Showing papers on "Scalar (physics) published in 2003"

Bulk Parameterization of Air–Sea Fluxes: Updates and Verification for the COARE Algorithm

Abstract: In 1996, version 2.5 of the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk algorithm was published, and it has become one of the most frequently used algorithms in the air–sea interaction community. This paper describes steps taken to improve the algorithm in several ways. The number of iterations to solve for stability has been shortened from 20 to 3, and adjustments have been made to the basic profile stability functions. The scalar transfer coefficients have been redefined in terms of the mixing ratio, which is the fundamentally conserved quantity, rather than the measured water vapor mass concentration. Both the velocity and scalar roughness lengths have been changed. For the velocity roughness, the original fixed value of the Charnock parameter has been replaced by one that increases with wind speeds of between 10 and 18 m s−1. The scalar roughness length parameterization has been simplified to fit both an early set of NOAA/Environmental Technology Laboratory (ETL) experiments and...

Chameleon Cosmology

Abstract: The evidence for the accelerated expansion of the universe and the time-dependence of the fine-structure constant suggests the existence of at least one scalar field with a mass of order H_0 If such a field exists, then it is generally assumed that its coupling to matter must be tuned to unnaturally small values in order to satisfy the tests of the Equivalence Principle (EP) In this paper, we present an alternative explanation which allows scalar fields to evolve cosmologically while having couplings to matter of order unity In our scenario, the mass of the fields depends on the local matter density: the interaction range is typically of order 1 mm on Earth (where the density is high) and of order 10-10^4 AU in the solar system (where the density is low) All current bounds from tests of General Relativity are satisfied Nevertheless, we predict that near-future experiments that will test gravity in space will measure an effective Newton's constant different by order unity from that on Earth, as well as EP violations stronger than currently allowed by laboratory experiments Such outcomes would constitute a smoking gun for our scenario

The scalar-tensor theory of gravitation

Abstract: The scalar-tensor theory of gravitation is one of the most popular alternatives to Einstein's theory of gravitation. This book provides a clear and concise introduction to the theoretical ideas and developments, exploring scalar fields and placing them in context with a discussion of Brans-Dicke theory. Topics covered include the cosmological constant problem, time variability of coupling constants, higher dimensional space-time, branes and conformal transformations. The authors emphasize the physical applications of the scalar-tensor theory and thus provide a pedagogical overview of the subject, keeping more mathematically detailed sections for the appendices. This book is suitable for graduate courses in cosmology, gravitation and relativity. It will also provide a valuable reference for researchers.

A re-evaluation of long-term flux measurement techniques - Part I: Averaging and coordinate rotation

Abstract: Experience of long term flux measurements over tall canopies during the last two decades has revealed that the eddy flux of sensible plus latent heat is typically 30% smaller than the available radiant energy flux. This failure to close the energy balance is less common close to the surface over short roughness but is still sometimes seen, especially in complex topography. These observations cast doubt on the results obtained from long term flux studies where daily and annual net ecosystem exchange is usually the small difference between large positive and negative fluxes over 24 h. In this paper we investigate this problem by examining some fundamental assumptions entailed in analysis of surface exchange by the eddy flux method. In particular, we clarify the form and use of the scalar conservation equation that underlies this analysis and we examine the links between averaging period and rotation of coordinates in the situation where coordinates are aligned with the wind vector. We show that rotating coordinates so that the x axis is aligned with the mean wind vector has the effect of high pass filtering the scalar covariance, wc, such that contributions to the aerodynamic flux from atmospheric motions with periods longer than the averaging period are lost while those of shorter period are distorted. We compare the effect of computing surface exchange by averaging many short periods, in each of which the coordinates are rotated so that the mean vertical velocity is zero (the method currently adopted in most long-term flux studies), with analysis in long-term coordinates and show a systematic underestimation of surface exchange in the former case. This is illustrated with data from three long- term forest field sites where underestimations of sensible and latent heat fluxes of 10-15% averaged over many days are seen. Crucial factors determining the loss of flux are the averaging period T , the measurement height and the content of the scalar cospectrum at periods longer than T. The properties of this cospectrum over tall canopies in both homogeneous and complex terrain are illustrated by measurements at our three sites and we see that over tall canopies on flat ground in convective conditions, or on hilly sites in near neutral flow, the scalar cospectra have much more low frequency content than classical surface-layer spectral forms would predict. We believe that the filtering of this low frequency covari- ance by the averaging-rotation operations in common use is a large contributory factor to the failure to close the energy balance over tall canopies.

