Publications (21)33.38 Total impact
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ABSTRACT: A spacetime is locally flat if and only if no geodesical deviation exists for congruences of all kinds of geodesics. However, while for causal geodesics the deviation can be measured observing the motion of (infinitesimal) falling bodies, it does not seem possible to evaluate the geodesical deviation of spacelike geodesic. So a physical problem may arise. To tackle this problem we analyze the interplay of local flatness and geodesic deviation measured for causal geodesics. We establish that a generic spacetime is (locally) flat if and only if there is no geodesic deviation for timelike geodesics or, equivalently, there is no geodesic deviation for null geodesics.  [Show abstract] [Hide abstract]
ABSTRACT: Much attention has been recently devoted to modified theories of gravity in the attempt to efficiently describe both early inflation and latetime acceleration of our universe without referring to the cosmological constant or other ad hoc kinds of fluids. The simplest models overcome General Relativity simply by replacing $R$ with $F(R)$ in the EinsteinHilbert action. Unfortunately, such models typically lack most of the beautiful solutions discovered in Einstein's gravity. Nonetheless, in $F(R)$ gravity, it has been possible to get at least few black holes, but still we do not know any empty wormholelike spacetime solution. The present paper aims to explain why it is so hard to get such solutions (given that they exist). Few solutions are derived in the simplest cases while only an implicit form has been obtained in the nontrivial case. 
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ABSTRACT: In the present paper, UnruhDeWitt detectors are used in order to investigate the issue of temperature associated with a spherically symmetric dynamical spacetimes. Firstly, we review the semiclassical tunneling method, then we introduce the UnruhDeWitt detector approach. We show that for the generic static black hole case and the FRW de Sitter case, making use of peculiar Kodama trajectories, semiclassical and quantum field theoretic techniques give the same standard and well known thermal interpretation, with an associated temperature, corrected by appropriate Tolman factors. For a FRW spacetime interpolating de Sitter space with the Einsteinde Sitter universe (that is a more realistic situation in the frame of $\Lambda$CDM cosmologies), we show that the detector response splits into a de Sitter contribution plus a fluctuating term containing no trace of Boltzmannlike factors, but rather describing the way thermal equilibrium is reached in the late time limit. As a consequence, and unlike the case of black holes, the identification of the dynamical surface gravity of a cosmological trapping horizon as an effective temperature parameter seems lost, at least for our comoving simplified detectors. The possibility remains that a detector performing a proper motion along a Kodama trajectory may register something more, in which case the horizon surface gravity would be associated more likely to vacuum correlations than to particle creation.  [Show abstract] [Hide abstract]
ABSTRACT: The aim of this work is to review the tunnelling method as an alternative description of the quantum radiation from black holes and cosmological horizons. The method is first formulated and discussed for the case of stationary black holes, then a foundation is provided in terms of analytic continuation throughout complex spacetime. The two principal implementations of the tunnelling approach, which are the null geodesic method and the HamiltonJacobi method, are shown to be equivalent in the stationary case. The HamiltonJacobi method is then extended to cover spherically symmetric dynamical black holes, cosmological horizons and naked singularities. Prospects and achievements are discussed in the conclusions.  [Show abstract] [Hide abstract]
ABSTRACT: In the paper, the temperature associated with a dynamical spherically symmetric black hole or with a cosmological horizon is investigated from the point of view of a pointlike detector. First, we briefly review the HamiltonJacobi tunneling method for a generic dynamical spherically symmetric spacetime, and present two applications of the tunneling method. Then, we apply a wellknown relativistic quantum theoretical technique, namely the UnruhDeWitt detector formalism for a conformally coupled scalar field in a generic FRW spacetime. As an application, for the generic static black hole case and the FRW de Sitter case, making use of peculiar Kodama observer trajectories, the tunneling semiclassical results are fully recovered, automatically corrected by Tolman factors. Some remarks on the temperature of FRW universe are presented. For more general spaces interpolating de Sitter space with the Einsteinde Sitter universe a second set of poles is present, whose exact role remains to be clarified, plus an extra fluctuating term describing the way equilibrium is reached, similarly to de Sitter space. The simple thermal interpretation found for de Sitter space is lost and forces, at a same time, a different quantum interpretation of the horizon surface gravity for the cosmological FRW models.  [Show abstract] [Hide abstract]
