336 research outputs found
Experimental quantum cosmology in time-dependent optical media
No full textIt is possible to construct artificial spacetime geometries for light by using intense laser pulses that modify the spatiotemporal properties of an optical medium. Here we theoretically investigate experimental possibilities for studying spacetime metrics of the form . By tailoring the laser pulse shape and medium properties, it is possible to create a refractive index variation that can be identified with . Starting from a perturbative solution to a generalized Hopfield model for the medium described by an , we provide estimates for the number of photons generated by the time-dependent spacetime. The simplest example is that of a uniformly varying that therefore describes the Robertson–Walker metric, i.e. a cosmological expansion. The number of photon pairs generated in experimentally feasible conditions appears to be extremely small. However, large photon production can be obtained by periodically modulating the medium and thus resorting to a resonant enhancement similar to that observed in the dynamical Casimir effect. Curiously, the spacetime metric in this case closely resembles that of a gravitational wave. Motivated by this analogy, we show that a periodic gravitational wave can indeed act as an amplifier for photons. The emission for an actual gravitational wave will be very weak but should be readily observable in the laboratory analogue
Tunneling method for Hawking radiation in the Nariai case
No full textWe revisit the tunneling picture for the Hawking effect in light of the charged Nariai manifold, because this general relativistic solution, which displays two horizons, provides the bonus to allow the knowledge of exact solutions of the field equations. We first perform a revisitation of the tunneling ansatz in the framework of particle creation in external fields à la Nikishov, which corroborates the interpretation of the semiclassical emission rate Γemission as the conditional probability rate for the creation of a couple of particles from the vacuum. Then, particle creation associated with the Hawking effect on the Nariai manifold is calculated in two ways. On the one hand, we apply the Hamilton–Jacobi formalism for tunneling, in the case of a charged scalar field on the given background. On the other hand, the knowledge of the exact solutions for the Klein–Gordon equations on Nariai manifold, and their analytic properties on the extended manifold, allow us a direct computation of the flux of particles leaving the horizon, and, as a consequence, we obtain a further corroboration of the semiclassical tunneling picture from the side of S-matrix formalism
Analog Hawking effect: A master equation
No full textWe consider further the problem of the analog Hawking radiation. We propose a fourth order ordinary
differential equation, which allows us to discuss the problem of Hawking radiation in analog gravity in a
unified way, encompassing fluids and dielectric media. In a suitable approximation, involving weak
dispersive effects, Wentzel-Kramers-Brillouin solutions are obtained far from the horizon (turning point),
and furthermore an equation governing the behavior near the horizon is derived, and a complete set of
analytical solutions is obtained also near the horizon. The subluminal case of the original fluid model
introduced by Corley and Jacobson and the case of dielectric media are discussed. We show that in this
approximation scheme there is a mode which is not directly involved in the pair-creation process.
Thermality is verified and a framework for calculating the gray-body factor is provided
Spectral properties for the Klein-Gordon Hamiltonian in charged black hole backgrounds
No full textCharged massive scalar fields on charged black hole backgrounds are investigated through methods of spectral analysis in Krein spaces. We consider, on the three charged black hole backgrounds (Nariai, Reissner-Nordström, ultracold-II) taken into account, a necessary condition for the existence of complex eigenvalues. We show that even if it is satisfied, in two cases (Nariai and ultracold-II), by direct calculation, they actually cannot exist. In both cases, the Klein paradox occurs without restriction on the parameters. In the third case, the condition for their existence is shown to coincide with the condition, allowing the quantum discharge phenomenon associated with the Klein paradox. We also clarify the role of “classical potentials,” which appear in the physical literature on the subject, giving rise to the so-called level-crossing appearing in semiclassical calculations, and we comment on problems occurring in quantum field theory in the presence of complex eigenvalues
Analogous Hawking Effect: S-Matrix and Thermofield Dynamics