Curvature-based transfer functions for direct volume rendering: methods and applications

Abstract: Direct volume rendering of scalar fields uses a transfer function to map locally measured data properties to opacities and colors. The domain of the transfer function is typically the one-dimensional space of scalar data values. This paper advances the use of curvature information in multi-dimensional transfer functions, with a methodology for computing high-quality curvature measurements. The proposed methodology combines an implicit formulation of curvature with convolution-based reconstruction of the field. We give concrete guidelines for implementing the methodology, and illustrate the importance of choosing accurate filters for computing derivatives with convolution. Curvature-based transfer functions are shown to extend the expressivity and utility of volume rendering through contributions in three different application areas: nonphotorealistic volume rendering, surface smoothing via anisotropic diffusion, and visualization of isosurface uncertainty.

Asymptotic safety of gravity coupled to matter

Abstract: Nonperturbative treatments of the UV limit of pure gravity suggest that it admits a stable fixed point with a positive Newton constant and a cosmological constant. We prove that this result is stable under the addition of a scalar field with a generic potential and nonminimal coupling to the scalar curvature. There is a fixed point where the mass and all nonminimal scalar interactions vanish, while the gravitational couplings have values which are almost identical to those in the pure gravity case. We discuss the linearized flow around this fixed point and find that the critical surface is four dimensional. In the presence of other, arbitrary, massless minimally coupled matter fields, the existence of the fixed point, the sign of the cosmological constant, and the dimension of the critical surface depend on the type and number of fields. In particular, for some matter content, there exist polynomial asymptotically free scalar potentials, suggesting a gravitational solution to the well-known problem of triviality.

The structure of fine-scale scalar mixing in gas-phase planar turbulent jets

Abstract: Fine-scale scalar mixing in gas-phase planar turbulent jets is studied using measurements of three-component scalar gradient and scalar energy dissipation rate fields. Simultaneous planar Rayleigh scattering and planar laser-induced fluorescence, applied in parallel planes, yield the three-dimensional scalar field measurements. The spatial resolution is sufficient to permit differentiation in all three spatial directions. The data span a range of outer-scale Reynolds numbers from 3290 to 8330. Direct measurement of the thicknesses of scalar dissipation structures (layers) shows that the thicknesses scale with outer-scale Reynolds number as ) plays a significant role in the scalar dissipation process. The present data resolve a range of length scales from the dissipation scales up to nearly the jet full width, and thus can be used in a priori testing of subgrid models for scalar mixing in large-eddy simulations (LES). Comparison of two models for subgrid scalar variance, a scale-similarity model and a gradient-based model, indicates that the scale-similarity model is more accurate at larger LES filter sizes.

Velocity-scalar filtered density function for large eddy simulation of turbulent flows

Abstract: A methodology termed the “velocity-scalar filtered density function” (VSFDF) is developed and implemented for large eddy simulation (LES) of turbulent flows. In this methodology, the effects of the unresolved subgrid scales (SGS) are taken into account by considering the joint probability density function (PDF) of the velocity and scalar fields. An exact transport equation is derived for the VSFDF in which the effects of the SGS convection and chemical reaction are closed. The unclosed terms in this equation are modeled in a fashion similar to that typically used in Reynolds-averaged simulation procedures. A system of stochastic differential equations (SDEs) which yields statistically equivalent results to the modeled VSFDF transport equation is constructed. These SDEs are solved numerically by a Lagrangian Monte Carlo procedure in which the Ito–Gikhman character of the SDEs is preserved. The consistency of the proposed SDEs and the convergence of the Monte Carlo solution are assessed by comparison with results obtained by a finite difference LES procedure in which the corresponding transport equations for the first two SGS moments are solved. The VSFDF results are compared with those obtained by the Smagorinsky model, and all the results are assessed via comparison with data obtained by direct numerical simulation of a temporally developing mixing layer involving transport of a passive scalar. It is shown that the values of both the SGS and the resolved components of all second order moments including the scalar fluxes are predicted well by VSFDF. The sensitivity of the calculations to the model’s (empirical) constants are assessed and it is shown that the magnitudes of these constants are in the same range as those employed in PDF methods.

