3rd Italian-Sino Italian Participants |

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.
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.
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.
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.
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.
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.
*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.
Centre
for Space physics
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.
Relativstic Thomas Fermi approximation for nuclear matter in bulk
Authors Remo Ruffini, Michael Rotondo, Xue
She Sheng
*University of Rome "La Sapienza" *I.C.R.A.- International Center for Relativistic Astrophysics Title: Hidden invariance in Gurzadyan朮ue cosmological models Authors: G.V. Vereshchagin, G. Yegorian Abstract: The dark energy formula derived by Gurzadyan and Xue which leads to a value fitting the SN data, provides a scaling relation between physical constants and cosmological parameters and defines a set of cosmological models. In previous works we considered several of those models and derived cosmological equations for each of them. In this Letter we present the phase portrait analysis of those models. Surprisingly, we found that the separatrix in the phase space which determines the character of solutions depends solely on the value of the current matter density, namely: at Om > 2/3 the equations describe Friedmannian Universe with the classical singularity at the beginning, while at Om < 2/3 all solutions for the considered models start with zero density and non-vanishing scale factor. Even more remarkable, the value Osep = 2/3, which defines the separatrix, is the same for all models. The latter reveals an underlying invariance hidden in the models, possibly, due to the basic nature of the GX-scaling. |