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Sebastiano Sonego

Publications and source records attributed to Sebastiano Sonego.

At least 19 recordsLinked to original sources
arXiv2015

Between quantum and classical gravity: Is there a mesoscopic spacetime?

Between the microscopic domain ruled by quantum gravity, and the macroscopic scales described by general relativity, there might be an intermediate, "mesoscopic" regime, where spacetime can still be approximately treated as a differentiable pseudo-Riemannian manifold, with small corrections of quantum gravitational origin. We argue that, unless one accepts to give up the relativity principle, either such a regime does not exist at all (hence, the quantum-to-classical transition is sharp), or the only mesoscopic, tiny corrections conceivable are on the behaviour of physical fields, rather than on the geometric structures.

gr-qc↗
arXiv2015

Nonequivalence of equivalence principles

Equivalence principles played a central role in the development of general relativity. Furthermore, they have provided operative procedures for testing the validity of general relativity, or constraining competing theories of gravitation. This has led to a flourishing of different, and inequivalent, formulations of these principles, with the undesired consequence that often the same name, "equivalence principle", is associated with statements having a quite different physical meaning. In this paper we provide a precise formulation of the several incarnations of the equivalence principle, clarifying their uses and reciprocal relations. We also discuss their possible role as selecting principles in the design and classification of viable theories of gravitation.

gr-qc↗
arXiv2014

Weak equivalence principle for self-gravitating bodies: A sieve for purely metric theories of gravity

We propose the almost-geodesic motion of self-gravitating test bodies as a possible selection rule among metric theories of gravity. Starting from a heuristic statement, the "gravitational weak equivalence principle", we build a formal, operative test able to probe the validity of the principle for any metric theory of gravity, in an arbitrary number of spacetime dimensions. We show that, if the theory admits a well-posed variational formulation, this test singles out only the purely metric theories of gravity. This conclusion reproduces known results in the cases of general relativity (also with a cosmological constant term), and scalar-tensor theories, but extends also to debated or unknown scenarios, such as f(R) and Lanczos-Lovelock theories. We thus provide new tools going beyond the standard methods, where the latter turn out to be inconclusive or inapplicable.

gr-qc↗
arXiv2011

Qualitative study of perfect-fluid Friedmann-Lema\^ıtre-Robertson-Walker models with a cosmological constant

The evolution of spatially homogeneous and isotropic cosmological models containing a perfect fluid with equation of state p=wρ and a cosmological constant Λ is investigated for arbitrary combinations of w and Λ, using standard qualitative analysis borrowed from classical mechanics. This approach allows one to consider a large variety of situations, appreciating similarities and differences between models, without solving the Friedmann equation, and is suitable for an elementary course in cosmology.

physics.ed-ph↗
arXiv2011

Hawking-like radiation from evolving black holes and compact horizonless objects

Usually, Hawking radiation is derived assuming (i) that a future eternal event horizon forms, and (ii) that the subsequent exterior geometry is static. However, one may be interested in either considering quasi-black holes (objects in an ever-lasting state of approach to horizon formation, but never quite forming one), where (i) fails, or, following the evolution of a black hole during evaporation, where (ii) fails. We shall verify that as long as one has an approximately exponential relation between the affine parameters on the null generators of past and future null infinity, then subject to a suitable adiabatic condition being satisfied, a Planck-distributed flux of Hawking-like radiation will occur. This happens both for the case of an evaporating black hole, as well as for the more dramatic case of a collapsing object for which no horizon has yet formed (or even will ever form). In this article we shall cast the previous statement in a more precise and quantitative form, and subsequently provide several explicit calculations to show how the time-dependent Bogoliubov coefficients can be calculated.

gr-qc↗
arXiv2010

Minimal conditions for the existence of a Hawking-like flux

We investigate the minimal conditions that an asymptotically flat general relativistic spacetime must satisfy in order for a Hawking-like Planckian flux of particles to arrive at future null infinity. We demonstrate that there is no requirement that any sort of horizon form anywhere in the spacetime. We find that the irreducible core requirement is encoded in an approximately exponential "peeling" relationship between affine coordinates on past and future null infinity. As long as a suitable adiabaticity condition holds, then a Planck-distributed Hawking-like flux will arrive at future null infinity with temperature determined by the e-folding properties of the outgoing null geodesics. The temperature of the Hawking-like flux can slowly evolve as a function of time. We also show that the notion of "peeling" of null geodesics is distinct, and in general different, from the usual notion of "inaffinity" used in Hawking's definition of surface gravity.

gr-qc↗
arXiv2010

Ultrastatic spacetimes

Several calculations in conformally static spacetimes rely on the introduction of an ultrastatic background. I describe the general properties of ultrastatic spacetimes, and then focus on the problem of whether a given spacetime can be ultrastatic, or conformally ultrastatic, in more than one way. I show that the first possibility arises iff the spacetime is a product containing a Minkowskian factor, and that the second arises iff the spatial sections are conformal to a product space.

gr-qc↗
arXiv2009

Linking the trans-Planckian and the information loss problems in black hole physics

The trans-Planckian and information loss problems are usually discussed in the literature as separate issues concerning the nature of Hawking radiation. Here we instead argue that they are intimately linked, and can be understood as "two sides of the same coin" once it is accepted that general relativity is an effective field theory.