Polymer and Fock representations for a scalar field

Abstract: In loop quantum gravity, matter fields can have support only on the 'polymer-like' excitations of quantum geometry, and their algebras of observables and Hilbert spaces of states cannot refer to a classical, background geometry. Therefore, to adequately handle the matter sector, one has to address two issues already at the kinematic level. First, one has to construct the appropriate background-independent operator algebras and Hilbert spaces. Second, to make contact with low-energy physics, one has to relate this 'polymer description' of matter fields to the standard Fock description in Minkowski space. While this task has been completed for gauge fields, important gaps remained in the treatment of scalar fields. The purpose of this letter is to fill these gaps.

Thermal Conduction in Magnetized Turbulent Gas

Abstract: Using numerical methods, we systematically study in the framework of ideal MHD the effect of magnetic fields on heat transfer within a turbulent gas. We measure the rates of passive scalar diffusion within magnetized fluids and make the comparisons (1) between MHD and hydrodynamic simulations, (2) between different MHD runs with different values of the external magnetic field (up to the energy equipartition value), and (3) between thermal conductivities parallel and perpendicular to the magnetic field. We do not find apparent suppression of diffusion rates by the presence of magnetic fields, which implies that magnetic fields do not suppress heat diffusion by turbulent motions.

An Intercomparison Between Divergence-Cleaning and Staggered Mesh Formulations for Numerical Magnetohydrodynamics

Abstract: In recent years, several different strategies have emerged for evolving the magnetic field in numerical MHD. Some of these methods can be classified as divergence-cleaning schemes, where one evolves the magnetic field components just like any other variable in a higher order Godunov scheme. The fact that the magnetic field is divergence-free is imposed post-facto via a divergence-cleaning step. Other schemes for evolving the magnetic field rely on a staggered mesh formulation which is inherently divergence-free. The claim has been made that the two approaches are equivalent. In this paper we cross-compare three divergence-cleaning schemes based on scalar and vector divergence-cleaning and a popular divergence-free scheme. All schemes are applied to the same stringent test problem. Several deficiencies in all the divergence-cleaning schemes become clearly apparent with the scalar divergence-cleaning schemes performing worse than the vector divergence-cleaning scheme. The vector divergence-cleaning scheme also shows some deficiencies relative to the staggered mesh divergence-free scheme. The differences can be explained by realizing that all the divergence-cleaning schemes are based on a Poisson solver which introduces a non-locality into the scheme, though other subtler points of difference are also catalogued. By using several diagnostics that are routinely used in the study of turbulence, it is shown that the differences in the schemes produce measurable differences in physical quantities that are of interest in such studies.

Scalar Field Theory on Fuzzy S 4

Abstract: Scalar fields are studied on fuzzy S 4 and a solution is found for the elimination of the unwanted degrees of freedom that occur in the model. The resulting theory can be interpreted as a Kaluza-Klein reduction of P3 to S 4 in the fuzzy context.

The influence of a forest canopy on top‐down and bottom‐up diffusion in the planetary boundary layer

Abstract: Results from two nested-grid large-eddy simulations comparing cases with and without a plant canopy are presented. Through comparisons of numerically generated mean and turbulence statistics, the influence of a plant canopy with a leaf area index of two is shown to modify the air flow compared with an identical case without plants. Investigations of instantaneous fields and spatial spectra show that a plant canopy alters the spatial structure of turbulence and acts to shift the dominant scale to a scale on the order of the canopy height. Distributed drag and scalar sources, representing the presence of a scalar emitting deciduous forest, have little influence on top-down diffusion, but enhanced mixing and increased turbulence intensities result in a dramatic modification to bottom-up scalar diffusion up to ∼4.5 times the height of the canopy. Use of previously proposed bottom-up gradient function with observations of scalar gradients under unstable stability conditions at 50 m over a 25 m tall forest (leaf area index of two) are shown to lead to an underestimate of the scalar emission flux by a factor of four. New top-down and bottom-up functions are proposed to include these canopy-induced effects for this particular canopy. Copyright © 2003 Royal Meteorological Society

Two photon decay rates of heavy quarkonia in the relativistic quark model

Abstract: Two-photon decay rates of pseudoscalar, scalar and tensor states of charmonium and bottomonium are calculated in the framework of the relativistic quark model. Both relativistic effects and one-loop radiative corrections are taken into account. The obtained results are compared with other theoretical predictions and available experimental data.

