arXiv subjects

Giovanni Imponente

Publications and source records attributed to Giovanni Imponente.

16 recordsLinked to original sources

arXiv2008

Classical and Quantum Features of the Mixmaster Singularity

This review article is devoted to analyze the main properties characterizing the cosmological singularity associated to the homogeneous and inhomogeneous Mixmaster model. After the introduction of the main tools required to treat the cosmological issue, we review in details the main results got along the last forty years on the Mixmaster topic. We firstly assess the classical picture of the homogeneous chaotic cosmologies and, after a presentation of the canonical method for the quantization, we develop the quantum Mixmaster behavior. Finally, we extend both the classical and quantum features to the fully inhomogeneous case. Our survey analyzes the fundamental framework of the Mixmaster picture and completes it by accounting for recent and peculiar outstanding results.

gr-qc↗

arXiv2006

Production mechanisms of multiple primaries for Cosmic Rays Showers

We investigate the physical mechanism of the GZ-effect that could explain the production of multiple primaries from an event initiated outside the Earth's atmosphere. In this case, there would correspondingly be multiple extensive air showers in temporal coincidence at ground, even for detectors separated by many kilometers, and also showers initiated by primaries of different energies could consequently have a common source. We analyse the perspectives and limits of some models and discuss the experimental counterparts.

astro-ph↗

arXiv2006

Classical and quantum behavior of the generic cosmological solution

In the present paper we generalize the original work of C.W. Misner \cite{M69q} about the quantum dynamics of the Bianchi type IX geometry near the cosmological singularity. We extend the analysis to the generic inhomogeneous universe by solving the super-momentum constraint and outlining the dynamical decoupling of spatial points. Firstly, we discuss the classical evolution of the model in terms of the Hamilton-Jacobi approach as applied to the super-momentum and super-Hamiltonian constraints; then we quantize it in the approximation of a square potential well after an ADM reduction of the dynamics with respect to the super-momentum constraint only. Such a reduction relies on a suitable form for the generic three-metric tensor which allows the use of its three functions as the new spatial coordinates. We get a functional representation of the quantum dynamics which is equivalent to the Misner-like one when extended point by point, since the Hilbert space factorizes into $\infty^3$ independent components due to the parametric role that the three-coordinates assume in the asymptotic potential term. Finally, we discuss the conditions for having a semiclassical behavior of the dynamics and we recognize that this already corresponds to having mean occupation numbers of order $\mathcal{O}(10^2)$.

gr-qc↗

arXiv2004

New Issues in the Inflationary Scenario

We briefly review the arising of an inflationary phase in the Universe evolution in order to discuss an inhomogeneous cosmological solution in presence of a real self interacting scalar field minimally coupled to gravity in the region of a slow rolling potential plateau. During the inhomogeneous de Sitter phase the scalar field dominant term is a function of the spatial coordinates only. We apply this generic solution to the Coleman-Weinberg potential and to the Lemaitre-Tolman metric. This framework specialized nearby the FLRW model allows a classical origin for the inhomogeneous perturbations spectrum.

gr-qc↗

arXiv2004

Mixmaster Chaoticity as Semiclassical Limit of the Canonical Quantum Dynamics

Within a cosmological framework, we provide a Hamiltonian analysis of the Mixmaster Universe dynamics on the base of a standard Arnowitt-Deser-Misner approach, showing how the chaotic behavior characterizing the evolution of the system near the cosmological singularity can be obtained as the semiclassical limit of the canonical quantization of the model in the same dynamical representation. The relation between this intrinsic chaotic behavior and the indeterministic quantum dynamics is inferred through the coincidence between the microcanonical probability distribution and the semiclassical quantum one.

gr-qc↗

arXiv2004

Inhomogeneous de Sitter Solution with Scalar Field and Perturbations Spectrum

We provide an inhomogeneous solution concerning the dynamics of a real self interacting scalar field minimally coupled to gravity in a region of the configuration space where it performs a slow rolling on a plateau of its potential. During the inhomogeneous de Sitter phase the scalar field dominant term is a function of the spatial coordinates only. This solution specialized nearby the FLRW model allows a classical origin for the inhomogeneous perturbations spectrum.

gr-qc↗

arXiv2004

Pre-inflationary perturbations spectrum

In the framework of a flat FLRW model we derive an inflationary regime in which the scalar field, laying on the plateau of its potential, admits a linear time dependence and remains close to a constant value. The behaviour of inhomogeneous perturbations is determined on the background metric in agreement to the "slow-rolling" approximation. We show that the inhomogeneous scales which before inflation were not much greater then the physical horizon, conserve their spectrum (almost) unaltered after the de Sitter phase.

gr-qc↗

arXiv2004

Mixmaster Chaos via the Invariant Measure

The chaoticity of the Mixmaster is discussed in the framework of Statistical Mechanics by using Misner--Chitre-like variables and an ADM reduction of its dynamics. We show that such a system is well described by a microcanonical ensemble whose invariant measure is induced by the corresponding Liouville one and is uniform. The covariance with respect to the choice of the temporal gauge of the obtained invariant measure is outlined.