ABSTRACT: We give an interpretation of the temperature in de Sitter universe in terms of a dynamical Unruh effect associated with the Hubble sphere. As with the quantum noise perceived by a uniformly accelerated observer in static spacetimes, observers endowed with a proper motion can in principle detect the effect. In particular, we study a "Kodama observer" as a twofield Unruh detector for which we show the effect is approximately thermal. We also estimate the backreaction of the emitted radiation and find trajectories associated with the Kodama vector fields are stable.  [Show abstract] [Hide abstract]
ABSTRACT: Studying the behaviour of a quantum field in a classical, curved, spacetime is an extraordinary task which nobody is able to take on at present time. Independently by the fact that such problem is not likely to be solved soon, still we possess the instruments to perform exact predictions in special, highly symmetric, conditions. Aim of the present contribution is to show how it is possible to extract quantitative information about a variety of physical phenomena in very general situations by virtue of the socalled HamiltonJacobi method. In particular, we shall prove the agreement of such semiclassical method with exact results of quantum field theoretic calculations. Comment: To appear in the proceedings of "Cosmology, the Quantum Vacuum, and Zeta Functions": A workshop with a celebration of Emilio Elizalde's Sixtieth birthday, Bellaterra, Barcelona, Spain, 810 Mar 2010  [Show abstract] [Hide abstract]
ABSTRACT: The dark energy issue is focusing the attention of an incresing number of physicists all over the world. Among the possible alternatives in order to explain what as been named the "Mystery of the Millennium" are the socalled Modified Theories of Gravity. A crucial test for such models is represented by the existence and (if this is the case) the properties of their black hole solutions. Nowadays, to our knowledge, only two nontrivial, spherically symmetric, solutions with vanishing cosmological constant are known by Barrow & Clifton (2005) and Deser, Sarioglu & Tekin (2008). Aim of the paper is to discuss some features of such solutions, with emphasis on their thermodynamic properties such as entropy and temperature, little progress being possible along the way which leads to a consistent definition of mass.  [Show abstract] [Hide abstract]
ABSTRACT: Previous work on dynamical black hole instability is further elucidated within the HamiltonJacobi method for horizon tunnelling and the reconstruction of the classical action by means of the nullexpansion method. Everything is based on two natural requirements, namely that the tunneling rate is an observable and therefore it must be based on invariantly defined quantities, and that coordinate systems which do not cover the horizon should not be admitted. These simple observations can help to clarify some ambiguities, like the doubling of the temperature occurring in the static case when using singular coordinates, and the role, if any, of the temporal contribution of the action to the emission rate. The formalism is also applied to FRW cosmological models, where it is observed that it predicts the positivity of the temperature naturally, without further assumptions of the sign of energy. PACS numbers:  [Show abstract] [Hide abstract]
ABSTRACT: Abstract unavailable. 
Article: Applications of the Tunneling Method to Particle Decay and Radiation from Naked Singularities
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ABSTRACT: Following recent literature on dS instability in presence of interactions, we study the decay of massive particles in general FRW models and the emission from naked singularities either associated with 4D charged black holes or with 2D shock waves, by means of the HamiltonJacobi tunneling method. It is shown that the twodimensional semiclassical tunneling amplitude from a naked singularity computed in that way is the same as the oneloop result of quantum field theory. Comment: LaTex document, 14 pages, two figures 
Article: Comment on
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ABSTRACT: The arguments of the above article [arXiv:0910.3934] do not apply to the papers which it criticizes, and contain several key errors, including a fundamental misunderstanding about the equivalence principle. 
Article: Reply to Comments on
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ABSTRACT: We point out basic misunderstandings about quantum field theory, general relativity and partial derivatives in the above Comments. In reply to a second comment on our first reply by the same author, we also identify precisely where the author's original calculation goes wrong and correct it, yielding the same local Hawking temperature as obtained by the HamiltonJacobi method.  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by the considerable success of alternative theories of gravity, we consider the toy model of a higher derivative Lagrangian theory, namely the PaisUhlenbeck oscillator studied in a recent paper by HawkingHertog. Its Euclidean Path Integral is studied with a certain detail and a pedagogical derivation of the propagator, which makes use of a Theorem due to Forman, is consequently proposed Comment: 12 pages, no figures, error sign corrected in section 3, conclusions unchanged, one reference added  [Show abstract] [Hide abstract]