The influence of probe resolution on the measurement of a passive scalar and its derivatives

Abstract: The influence of probe resolution on the statistical measurement of a passive scalar is reported. A spectral method is employed to simulate degradation of the spatial resolution of a probe on the measured variances of a fluctuating scalar and its streamwise derivative by low-pass filtering a time-series of data at different cutoff frequencies. Direct measurements are also employed by varying probe sensor separation. The far field of a circular jet and the near wake of a circular cylinder are both investigated using air as the working fluid. The use of this low-Schmidt number working fluid and relatively low turbulence Reynolds numbers allows for good resolution of small scales of scalar fluctuations. By comparison, the same level of resolution is much more difficult to achieve when utilising a high-Schmidt number working fluid. A small temperature differential above ambient is used to mark the passive scalar, which is measured using a cold-wire anemometer. Taylor's hypothesis is employed to determine length scales. The present results are in good agreement with previous direct measurements using both optical techniques and cold-wire probes. It is found that the spatial resolution required for accurate measurement of the scalar dissipation rate is well described by the characteristic smallest scale of the scalar fluctuation, i.e. 'the Batchelor scale'. However, an order of magnitude less resolution is required for the scalar variance. The effect of degrading resolution on the variance measurements is more significant in the near wake than the far-field jet, suggesting that these requirements may be flow-dependent.

Low Dimensional Modeling of Flow for Closed-Loop Flow Control

Abstract: The focus of this work is on the use of detailed numerical simulation data to develop low dimensional models of a flow that can be used for the design of a control law for a feedback control-loop system. The subsonic flow over a shallow cavity was selected as the test bed. This research is an integral part of the closedloop flow control group at the Collaborative Center of Control Science at The Ohio State University. Numerical simulations were carried out using different modeling options and the preliminary results were compared with the experimental data. The simulation results showed that a quasi-3-D simulation, which solves the full 3-D Navier-Stokes equations but neglects the effects of the sidewalls, is a good compromise between providing sufficiently accurate data of the flow and quick turn around. The Proper Orthogonal Decomposition (POD) and Galerkin projection were used to obtain the spatial basis and the time evolution coefficient required for the low dimensional model. The results for the baseline case showed that four POD modes, using either a scalar or vector approach to determine norms, contain 96% of the energy and capture the essential dynamics of the flow. However, the system of ordinary differential equations for deriving the time coefficients for the reduced order model requires at least 8 modes with the vector approach, but does not converge with the scalar approach.

A new approach to turbulent transport of a mean scalar

Abstract: We develop a simple mean field approach to the transport of a passive scalar for which the fundamental equation is a second order differential equation in the transported quantity, not a first order equation Triple correlations are included, as they must be for any realistic description of turbulence No correlation time enters the theory, only an eddy turnover time The approach is simpler than standard approaches which incorporate triple correlations, but more realistic than Gaussian or short correlation time closures which do not A similar approach has proven useful in magnetohydrodynamics

Mixed velocity–passive scalar statistics in high-Reynolds-number turbulence

Abstract: Statistics of the mixed velocity–passive scalar field and its Reynolds number dependence are studied in quasi-isotropic decaying grid turbulence with an imposed mean temperature gradient. The turbulent Reynolds number (using the Taylor microscale as the length scale), Rλ, is varied over the range 85 [les ] Rλ [les ] 582. The passive scalar under consideration is temperature in air. The turbulence is generated by means of an active grid and the temperature fluctuations result from the action of the turbulence on the mean temperature gradient. The latter is created by differentially heating elements at the entrance to the wind tunnel plenum chamber. The mixed velocity–passive scalar field evolves slowly with Reynolds number. Inertial-range scaling exponents of the co-spectra of transverse velocity and temperature, Evθ(k1), and its real-space analogue, the ‘heat flux structure function,’ 〈Δv(r)Δθ(r)〉, show a slow evolution towards their theoretical predictions of −7/3 and 4/3, respectively. The sixth-order longitudinal mixed structure functions, 〈(Δu(r))2(Δθ(r))4〉, exhibit inertial-range structure function exponents of 1.36–1.52. However, discrepancies still exist with respect to the various methods used to estimate the scaling exponents, the value of the scalar intermittency exponent, μθ, and the effects of large-scale phenomena (namely shear, decay and turbulent production of 〈θ2〉) on 〈(Δu(r))2(Δθ(r))4〉. All the measured fine-scale statistics required to be zero in a locally isotropic flow are, or tend towards, zero in the limit of large Reynolds numbers. The probability density functions (PDFs) of Δv(r)Δθ(r) exhibit roughly exponential tails for large separations and super-exponential tails for small separations, thus displaying the effects of internal intermittency. As the Reynolds number increases, the PDFs become symmetric at the smallest scales – in accordance with local isotropy. The expectation of the transverse velocity fluctuation conditioned on the scalar fluctuation is linear for all Reynolds numbers, with slope equal to the correlation coefficient between v and θ. The expectation of (a surrogate of) the Laplacian of the scalar reveals a Reynolds number dependence when conditioned on the transverse velocity fluctuation (but displays no such dependence when conditioned on the scalar fluctuation). This former Reynolds number dependence is consistent with Taylor’s diffusivity independence hypothesis. Lastly, for the statistics measured, no violations of local isotropy were observed.

