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Stefano Ansoldi

Publications and source records attributed to Stefano Ansoldi.

At least 37 records · Page 2Linked to original sources
arXiv2011

The environment and distribution of emitting electrons as a function of source activity in Markarian 421

For the high-frequency peaked BL Lac object Mrk 421 we study the variation of the spectral energy distribution (SED) as a function of source activity, from quiescent to active. We use a fully automatized chi-squared minimization procedure, instead of the "eyeball" procedure more commonly used in the literature, to model nine SED datasets with a one-zone Synchrotron-Self-Compton (SSC) model and examine how the model parameters vary with source activity. The latter issue can finally be addressed now, because simultaneous broad-band SEDs (spanning from optical to VHE photon energies) have finally become available. Our results suggest that in Mrk 421 the magnetic field decreases with source activity, whereas the electron spectrum's break energy and the Doppler factor increase -- the other SSC parameters turn out to be uncorrelated with source activity. In the SSC framework these results are interpreted in a picture where the synchrotron power and peak frequency remain constant with varying source activity, through a combination of decreasing magnetic field and increasing number density of electrons below the break energy: since this leads to an increased electron-photon scattering efficiency, the resulting Compton power increases, and so does the total (= synchrotron plus Compton) emission.

astro-ph.HE↗
arXiv2007

Vacuum and semiclassical gravity: a difficulty and its bewildering significance

We review a long-standing difficulty in some semiclassical models of vacuum and vacuum decay. Surprisingly enough these models, careless of their transparent formulation, are affected by, both, technical and conceptual issues. After proving some general results that are relevant for, both, the Euclidean and Lorentzian sectors of their dynamics, we briefly highlight their importance in connection with the issues discussed before, arguing that their solution might be interesting in our quest for quantum gravity.

gr-qc↗
arXiv2007

Universes out of almost empty space

Baby universes (inflationary or non--inflationary) are regions of spacetime that disconnect from the original ambient spacetime, which we take to be asymptotically flat spacetime. A particular kind of baby universe solution, involving string--like matter, is studied to show that it can be formed by ``investing'' an arbitrarily small amount of energy, i.e. it can appear from an almost flat space at the classical level. Since this possibility has not yet been clearly recognized in the literature, we then discuss in detail its properties, relevance and possible generalizations.

gr-qc↗
arXiv2007

Shell-mediated tunnelling between (anti-)de Sitter vacua

We give an extensive study of the tunnelling between arbitrary (anti-)de Sitter spacetimes separated by an infinitesimally thin relativistic shell in arbitrary spacetime dimensions. In particular, we find analytically an exact expression for the tunnelling amplitude. The detailed spacetime structures that can arise are discussed, together with an effective "regularization scheme" for "before tunnelling" configurations.

gr-qc↗
arXiv2007

Bubbles and Quantum Tunnelling in Inflationary Cosmology

We review a procedure to use semiclassical methods in the quantization of General Relativistic shells and apply these techniques in some simplified models of inflationary cosmology. Some interesting open issues are introduced and the relevance of their solution in the broader context of Quantum Gravity is discussed.

gr-qc↗
arXiv2007

Braneworlds and Quantum States of Relativistic Shells

We review some applications of relativistic shells that are relevant in the context of quantum gravity/quantum cosmology. Using a recently developed approach, the stationary states of this general relativistic system can be determined in the semiclassical approximation. We suggest that this technique might be of phenomenological relevance in the context of the brane-world scenario and we draw a picture of the general set-up and of the possible developments.

gr-qc↗
arXiv2006

Noncommutative geometry inspired charged black holes

We find a new, non-commutative geometry inspired, solution of the coupled Einstein-Maxwell field equations describing a variety of charged, self-gravitating objects, including extremal and non-extremal black holes. The metric smoothly interpolates between deSitter geometry, at short distance, and Reissner-Nordstroem geometry far away from the origin. Contrary to the ordinary Reissner-Nordstroem spacetime there is no curvature singularity in the origin neither "naked" nor shielded by horizons. We investigate both the Hawking process and pair creation in this new scenario.

gr-qc↗
arXiv2006

Child Universes in the Laboratory

Although cosmology is usually considered an observational science, where there is little or no space for experimentation, other approaches can (and have been) also considered. In particular, we can change rather drastically the above, more passive, observational perspective and ask the following question: could it be possible, and how, to create a universe in a laboratory? As a matter of fact, this seems to be possible, according to at least two different paradigms; both of them help to evade the consequences of singularity theorems. In this contribution we will review some of these models and we will also discuss possible extensions and generalizations, by paying a critical attention to the still open issues as, for instance, the detectability of child universes and the properties of quantum tunnelling processes.

