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We review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark energy models in addition to the conventional cosmological constant, paying particular attention to scalar field models such as quintessence, K-essence, tachyon, phantom and dilatonic models. The importance of cosmological scaling solutions is emphasized when studying the dynamical system of scalar fields including coupled dark energy. We study the evolution of cosmological perturbations allowing us to confront them with the observation of the Cosmic Microwave Background and Large Scale Structure and demonstrate how it is possible in principle to reconstruct the equation of state of dark energy by also using Supernovae Ia observational data. We also discuss in detail the nature of tracking solutions in cosmology, particle physics and braneworld models of dark energy, the nature of possible future singularities, the effect of higher order curvature terms to avoid a Big Rip singularity, and approaches to modifying gravity which leads to a late-time accelerated expansion without recourse to a new form of dark energy.

Dynamics of dark energy

Coherent preparation by laser light of quantum states of atoms and molecules can lead to quantum interference in the amplitudes of optical transitions. In this way the optical properties of a medium can be dramatically modified, leading to electromagnetically induced transparency and related effects, which have placed gas-phase systems at the center of recent advances in the development of media with radically new optical properties. This article reviews these advances and the new possibilities they offer for nonlinear optics and quantum information science. As a basis for the theory of electromagnetically induced transparency the authors consider the atomic dynamics and the optical response of the medium to a continuous-wave laser. They then discuss pulse propagation and the adiabatic evolution of field-coupled states and show how coherently prepared media can be used to improve frequency conversion in nonlinear optical mixing experiments. The extension of these concepts to very weak optical fields in the few-photon limit is then examined. The review concludes with a discussion of future prospects and potential new applications.

/pdf/electromagnetically-induced-transparency-optics-in-coherent-2dhges92td.pdf

Electromagnetically induced transparency : Optics in coherent media

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Table Of Integrals Series And Products

Modified Gravity and Cosmology

Several years ago Schwinger proposed a physical mechanism for sonoluminescence in terms of changes in the properties of the quantum-electrodynamic vacuum state during collapse of the bubble This mechanism is most often phrased in terms of changes in the Casimir energy (ie, changes in the distribution of zero-point energies) and has recently been the subject of considerable controversy The present paper further develops this quantum-vacuum approach to sonoluminescence: We calculate Bogolubov coefficients relating the QED vacuum states in the presence of a homogeneous medium of changing dielectric constant In this way we derive an estimate for the spectrum, number of photons, and total energy emitted We emphasize the importance of rapid spatio-temporal changes in refractive indices and the delicate sensitivity of the emitted radiation to the precise dependence of the refractive index as a function of wave number, pressure, temperature, and noble gas admixture Although the basic physics of the dynamical Casimir effect is a universal phenomenon of QED, specific and particular experimental features are encoded in the condensed matter physics controlling the details of the refractive index This calculation places rather tight constraints on the possibility of using the dynamical Casimir effect as an explanation for sonoluminescence, and we are hopeful that this scenario will soon be amenable to direct experimental probes In the following paper we discuss the technical complications due to finite-size effects, but for reasons of clarity in this paper we confine our attention to bulk effects

Sonoluminescence as a QED vacuum effect. I. The physical scenario

We develop a scenario in which feeble intermediate-range forces emerge as an effect resulting from the compactification (a-italic-grave-accent l-italica-italic Kaluza-Klein) of a five-dimensional model. It is conjectured that this general strategy will prove applicable to more realistic multidimensional theories such as the ten-dimensional superstring. We speculate that deviations from the standard gravitational force: similar to the ones reported recently as a ''fifth force'': may be interpreted as evidence for higher dimensions.

Feeble intermediate-range forces from higher dimensions.

The Raychaudhuri equation has seen extensive use in general relativity, most notably in the development of various singularity theorems. In this rather technical article we shall generalize the Raychaudhuri equation in several ways. First an improved version of the standard timelike Raychaudhuri equation is developed, where several key terms are lumped together as a divergence. This already has a number of interesting applications, both within the Arnowitt-Deser-Misner formalism and elsewhere. Second, a spacelike version of the Raychaudhuri equation is briefly discussed. Third, a version of the Raychaudhuri equation is developed that does not depend on the use of normalized congruences. This leads to useful formulae for the 'diagonal' part of the Ricci tensor. Fourth, a 'two vector' version of the Raychaudhuri equation is developed that uses two congruences to effectively extract 'off-diagonal' information concerning the Ricci tensor.

Some generalizations of the Raychaudhuri equation

Transmission through a potential barrier, and the related issue of particle production from a parametric resonance, are topics of considerable general interest in quantum physics. The authors have developed a rather general bound on quantum transmission probabilities, and recently applied it to bounding the greybody factors of a Schwarzschild black hole. In the current paper, we take a different tack—we use the Miller–Good transformation (which maps an initial Schrodinger equation to a final Schrodinger equation for a different potential) to significantly generalize the previous bound.

http://iopscience.iop.org/article/10.1088/1751-8113/42/4/045301/pdf

Transmission probabilities and the Miller-Good transformation

Regular black holes with nonsingular cores have been considered in several approaches to quantum gravity, and as agnostic frameworks to address the singularity problem and Hawking’s information paradox. While in a recent work we argued that the inner core is destabilized by linear perturbations, opposite claims were raised that regular black holes have in fact stable cores. To reconcile these arguments, we discuss a generalization of the geometrical framework, originally applied to Reissner-Nordtsrom black holes by Ori, and show that regular black holes have an exponentially growing Misner-Sharp mass at the inner horizon. This result can be taken as an indication that stable nonsingular black hole spacetimes are not the definitive endpoint of a quantum gravity regularization mechanism, and that nonperturbative backreation effects must be taken into account in order to provide a consistent description of the quantum-gravitational endpoint of gravitational stellar collapse.

/pdf/inner-horizon-instability-and-the-unstable-cores-of-regular-2lxhtsz7kz.pdf

Matt Visser

Papers

Sonoluminescence as a QED vacuum effect. I. The physical scenario

Feeble intermediate-range forces from higher dimensions.

Some generalizations of the Raychaudhuri equation

Transmission probabilities and the Miller-Good transformation

Inner horizon instability and the unstable cores of regular black holes