On the Singularity Structure and Stability of Plane Waves

Abstract: We describe various aspects of plane wave backgrounds. In particular, we make explicit a simple criterion for singularity by establishing a relation between Brinkmann metric entries and diffeomorphism-invariant curvature information. We also address the stability of plane wave backgrounds by analyzing the fluctuations of generic scalar modes. We focus our attention on cases where after fixing the light-cone gauge the resulting world sheet fields appear to have negative ``mass terms. We nevertheless argue that these backgrounds may be stable.

Spherically symmetric steady states of galactic dynamics in scalar gravity

Abstract: The kinetic motion of the stars of a galaxy is considered within the framework of a relativistic scalar theory of gravitation. This model, even though unphysical, may represent a good laboratory to study in a rigorous, mathematical way those problems, such as the influence of the gravitational radiation on the dynamics, which are still beyond our present understanding of the physical model represented by the Einstein–Vlasov system. The present paper is devoted to deriving the equations of the model and to proving the existence of spherically symmetric equilibria with finite radius.

Neutralino-nucleon cross section and charge and colour breaking constraints

Abstract: We compute the neutralino-nucleon cross section in several supersymmetric scenarios, taking into account all kind of constraints. In particular, the constraints that the absence of dangerous charge and colour breaking minima imposes on the parameter space are studied in detail. In addition, the most recent experimental constraints, such as the lower bound on the Higgs mass, the b→sγ branching ratio, and the muon g−2 are considered. The astrophysical bounds on the dark matter density are also imposed on the theoretical computation of the relic neutralino density, assuming thermal production. This computation is relevant for the theoretical analysis of the direct detection of dark matter in current experiments. We consider first the supergravity scenario with universal soft terms and GUT scale. In this scenario the charge and colour breaking constraints turn out to be quite important, and tan β20 is forbidden. Larger values of tan β can also be forbidden, depending on the value of the trilinear parameter A. Finally, we study supergravity scenarios with an intermediate scale, and also with non-universal scalar and gaugino masses where the cross section can be very large.

Viscous cosmological models and accelerated universes

Abstract: It is shown that a present acceleration with a past deceleration is a possible solution to the Friedmann equation by considering the Universe as a mixture of a scalar with a matter field and by including a nonequilibrium pressure term in the energy-momentum tensor. The dark energy density decays more slowly with respect to the time than the matter energy density does. The inclusion of the nonequilibrium pressure leads to a less pronounced decay of the matter field with a shorter period of past deceleration.

A comparison of subgrid‐scale models for large‐eddy simulations of convection in the Earth's core

Abstract: SUMMARY Large-eddy simulations provide a strategy for modelling large-scale flow when the smallest scales are not resolved. The approach relies on spatial filtering to eliminate scales smaller than the grid spacing, but requires models for the influence of the subgrid scales. We investigate four subgrid-scale models in numerical calculations of magnetoconvection in the Earth's core. Three of the models are based on eddy diffusivities, while the fourth is the similarity model of Bardina et al. (1980). The predictions of the subgrid-scale models are tested using a direct numerical simulation (DNS), which resolves the smallest dissipative scales. In order to achieve the required resolution we restrict the calculations to a small volume of the core with periodic boundary conditions. The grid is a cube with 128 × 64 × 32 nodes, oriented so that the z-coordinate is aligned with the rotation axis and the y-coordinate is parallel to an imposed magnetic field. The direction of gravity may be oriented arbitrarily in the x–z plane and several representative cases are considered. Output from the DNS is filtered on to a coarser grid prior to evaluating the subgrid-scale models. The results are compared with estimates of the subgrid-scale heat and momentum fluxes calculated from the fully resolved solution. Substantial anisotropy in the subgrid-scale fluxes is caused by the influences of rotation and the imposed magnetic field. Models based on scalar eddy diffusivities are incapable of reproducing this anisotropy, whereas the similarity model gives a good match to the amplitude and spatial distribution of the subgrid-scale fluxes.

