical ones. A comparison between standard and modified FRW cosmology is provided, thus outlining the main achievement of our formulation. 

Samir Mandal 

Centre for Space physics
Kolkata, India.

Title: Spectral Properties of Accretion Disk Around Compact Objects in High Energies

Abstract: We have calculated the spectrum of an accretion disk around a compact object by a model flow which includes standing shocks.  This model self-consistently explains the transition between hard  and soft states, as well as the origin of high energy power-law  spectrum in Mev range. We have used the fact that the shocks may accelerate electrons to a non-thermal distribution, which in turn produce power-law synchrotron photons in presence of magnetic field.

We incorporated inverse-Comptonization of these photons to generate the self-consistent spectrum. Also we have studied the effect of different flow parameters, like shock location, compression ratio etc. on the emitted spectrum.

Michael Rotondo

Relativstic Thomas Fermi approximation for nuclear matter in bulk

Authors Remo Ruffini, Michael Rotondo, Xue She Sheng
A general approach to analyze the electromagnetic field of nuclear matter in bulk is examined using a relativistic Thomas Fermi approximation. The inference of electromagnetic fields close to the vacuum polarization limit close to neutron star surfaces are presented.

G.V. Vereshchagin

*University of Rome "La Sapienza"

*I.C.R.A.- International Center for Relativistic Astrophysics