Non-equilibrium Thermodynamics of Spacetime: the Role of Gravitational Dissipation

In arXiv:gr-qc/9504004 it was shown that the Einstein equation can be derived as a local constitutive equation for an equilibrium spacetime thermodynamics. More recently, in the attempt to extend the same approach to the case of $f(R)$ theories of gravity, it was found that a non-equilibrium setting is indeed required in order to fully describe both this theory as well as classical GR (arXiv:gr-qc/0602001). Here, elaborating on this point, we show that the dissipative character leading to a non-equilibrium spacetime thermodynamics is actually related -- both in GR as well as in $f(R)$ gravity -- to non-local heat fluxes associated with the purely gravitational/internal degrees of freedom of the theory. In particular, in the case of GR we show that the internal entropy production term is identical to the so called tidal heating term of Hartle-Hawking. Similarly, for the case of $f(R)$ gravity, we show that dissipative effects can be associated with the generalization of this term plus a scalar contribution whose presence is clearly justified within the scalar-tensor representation of the theory. Finally, we show that the allowed gravitational degrees of freedom can be fixed by the kinematics of the local spacetime causal structure, through the specific Equivalence Principle formulation. In this sense, the thermodynamical description seems to go beyond Einstein's theory as an intrinsic property of gravitation.Comment: 13 pages, 1 figur

Analogue model for quantum gravity phenomenology

So called "analogue models" use condensed matter systems (typically hydrodynamic) to set up an "effective metric" and to model curved-space quantum field theory in a physical system where all the microscopic degrees of freedom are well understood. Known analogue models typically lead to massless minimally coupled scalar fields. We present an extended "analogue space-time" programme by investigating a condensed-matter system - in and beyond the hydrodynamic limit - that is in principle capable of simulating the massive Klein-Gordon equation in curved spacetime. Since many elementary particles have mass, this is an essential step in building realistic analogue models, and an essential first step towards simulating quantum gravity phenomenology. Specifically, we consider the class of two-component BECs subject to laser-induced transitions between the components, and we show that this model is an example for Lorentz invariance violation due to ultraviolet physics. Furthermore our model suggests constraints on quantum gravity phenomenology in terms of the "naturalness problem" and "universality issue".Comment: Talk given at 7th Workshop on Quantum Field Theory Under the Influence of External Conditions (QFEXT 05), Barcelona, Catalonia, Spain, 5-9 Sep 200

Sonoluminescence as a QED vacuum effect: Probing Schwinger's proposal

Several years ago Schwinger proposed a physical mechanism for sonoluminescence in terms of photon production due to changes in the properties of the quantum-electrodynamic (QED) vacuum arising from a collapsing dielectric bubble. This mechanism can be re-phrased in terms of the Casimir effect and has recently been the subject of considerable controversy. The present paper probes Schwinger's suggestion in detail: Using the sudden approximation we calculate Bogolubov coefficients relating the QED vacuum in the presence of the expanded bubble to that in the presence of the collapsed bubble. In this way we derive an estimate for the spectrum and total energy emitted. We verify that in the sudden approximation there is an efficient production of photons, and further that the main contribution to this dynamic Casimir effect comes from a volume term, as per Schwinger's original calculation. However, we also demonstrate that the timescales required to implement Schwinger's original suggestion are not physically relevant to sonoluminescence. Although Schwinger was correct in his assertion that changes in the zero-point energy lead to photon production, nevertheless his original model is not appropriate for sonoluminescence. In other works (see quant-ph/9805023, quant-ph/9904013, quant-ph/9904018, quant-ph/9905034) we have developed a variant of Schwinger's model that is compatible with the physically required timescales.Comment: 18 pages, ReV_TeX 3.2, 9 figures. Major revisions: This document is now limited to providing a probe of Schwinger's original suggestion for sonoluminescence. For details on our own variant of Schwinger's ideas see quant-ph/9805023, quant-ph/9904013, quant-ph/9904018, quant-ph/990503

Sonoluminescence: Two-photon correlations as a test of thermality

In this Letter we propose a fundamental test for probing the thermal nature of the spectrum emitted by sonoluminescence. We show that two-photon correlations can in principle discriminate between real thermal light and the quasi-thermal squeezed-state photons typical of models based on the dynamic Casimir effect. Two-photon correlations provide a powerful experimental test for various classes of sonoluminescence models.Comment: 6 pages, revtex 3; revised to include more discussion of finite volume effects; physics conclusions unchanged; to appear in Physics Letters

Lorentz symmetry breaking: phenomenology and constraints

In this talk I shall review several motivations for considering departures from exact Lorentz invariance and the different theoretical frameworks adopted to describe these departures. Among these, I shall focus on an effective field theory approach and discuss the phenomenology and constraints of Lorentz symmetry breaking in the Standard Model as well as in Gravity. In particular I will focus on current constraints on UV breaking inspired by quantum gravity scenarios and briefly discuss the open issues and future perspectives for this field of research

Vacuum Effects in Gravitational Fields: Theory and Detectability

In this thesis, we investigate quantum vacuum effects in the presence of gravitational fields. After discussing the general theory of vacuum effects in strong fields we apply it to the relevant issue of the interaction of the quantum vacuum with black hole geometries. In particular we consider the long-standing problem of the interpretation of gravitational entropy. After these investigations, we discuss the possible experimental tests of particle creation from the quantum vacuum. This leads us to study acoustic geometries and their way of ``simulating'' gravitational structures, such as horizons and black holes. We study the stability of these structures and the problems related to setting up experimental detection of ``phonon Hawking flux'' from acoustic horizons. This line of research then leads us to propose a new model for explaining the emission of light in the phenomenon of Sonoluminescence, based on the dynamical Casimir effect. This is possibly amenable to experimental investigation. Finally we consider high energy phenomena in the early universe. In particular we discuss inflation and possible alternative frameworks for solving the cosmological puzzles.Comment: Latex2e, 237 pages, 36 figures. Ph.D. Thesi

Sonoluminescence and the QED vacuum

In this talk I shall describe an extension of the quantum-vacuum approach to sonoluminescence proposed several years ago by J.Schwinger. We shall first consider a model calculation based on Bogolubov coefficients relating the QED vacuum in the presence of an expanded bubble to that in the presence of a collapsed bubble. In this way we shall derive an estimate for the spectrum and total energy emitted. This latter will be shown to be proportional to the volume of space over which the refractive index changes, as Schwinger predicted. After this preliminary check we shall deal with the physical constraints that any viable dynamical model for SL has to satisfy in order to fit the experimental data. We shall emphasize the importance of the timescale of the change in refractive index. This discussion will led us to propose a somewhat different version of dynamical Casimir effect in which the change in volume of the bubble is no longer the only source for the change in the refractive index.Comment: 15 pages, 1 figure, uses sprocl.sty. Talk at the 4th Workshop on Quantum Field Theory Under the Influence of External Conditions, Leipzig, 14-18 September, 199

Particle creation by moving spherical shell in the dynamical Casimir effect

The creation of massless scalar particles from the quantum vacuum by spherical shell with time varying radius is studied. In the general case of motion the equations are derived for the instantaneous basis expansion coefficients. The examples are considered when the mean number of particles can be explicitly evaluated in the adiabatic approximation.Comment: 9 pages, LaTeX, no figures, typos corrected, discussion added. Journal-ref adde