gr-qc↗
arXiv2006

Solitons as Key Parts to Produce a Universe in the Laboratory

Cosmology is usually understood as an observational science, where experimentation plays no role. It is interesting, nevertheless, to change this perspective addressing the following question: what should we do to create a universe, in a laboratory? It appears, in fact, that this is, in principle, possible according to at least two different paradigms; both allow to circumvent singularity theorems, i.e. the necessity of singularities in the past of inflating domains which have the required properties to generate a universe similar to ours. The first of them is substantially classical, and is built up considering solitons which collide with surrounding topological defects, generating an inflationary domain of space-time. The second is, instead, partly quantum and considers the possibility of tunnelling of past-non-singular regions of spacetime into an inflating universe, following a well-known instanton proposal. We are, here, going to review some of these models, as well as highlight possible extensions, generalizations and the open issues (as for instance the detectability of child universes and the properties of quantum tunnelling processes) that still affect the description of their dynamics. In doing so we will remark how the works on this subject can represent virtual laboratories to test the role that fundamental principles of physics (particularly, the interplay of quantum and general relativistic realms) played in the formation of our universe.

gr-qc↗
arXiv2005

The role of a dynamical measure and dynamical tension in brane creation and growth

The use of a non-Riemannian measure of integration in the action of strings and branes allows the possibility of dynamical tension. In particular, lower dimensional objects living in the string/brane can induce discontinuities in the tension: the effect of pair creation on the string tension is studied. We investigate then the role that these new features can play in string and brane creation and growth. A mechanism is studied by means of which a scalar field can transfer its energy to the tension of strings and branes. An infinite dimensional symmetry group of this theory is discussed. Creation and growth of bubbles in a formulation that requires mass generation for the bulk gauge fields coupled to the branes is also discussed.

hep-th↗
arXiv2004

Gravitational Tunnelling of Relativistic Shells

Thin shells in general relativity have been used in the past as keystones to obtain realistic models of cosmological and astrophysical situations. A crucial role for these developments was played by the compact description of their dynamics in terms of Israel's junction conditions. Starting from this geometrical formulation we present a problem related to the WKB regime of shell dynamics and suggest a possible solution.

gr-qc↗
arXiv2004

Vacuum decay by p-branes production

We present a generalization to the N-dimensional case for the nucleation coefficient of a spherical p-brane, separating two (anti-)de Sitter spacetimes. We use a semiclassical approximation based on the analytical continuation to the Euclidean sector of a suitable effective action describing a p-brane in General Relativity.

gr-qc↗
arXiv2004

Minisuperspace, WKB, Quantum States of General Relativistic Extended Objects

The dynamics of relativistic thin shells is a recurrent topic in the literature about the classical theory of gravitating systems and the still ongoing attempts to obtain a coherent description of their quantum behavior. Certainly, a good reason to make this system a preferred one for many models is the clear, synthetic description given by Israel junction conditions. Using some results from an effective Lagrangian approach to the dynamics of spherically symmetric shells, we show a general way to obtain WKB states for the system; a simple example is also analyzed.

gr-qc↗
arXiv2004

Boundary versus Bulk Dynamics of Extended Objects and the Fractal Structure of Quantum Spacetime

Subject of this thesis is the study of a closed bosonic string: the treatment will follow an approach alternative with respect to the traditional one and stems from a research line pursued in by Eguchi. A description of the dynamics using Nonstandard Analysis is given in chapter 10 with prerequisites in appendices B and C. Detailed calculation are carried out explicitly, although many of them are deferred to appendix A. Selected parts of this thesis have been published as separate articles by the author (ansoldi@trieste.infn.it) and collaborators.

hep-th↗
arXiv2003

WKB metastable quantum states of a de Sitter--Reissner-Nordstroem dust shell

We study the dynamics of a spherically symmetric dust shell separating two spacetime domains, the 'interior' one being a part of the de Sitter spacetime and the exterior one having the 'extremal' Reissner-Nordstroem geometry. Extending the ideas of previous works on the subject, we show that the it is possible to determine the (metastable) WKB quantum states of this gravitational system.

gr-qc↗
arXiv2003

Real and Imaginary Mass Generation in the Presence of External Fields and Axions

Small fluctuations around a constant electric or constant magnetic field F are analyzed in a theory with pseudo scalar [phi] with a coupling g[phi]F~F. It is found that a magnetic external field leads to mass generation for the small perturbations, while an electric field suffers from a tachyonic mass generation in the case in which the field strength is higher than a critical value (related to the pseudo scalar mass). The vacuum energy can be exactly evaluated and it is found that an imaginary part is present when the external electric field exceeds its critical value.

hep-th↗