ABSTRACT: Previous work on dynamical black hole instability is further elucidated within the HamiltonJacobi method for horizon tunneling and the reconstruction of the classical action by means of the nullexpansion method. Everything is based on two natural requirements, namely that the tunneling rate is an observable and therefore it must be based on invariantly defined quantities, and that coordinate systems which do not cover the horizon should not be admitted. These simple observations can help to clarify some ambiguities, like the doubling of the temperature occurring in the static case when using singular coordinates, and the role, if any, of the temporal contribution of the action to the emission rate. The formalism is also applied to FRW cosmological models, where it is observed that it predicts the positivity of the temperature naturally, without further assumptions on the sign of the energy. Comment: Standard Latex document, typos corrected, refined discussion of tunneling picture, subsection 5.1 removed  [Show abstract] [Hide abstract]
ABSTRACT: A local Hawking temperature is derived for any future outer trapping horizon in spherical symmetry, using a Hamilton Jacobi variant of the Parikh Wilczek tunneling method. It is given by a dynamical surface gravity as defined geometrically. The operational meaning of the temperature is that Kodama observers just outside the horizon measure an invariantly redshifted temperature, diverging at the horizon itself. In static, asymptotically flat cases, the Hawking temperature as usually defined by the Killing vector agrees in standard cases, but generally differs by a relative redshift factor between the horizon and infinity, this being the temperature measured by static observers at infinity. Likewise, the geometrical surface gravity reduces to the Newtonian surface gravity in the Newtonian limit, while the Killing definition instead reflects measurements at infinity. This may resolve a longstanding puzzle concerning the Hawking temperature for the extremal limit of the charged stringy black hole, namely that it is the local temperature which vanishes. In general, this confirms the quasistationary picture of blackhole evaporation in early stages. However, the geometrical surface gravity is generally not the surface gravity of a static black hole with the same parameters.  [Show abstract] [Hide abstract]
ABSTRACT: A local Hawking temperature was recently derived for any future outer trapping horizon in spherical symmetry, using a HamiltonJacobi tunneling method, and is given by a dynamical surface gravity as defined geometrically. Descriptions are given of the operational meaning of the temperature, in terms of what observers measure, and its relation to the usual Hawking temperature for static black holes. Implications for the final fate of an evaporating black hole are discussed. Comment: 7 pages, contribution to Proceedings of ERE2008  [Show abstract] [Hide abstract]
ABSTRACT: The instability against emission of fermionic particles by the trapping horizon of an evolving black hole is analyzed and confirmed using the HamiltonJacobi tunneling method. This method automatically selects one special expression for the surface gravity of a changing horizon. The results also apply to point masses embedded in an expanding universe. As a bonus of the tunneling method, we gain the insight that the surface gravity still defines a temperature parameter as long as the evolution is sufficiently slow that the blackhole pass through a sequence of quasiequilibrium states, and that black holes should be semiclassically unstable even in a hypothetical world without bosonic fields.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the interplay of local flatness and geodesic deviation measured for causal geodesics starting from the remark that, form a physical viewpoint, the geodesic deviation can be measured for causal geodesic, observing the motion of (infinitesimal) falling bodies, but it can hardly be evaluated on spacelike geodesics. We establish that a generic spacetime is (locally) flat if and only if there is no geodesic deviation for timelike geodesics or, equivalently, there is no geodesic deviation for null geodesics.
Publication Stats
407  Citations  
33.38  Total Impact Points  
Top Journals
Institutions

2013

Eurasian National University
Astana, Astana Qalasy, Kazakhstan


20102011

INFN  Istituto Nazionale di Fisica Nucleare
Frascati, Latium, Italy 
Nottingham Trent University
Nottigham, England, United Kingdom


20072010

Università degli Studi di Trento
 Department of Physics
Trient, TrentinoAlto Adige, Italy


2009

Shanghai Normal University
Shanghai, Shanghai Shi, China


20082009

University of Toronto
 Department of Physics
Toronto, Ontario, Canada
