315 research outputs found
Quantum Tetrahedra
No full textWe discuss in details the role of Wigner 6j symbol as the basic building block unifying such different fields as state sum models for quantum geometry, topological quantum field theory, statistical lattice models and quantum computing. The apparent 2-fold nature of the 6j symbol displayed in quantum field theory and quantum computing -a quantum tetrahedron and a computational gate- is shown to merge together in a unified quantum-computational SU(2)-state sum framework
Il paesaggio storico ai margini orientali della piana campana in periodo annibalico. Osservazioni topografiche dalle ricerche per la Carta archeologica della Campania.
No full textThe paper focuses on the territories on the est margins of the piana campana in the years of the annibal wars, in particular during the Second Punic War.
The results of a topographic research carried out in this area show useful data for the reconstruction of the historical landscape in those years and the ancient road system that crosses the area.
The author by employing archaeological data, analyze olso the role of Suessola montains quoted from Livius during the attempts of conquest of Nola by Hannibal in 216-214 BC
Classical and quantum perturbations to the primordial universe
No full textIn this Ph.D. thesis we analyse both classical and quantum effects relevant for the study of cosmological perturbations. We choose this particular topic because, through the analysis of cosmological perturbations, it is possible to explore a wide range of different physical phenomena. Moreover, they are a central and important piece in the puzzle of the history of the universe.
The most obvious relevance of cosmological perturbations is the study of structure formation and the large scale structure of the universe. In this regard, such perturbations are related to primordial gravitational waves and primordial magnetic fields. Given their dependence on pre-recombination phenomena, they could give us some information on the universe before hydrogen recombination.
Classical perturbations have been widely studied in literature, with the main focus on isotropic cosmological models. While this is usually a good approximation, the presence of a primordial magnetic field causes a coupling between different algebraic modes of the usual decomposition, connecting density perturbations, primordial magnetic fields and primordial gravitational waves. Moreover, the presence of the magnetic field requires the use of an anisotropic cosmological model. While small, these relations are important in the evolution of anisotropic structures. Furthermore, such primordial seeds of the magnetic fields are widely believed to be the origin of the magnetic fields measured today in galaxies. In the first part of this thesis, we analyse these relations, together with the possible effects that a non ideal, i.e. viscous, cosmological fluid could have on the growth of perturbations. We focus our attention to a Bianchi I model, improving the results of some preceding papers.
The second part of the thesis focuses on the semiclassical approximation of quantum gravity. Quantum effects are believed to influence the birth and dynamics of perturbation seeds and, in general, the dynamics of the primordial universe. This way, the mathematical scheme used to represent these effects is a central point in the description of quantum gravity regarding such seeds.
Furthermore, even more care is required to split the WKB action between embedding variables and physical degrees of freedom, and in many models the quantum gravity corrections to the Schrödinger equation violate the unitarity of the system evolution. This decomposition shares some similarities with the Born-Oppenheimer approximation of molecular physics.
We perform a critical analysis of two different ways to apply this decomposition. In particular, we analyse limits and perspectives of the different proposals to solve the non unitarity problem, even comparing expansions in different fundamental physical constants (Planck constant and mass). We find the source of non-unitary effects in a common assumption in the definition of WKB time, and we propose an alternative formulation. Also, we show how the usual assumptions of classicality of the physical quantities must be handled with care, focusing our attention to the implementation of the classical background in the perturbation scheme.