No full textWe consider the full S-matrix in the scattering giving rise to analogous Hawking radiation in dispersive media. We show the general structure of the scattering in the weak dispersion approximation and discuss some unnoticed features of the primary process, with a possible generalization of the phenomenology of the Hawking effect. In particular, we stress that the Hawking particle and its antiparticle partner a priori could also be produced with different rates. We provide a general parameterization of the S-matrix, adopting the Iwasawa decomposition for the matrix itself. Then, we assume that a perturbative structure in a suitable sense is allowed and display the corresponding expansion. In connection with the general structure of the S-matrix at the leading order, we also consider the thermofield dynamics (TFD) framework and show that the TFD picture is still available, with a doubling of the degrees of freedom emerging in a natural way, as for the astrophysical black hole case. Furthermore, we show that particles on the thermal vacuum can be identified with real particles appearing in the scattering
Perturbative Approach to Analog Hawking Radiation in dielectric media in subcritical regime
No full textWe take into account the subcritical case for dielectric media by exploiting
an approximation allowing us to perform perturbative analytical calculations
and still not implying low dispersive effects. We show that in the background
of a specific soliton-like solution, pair-creation occurs and can display a
thermal behaviour governed by an effective temperature. The robustness of the
approach is also corroborated by the analysis of the -model related
to the standard Hopfield model, for which analogous results are obtained.Comment: 22 pages, 6 figure
Exact solutions for analog Hawking effect in dielectric media
No full textIn the framework of the analog Hawking radiation for dielectric media, we analyze a toy model and also the 2D reduction of the Hopfield model for a specific monotone and realistic profile for the refractive index. We are
able to provide exact solutions, which do not require any weak dispersion approximation. The theory of Fuchsian ordinary differential equations is the basic tool for recovering exact solutions, which are rigorously
identified and involve the so-called generalized hypergeometric functions
.
A complete set of connection formulas are available, both for the subcritical case and for the transcritical one, and also the Stokes phenomenon occurring in the problem is fully discussed. From the physical point of view,
we focus on the problem of thermality. Under suitable conditions, the Hawking temperature is deduced, and we show that it is in full agreement with the expression deduced in other frameworks under various approximations
Laser pulse analogues for gravity
No full textIntense pulses of light may be used to create an effective flowing medium which mimics certain properties of black hole physics. It is possible to create the analogues of black and white hole horizons and a photon emission is predicted that is analogous to Hawking radiation. We give an overview of the current state of the art in the field of analogue gravity with laser pulses and of its implications and applications for optics.</p
The Dirac equation in Kerr-Newman-AdS black hole background
No full textWe consider the Dirac equation on the Kerr–Newman–AdS black hole background. We first perform the variable separation for the Dirac equation and define the Hamiltonian operator H. Then we show that for a massive Dirac field with mass \mu \geq 1 / ͑(2l), where l is linked to the cosmological constant \Lambda by \Lambda = -3 /(l^2), essential self-adjointness of H on
C_0^\infty ((r_+ ,\infty)\times S^2͒)^4 is obtained even in presence of the boundarylike behavior of infinity in an asymptotically AdS black hole background. Furthermore, qualitative spectral properties of the Hamiltonian are taken into account and in agreement with the existing results concerning the case of stationary axisymmetric asymptotically flat black holes we infer the absence of time-periodic and normalizable solutions of the Dirac equation around the black hole in the nonextremal case
Quantum Effects for the Dirac Field in Reissner-Nordstrom-AdS Black Hole Background
No full textThe behavior of a charged massive Dirac field on a Reissner–Nordstrom–AdS
black hole background is investigated. We first analyze the problem of the
essential self-adjointness of the Dirac Hamiltonian, which is made difficult by
the boundary-like behavior of spatial infinity, and we find that the Hamiltonian
is essentially self-adjoint iff \mu L \geq 1/2; moreover, we determine the essential
spectrum of the Hamiltonian. Then we focus on the analysis of the discharge
problem for the case \mu L \geq 1/2. We follow the Ruffini–Damour–Deruelle
approach and, as in the standard Reissner–Nordstrom black hole case, we
find that the existence of level-crossing between the positive and negative
energy solutions of the Dirac equation is at the root of the pair-creation process
associated with the discharge of the black hole
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