gr-qc↗
arXiv2009

Events and observables in generally invariant spacetime theories

We address the problem of observables in generally invariant spacetime theories such as Einstein's general relativity. Using the refined notion of an event as a ``point-coincidence'' between scalar fields that completely characterise a spacetime model, we propose a generalisation of the relational local observables that does not require the existence of four everywhere invertible scalar fields. The collection of all point-coincidences forms in generic situations a four-dimensional manifold, that is naturally identified with the physical spacetime.

gr-qc↗
arXiv2009

Coordinates, observables and symmetry in relativity

We investigate the interplay and connections between symmetry properties of equations, the interpretation of coordinates, the construction of observables, and the existence of physical relativity principles in spacetime theories. Using the refined notion of an event as a ``point-coincidence'' between scalar fields that completely characterise a spacetime model, we also propose a natural generalisation of the relational local observables that does not require the existence of four everywhere invertible scalar fields. The collection of all point-coincidences forms in generic situations a four-dimensional manifold, that is naturally identified with the physical spacetime.

gr-qc↗
arXiv2009

Foundations of anisotropic relativistic mechanics

We lay down the foundations of particle dynamics in mechanical theories that satisfy the relativity principle and whose kinematics can be formulated employing reference frames of the type usually adopted in special relativity. Such mechanics allow for the presence of anisotropy, both conventional (due to non-standard synchronisation protocols) and real (leading to detectable chronogeometrical effects, independent of the choice of synchronisation). We give a general method for finding the fundamental dynamical quantities (Lagrangian, energy and momentum), and write their explicit expression in all the kinematics compatible with the basic requirements. We also write the corresponding dispersion relations and outline a formulation of these theories in terms of a pseudo-Finslerian spacetime geometry. Although the treatment is restricted to the case of one spatial dimension, an extension to three dimensions is almost straightforward.

gr-qc↗
arXiv2009

Small, dark, and heavy: But is it a black hole?

Astronomers have certainly observed things that are small, dark, and heavy. But are these objects really black holes in the sense of general relativity? The consensus opinion is simply "yes", and there is very little "wriggle room". We discuss one of the specific alternatives.

gr-qc↗
arXiv2007

Fate of gravitational collapse in semiclassical gravity

While the outcome of gravitational collapse in classical general relativity is unquestionably a black hole, up to now no full and complete semiclassical description of black hole formation has been thoroughly investigated. Here we revisit the standard scenario for this process. By analyzing how semiclassical collapse proceeds we show that the very formation of a trapping horizon can be seriously questioned for a large set of, possibly realistic, scenarios. We emphasise that in principle the theoretical framework of semiclassical gravity certainly allows the formation of trapping horizons. What we are questioning here is the more subtle point of whether or not the standard black hole picture is appropriate for describing the end point of realistic collapse. Indeed if semiclassical physics were in some cases to prevent formation of the trapping horizon, then this suggests the possibility of new collapsed objects which can be much less problematic, making it unnecessary to confront the information paradox or the run-away end point problem.

gr-qc↗
arXiv2006

Hawking-like radiation does not require a trapped region

We discuss the issue of quasi-particle production by ``analogue black holes'' with particular attention to the possibility of reproducing Hawking radiation in a laboratory. By constructing simple geometric acoustic models, we obtain a somewhat unexpected result: We show that in order to obtain a stationary and Planckian emission of quasi-particles, it is not necessary to create a trapped region in the acoustic spacetime (corresponding to a supersonic regime in the fluid flow). It is sufficient to set up a dynamically changing flow asymptotically approaching a sonic regime with sufficient rapidity in laboratory time.

gr-qc↗
arXiv2006

Quasi-particle creation by analogue black holes

We discuss the issue of quasi-particle production by ``analogue black holes'' with particular attention to the possibility of reproducing Hawking radiation in a laboratory. By constructing simple geometric acoustic models, we obtain a somewhat unexpected result: We show that in order to obtain a stationary and Planckian emission of quasi-particles, it is not necessary to create an ergoregion in the acoustic spacetime (corresponding to a supersonic regime in the flow). It is sufficient to set up a dynamically changing flow either eventually generating an arbitrarily small sonic region v=c, but without any ergoregion, or even just asymptotically, in laboratory time, approaching a sonic regime with sufficient rapidity.

gr-qc↗
arXiv2006

Optical geometry analysis of the electromagnetic self-force

We present an analysis of the behaviour of the electromagnetic self-force for charged particles in a conformally static spacetime, interpreting the results with the help of optical geometry. Some conditions for the vanishing of the local terms in the self-force are derived and discussed.

gr-qc↗
arXiv2004

Interpreting doubly special relativity as a modified theory of measurement

In this article we develop a physical interpretation for the deformed (doubly) special relativity theories (DSRs), based on a modification of the theory of measurement in special relativity. We suggest that it is useful to regard the DSRs as reflecting the manner in which quantum gravity effects induce Planck-suppressed distortions in the measurement of the "true" energy and momentum. This interpretation provides a framework for the DSRs that is manifestly consistent, non-trivial, and in principle falsifiable. However, it does so at the cost of demoting such theories from the level of "fundamental" physics to the level of phenomenological models -- models that should in principle be derivable from whatever theory of quantum gravity one ultimately chooses to adopt.

gr-qc↗