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Roberto Casadio

Publications and source records attributed to Roberto Casadio.

141 records · Page 8Linked to original sources
arXiv2002

New black holes in the brane-world?

It is known that the Einstein field equations in five dimensions admit more general spherically symmetric black holes on the brane than four-dimensional general relativity. We propose two families of analytic solutions (with g_tt\not=-1/g_rr), parameterized by the ADM mass and the PPN parameter beta, which reduce to Schwarzschild for beta=1. Agreement with observations requires |β-1| |η|<<1. The sign of eta plays a key role in the global causal structure, separating metrics which behave like Schwarzschild (eta<0) from those similar to Reissner-Nordstroem (eta>0). In the latter case, we find a family of black hole space-times completely regular.

gr-qc↗
arXiv2001

Gravitational collapse of a radiating shell and radiation bursts

We study the collapse of a self-gravitating thick shell of bosons coupled to a scalar radiation field. Due to the non-adiabaticity of the collapse, the shell (quantum) internal degrees of freedom absorb energy from the (classical) gravitational field and are excited. The excitation energy is then emitted in the form of bursts of (thermal) radiation and the corresponding backreaction on the trajectory is estimated.

gr-qc↗
arXiv2001

On time evolution of quantum black holes

The time evolution of black holes involves both the canonical equations of quantum gravity and the statistical mechanics of Hawking radiation, neither of which contains a time variable. In order to introduce the time, we apply the semiclassical approximation to the Hamiltonian constraint on the apparent horizon and show that, when the backreaction is included, it suggests the existence of a long-living remnant, similarly to what is obtained in the microcanonical picture for the Hawking radiation.

gr-qc↗
arXiv2001

Black Hole Evaporation and Compact Extra Dimensions

We study the evaporation of black holes in space-times with extra dimensions of size L. We first obtain a description which interpolates between the expected behaviors of very large and very small black holes and then show that the luminosity is greatly damped when the horizon shrinks towards L from a larger value. Analogously, black holes born with an initial size smaller than L are almost stable. This effect is due to the dependence of both the Hawking temperature and the grey-body factor of a black hole on the dimensionality of space. Although the picture of what happens when the horizon becomes of size L is still incomplete, we argue that there occurs a (first order) phase transition, possibly signaled by an outburst of energy which leaves a quasi-stable remnant.

hep-th↗
arXiv2000

Black Hole Evaporation and Large Extra Dimensions

We study the evaporation of black holes in space-times with extra dimensions of size L. We first obtain a potential which describes the expected behaviors of very large and very small black holes and then show that a (first order) phase transition, possibly signaled by an outburst of energy, occurs in the system when the horizon shrinks below L from a larger value. This is related to both a change in the topology of the horizon and the restoring of the translational symmetry along the extra dimensions.

hep-th↗
arXiv2000

Quantum gravitational fluctuations and the semi-classical limit

An attempt is made to go beyond the standard semi-classical approximation for gravity in the Born-Oppenheimer decomposition of the wave-function in minisuperspace. New terms are included which correspond to quantum gravitational fluctuations on the background metric. Their existence renders the definition of the semi-classical limit rather delicate and can lead to the avoidance of the singularities the classical theory predicts in cosmology and in the gravitational collapse of compact objects.

gr-qc↗
arXiv1999

Quantized shells as a tool for studying semiclassical effects in general relativity

Thin shells in general relativity can be used both as models of collapsing objects and as probes in the space-time outside compact sources. Therefore they provide a useful tool for the analysis of the final fate of collapsing matter and of the effects induced in the matter by strong gravitational fields. We describe the radiating shell as a (second quantized) many-body system with one collective degree of freedom, the (average) radius, by means of an effective action which also entails a thermodynamic description. Then we study some of the quantum effects that occur in the matter when the shell evolves from an (essentially classical) large initial radius towards the singularity and compute the corresponding backreaction on its trajectory.