gr-qc↗

arXiv2004

Complex dynamics of the biological rhythms: gallbladder and heart cases

A theoretical analysis of the mechanisms underlying the dynamics of gallbladder and heart pulsation could clarify the question regarding the classification as chaotic of the associated behaviour, eventually related to a normal and healthy beat; this analysis is particularly relevant in view of the control of dynamics bifurcations arising in situations of disease. In this work is presented a summary of the DFA method applied to gallbladder volume data for a modest number of healthy and ill patients: the presence of signal correlation is found in both cases, but the fit shapes differ from some critical values.

q-bio.TO↗

arXiv2004

Covariant Lyapunov Exponents for the Mixmaster

The dynamics of the Mixmaster Universe is analized in a covariant picture via Misner--Chitre-like variables for an ADM Hamiltonian approach. The system outcomes as isomorphic to a billiard on the Lobachevsky plane and Lyapunov exponents are calculated explicitly.

gr-qc↗

arXiv2004

Bianchi IX Chaoticity: BKL Map and Continuous Flow

We analyze the Bianchi IX dynamics (Mixmaster) in view of its stochastic properties; in the present paper we address either the original approach due to Belinski, Khalatnikov and Lifshitz (BKL) as well as a Hamiltonian one relying on the Arnowitt--Deser--Misner (ADM) reduction. We compare these two frameworks and show how the BKL map is related to the geodesic flow associated with the ADM dynamics. In particular, the link existing between the \textit{anisotropy parameters} and the \textit{Kasner indices} is outlined.

gr-qc↗

arXiv2004

General Aspects of the de Sitter phase

We present a detailed discussion of the inflationary scenario in the context of inhomogeneous cosmologies. After a review of the fundamental features characterizing the inflationary model, as referred to a homogeneous and isotropic Universe, we develop a generalization in view of including small inhomogeneous corrections in the theory. A second step in our discussion is devoted to show that the inflationary scenario provides a valuable dynamical ``bridge'' between a generic Kasner-like regime and a homogeneous and isotropic Universe in the horizon scale. This result is achieved by solving the Hamilton-Jacobi equation for a Bianchi IX model in the presence of a cosmological space-dependent term. In this respect, we construct a quasi-isotropic inflationary solution based on the expansion of the Einstein equations up to first-two orders of approximation, in which the isotropy of the Universe is due to the dominance of the scalar field kinetic term; the first order of approximation corresponds to the inhomogeneous corrections and is driven by the matter evolution. We show how such a quasi-isotropic solution contains a certain freedom in fixing the space functions involved in the problem. The main physical issue of this analysis corresponds to outline the impossibility for the classical origin of density perturbations, due to the exponential decay of the matter term during the de Sitter phase.

gr-qc↗

arXiv2002

The Mixmaster Model as a Cosmological Framework and Aspects of its Quantum Dynamics

This paper provides a review of some recent issues on the Mixmaster dynamics concerning the features of its stochasticity. After a description of the geometrical structure characterizing the homogeneous cosmological models in the Bianchi classification and the Belinsky-Khalatnikov-Lifshitz piecewise representation of the types VIII and IX oscillatory regime, we face the question regarding the time covariance of the resulting chaos as viewed in terms of continuous Misner-Chitré like variables. Finally we show how in the statistical mechanics framework the Mixmaster chaos raises as semiclassical limit of the quantum dynamics in the Planckian era.

gr-qc↗

arXiv2002

Covariant Mixmaster Dynamics

We provide a Hamiltonian analysis of the Mixmaster Universe dynamics on the base of a standard Arnowitt-Deser-Misner Hamiltonian approach, showing the covariant nature of its chaotic behaviour with respect to the choice of any time variable, from the point of view either of the dynamical systems theory, either of the statistical mechanics one.

gr-qc↗

arXiv2001

Covariant Formulation of the Invariant Measure for the Mixmaster Dynamics

We provide a Hamiltonian analysis of the Mixmaster Universe dynamics showing the covariant nature of its chaotic behavior with respect to any choice of time variable. We construct the appropriate invariant measure for the system (which relies on the existence of an ``energy-like'' constant of motion) without fixing the time gauge, i.e. the corresponding lapse function. The key point in our analysis consists of introducing generic Misner-Chitré-like variables containing an arbitrary function, whose specification allows one to set up the same dynamical scheme in any time gauge.

gr-qc↗

arXiv2000

The Observation of Lunar Impacts

The intense activity of cratering on the Moon and in the inner regions of the solar system was accomplished during the first 10^9 years [1]. Occasionally, some impact events occur even nowadays. In Section 1, we treat, from a historical point of view, the Earth-based observation of lunar impacts. In Section 2, we consider the visibility conditions of such events evaluating the luminosity of the background upon which an impact shines. In Section~3, the luminosity of an impact is discussed. The occurrence of lunar impact events outside of meteor shower periods is calculated using the hourly rate of the sporadic meteors and their population index. The evidence of a larger rate of impacts of meteoroids in the past under these hypotheses is presentend in the last section.

astro-ph↗