Studies in this research field are very important because they could bind CMB measurements and primordial gravitational waves to quantum gravity, bringing us finally an experimental playground
Telesia
No full textIl lavoro illustra dal punto di vista aerotopografico la città antica di Telesia, Campania settentrionale, alla luce dello stato dell'arte
Singular euclidean structures and riemann surfaces
No full textEuclidean triangulated surface .Tl;M/ characterizes
a polyhedral metric with conical singularities associated with the vertices of the
triangulation. In this chapter we show that around any such a vertex we can introduce
complex coordinates in terms of which we can write down the conformal conical
metric, locally parametrizing the singular structure of .Tl;M/. This makes available
a powerful dictionary between 2–dimensional triangulations and complex geometry
General Relativity and Gravitational Physics : Proceedings of the 11th Italian Conference, SISSA, Trieste, Italy, 26 – 30 September 1994
No full textLinked Gauss-Diffusion processes for modeling a finite-size neuronal network
No full textA Leaky Integrate-and-Fire (LIF) model with stochastic current-based linkages is considered to describe the firing activity of neurons interacting in a (2 × 2)-size feed-forward network. In the subthreshold regime and under the assumption that no more than one spike is exchanged between coupled neurons, the stochastic evolution of the neuronal membrane voltage is subject to random jumps due to interactions in the network. Linked Gauss-Diffusion processes are proposed to describe this dynamics and to provide estimates of the firing probability density of each neuron. To this end, an iterated integral equation-based approach is applied to evaluate numerically the first passage time density of such processes through the firing threshold. Asymptotic approximations of the firing densities of surrounding neurons are used to obtain closed-form expressions for the mean of the involved processes and to simplify the numerical procedure. An extension of the model to an (N × N)-size network is also given. Histograms of firing times obtained by simulations of the LIF dynamics and numerical firings estimates are compared
Su un mausoleo nell’agro nolano
No full textUn edificio funerario in opera incerta conservato nel comune di Cicciano, in località San Nicola, nella Agro di Nola, viene riletto attraverso il rilievo e l’analisi della tecnica edilizia adoperata.
Lo studio offre l’occasione di compiere alcune considerazioni sui mausolei del tipo “a cuspide” e sulla loro diffusione in questa parte della piana campana alla fine del I secolo a.C. Il tentativo di ricostruire il contesto topografico originario di pertinenza porta, inoltre, l’autrice ad alcune riflessioni sulla compresenza tomba ed edificio rurale sulla scorta dei molteplici esempi noti.
L’argomento, al fine della contestualizzazione topografica del monumento, conduce in margine anche ad alcune considerazioni sulla viabilità secondaria e sulla divisione centuriale ricostruibile nella zona nell’ambito della quale il mausoleo appare inserito.A mortuary building in Opus incertum preserved in the town of Cicciano, in the locality of San Nicola, in the countryside of Nola, is reread through detection and analysis of construction technique used.
The study provides an opportunity to make some considerations on mausoleums "cusp" and their dissemination in this part of the Campanian plain at the end of the 1st century BC. Attempting to reconstruct the original topographical context of relevance also leads the author to some reflections on coexistence grave and rural building on the basis of the numerous examples known.
The argument, in order of the topographic map of the monument, contextualization leads in some secondary roads and considerations on a centurial retreadable Division in the area within which the mausoleum appears
A comparison theorem for cosmological lightcones
No full textLet (M, g) denote a cosmological spacetime describing the evolution of a universe which is isotropic and homogeneous on large scales, but highly inhomogeneous on smaller scales. We consider two past lightcones, the first, C−L(p,g), is associated with the physical observer p∈M who describes the actual physical spacetime geometry of (M, g) at the length scale L, whereas the second, C−L(p,g^), is associated with an idealized version of the observer p who, notwithstanding the presence of local inhomogeneities at the given scale L, wish to model (M, g) with a member (M,g^) of the family of Friedmann–Lemaitre–Robertson–Walker spacetimes. In such a framework, we discuss a number of mathematical results that allows a rigorous comparison between the two lightcones C−L(p,g) and C−L(p,g^). In particular, we introduce a scale-dependent (L) lightcone-comparison functional, defined by a harmonic type energy, associated with a natural map between the physical C−L(p,g) and the FLRW reference lightcone C−L(p,g^). This functional has a number of remarkable properties, in particular it vanishes iff, at the given length-scale, the corresponding lightcone surface sections (the celestial spheres) are isometric. We discuss in detail its variational analysis and prove the existence of a minimum that characterizes a natural scale-dependent distance functional between the two lightcones. We also indicate how it is possible to extend our results to the case when caustics develop on the physical past lightcone C−L(p,g). Finally, by exploiting causal diamond theory, we show how the distance functional is related (to leading order in the scale L) to spacetime scalar curvature in the causal past of the two lightcones, and briefly illustrate a number of its possible applications
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