gr-qc↗
arXiv1998

Adiabatic Invariant Treatment of a Collapsing Sphere of Quantized Dust

The semiclassical collapse of a sphere of quantized dust is studied. A Born-Oppenheimer decomposition is performed for the wave function of the system and the semiclassical limit is considered for the gravitational part. The method of adiabatic invariants for time dependent Hamiltonians is then employed to find (approximate) solutions to the quantum dust equations of motions. This allows us to obtain corrections to the adiabatic approximation of the dust states associated with the time evolution of the metric. The diverse non-adiabatic corrections are generally associated with particle (dust) creation and related fluctuations. The back-reaction due to the dominant contribution to particle creation is estimated and seen to slow-down the collapse.

gr-qc↗
arXiv1998

Hamiltonian formalism for the Oppenheimer-Snyder model

A family of effective actions in Hamiltonian form is derived for a self-gravitating sphere of isotropic homogeneous dust. Starting from the Einstein-Hilbert action for barotropic perfect fluids and making use of the symmetry and equation of state of the matter distribution we obtain reduced actions for two canonical variables, namely the radius of the sphere and its ADM energy, the latter being conserved along trajectories of the former. These actions differ by the value of the (conserved) geodesic energy of the radius of the sphere which defines (disconnected) classes of solutions in correspondence to the inner geometry and proper volume of the sphere. Each class is thus treated as one constrained dynamical system and the union of all classes covers the full phase space of the model. Generalization to the (inhomogeneous) Tolman model is shown to be straightforward. Quantization is also discussed.

gr-qc↗
arXiv1997

The Poincare' coset models ISO(d-1,1)/R^n and T-duality

We generalize a family of Lagrangians with values in the Poincaré group ISO(d-1,1), which contain the description of spinning strings in flat (d-1)+1 dimensions, by including symmetric terms in the world-sheet coordinates. Then, by promoting a subgroup H=R^n, n less than or equal to d, which acts invariantly from the left on the element of ISO(d-1,1), to a gauge symmetry of the action, we obtain a family of sigma-models. They describe bosonic strings moving in (generally) curved, and in some cases degenerate, space-times with an axion field. Further, the space-times of the effective theory admit in general T-dual geometries. We give explicit results for two non degenerate cases.

hep-th↗
arXiv1996

Semiclassical Collapse of a Sphere of Dust and Hawking Radiation

The independent dynamical variables of a collapsing homogeneous sphere of dust are canonically quantised and coupled equations describing matter (dust) and gravitation are obtained. The conditions for the validity of the adiabatic (Born-Oppenheimer) and semiclassical approximations are derived. On neglecting back-reaction effects, it is also shown that in the vicinity of the horizon and inside the dust the Wightman function for a conformal scalar field coupled to a monopole emitter is thermal at the characteristic Hawking temperature.

gr-qc↗
arXiv1996

Semiclassical collapse of a sphere of dust

The semiclassical collapse of a homogeneous sphere of dust is studied. After identifying the independent dynamical variables, the system is canonically quantised and coupled equations describing matter (dust) and gravitation are obtained. The conditions for the validity of the adiabatic (Born--Oppenheimer) and semiclassical approximations are derived. Further on neglecting back--reaction effects, it is shown that in the vicinity of the horizon and inside the dust the Wightman function for a conformal scalar field coupled to a monopole emitter is thermal at the characteristic Hawking temperature.

gr-qc↗
arXiv1995

The accelerated observer and quantum effects

An extended monopole detector at constant acceleration coupled to a massless scalar field is allowed to evolve quantum mechanically. It is found that while in the classical, followed by the point particle, limit the usual result Unruh effect is recovered, in the point particle (before the classical) limit the detector decouples from the scalar field and therefore the effect disappears.

quant-ph↗
arXiv1995

Black hole and the adiabatic phase

An open system consisting of a scalar field bound to a Kerr black hole whose mass ($M$) and specific angular momentum ($a$) are slowly (adiabatically) perturbed is considered. The adiabatically induced phase and the conditions for the validity of the adiabatic approximation are obtained. The effect of closed cycles in parameter space ($a$, $M$ plane) on the energy levels of both stable and unstable scalar field bound states, together with other quantities of interest, is illustrated. Lastly it is noted that the black hole wavefunction will acquire an equal and opposite phase to that of matter thus leading to a change of its effective action (entropy).

gr-qc↗