A Bound on Mixing Efficiency for the Advection-Diffusion Equation

Abstract: An upper bound on the mixing efficiency is derived for a passive scalar under the influence of advection and diffusion with a body source. For a given stirring velocity field, the mixing efficiency is measured in terms of an equivalent diffusivity, which is the molecular diffusivity that would be required to achieve the same level of fluctuations in the scalar concentration in the absence of stirring, for the same source distribution. The bound on the equivalent diffusivity depends only on the functional "shape" of both the source and the advecting field. Direct numerical simulations performed for a simple advecting flow to test the bounds are reported.

Spherically symmetric steady states of galactic dynamics in scalar gravity

Abstract: The kinetic motion of the stars of a galaxy is considered within the framework of a relativistic scalar theory of gravitation. This model, even though unphysical, may represent a good laboratory where to study in a rigorous, mathematical way those problems, like the influence of the gravitational radiation on the dynamics, which are still beyond our present understanding of the physical model represented by the Einstein--Vlasov system. The present paper is devoted to derive the equations of the model and to prove the existence of spherically symmetric equilibria with finite radius.

Focal shift and the axial optical coordinate for high-aperture systems of finite Fresnel number.

Abstract: Analytic expressions are given for the on-axis intensity predicted by the Rayleigh–Sommerfeld and Kirchhoff diffraction integrals for a scalar optical system of high numerical aperture and finite value of Fresnel number. A definition of the axial optical coordinate is introduced that is valid for finite values of Fresnel number, for high-aperture systems, and for observation points distant from the focus. The focal shift effect is reexamined. For the case when the focal shift is small, explicit expressions are given for the focal shift and the axial peak in intensity.

On scalar dissipation and partially premixed flame propagation

Abstract: Measurements of the scalar dissipation rate in the region immediately upstream of a lifted jet flame are presented. The scalar dissipation is determined in this isothermal region from a planar measurement of a two-dimensional conserved scalar (jet fluid) using laser Rayleigh scattering. Fields of the scalar dissipation rate are presented in addition to tabulated values for three different liftoff heights ( Re d =4800, 6400, and 8300). Scalar dissipation rates do not reach levels thought to cause extinction of the leading edge based on comparison with extinction data for counterflow diffusion flames. Additionally, results are presented on the axial flame propagation velocities relative to the jet flow. The data indicate that over the three flow conditions, the flame velocity relative to the flow is approximately constant during the case of a quasi-stationary lifted flame. In light of these findings, it is suggested that concepts involving partially premixed flame propagation, rather than those of critical ...

Study of a Class of Four Dimensional Nonsingular Cosmological Bounces

Abstract: We study a novel class of nonsingular time-symmetric cosmological bounces. In this class of four dimensional models the bounce is induced by a perfect fluid with a negative energy density. Metric perturbations are solved in an analytic way all through the bounce. The conditions for generating a scale invariant spectrum of tensor and scalar metric perturbations are discussed.

Magnetic field generation from nonequilibrium phase transitions

Abstract: We study the generation of magnetic fields during the stage of particle production resulting from spinodal instabilities during phase transitions out of equilibrium. The main premise is that long-wavelength instabilities that drive the phase transition lead to strong nonequilibrium charge and current fluctuations which generate electromagnetic fields. We present a formulation based on the nonequilibrium Schwinger-Dyson equations that leads to an exact expression for the spectrum of electromagnetic fields valid for general theories and cosmological backgrounds and whose main ingredient is the transverse photon polarization out of equilibrium. This formulation includes the dissipative effects of the conductivity in the medium. As a prelude to cosmology, we study magnetogenesis in Minkowski spacetime in a theory of N charged scalar fields to lowest order in the gauge coupling and to leading order in the large N within two scenarios of cosmological relevance. The long-wavelength power spectrum for electric and magnetic fields at the end of the phase transition is obtained explicitly. It follows that equipartition between electric and magnetic fields does not hold out of equilibrium. In the case of a transition from a high-temperature phase, the conductivity of the medium severely hinders the generation of magnetic fields; however, the magnetic fields generated are correlated on scales of the order of the domain size, which is much larger than the magnetic diffusion length. The implications of the results to cosmological phase transitions driven by spinodal instabilities are discussed.

Study of a class of four-dimensional non-singular cosmological bounces

Abstract: We study a novel class of non-singular time-symmetric cosmological bounces. In this class of four-dimensional models the bounce is induced by a perfect fluid with a negative energy density. Metric perturbations are solved in an analytic way all through the bounce. The conditions for generating a scale invariant spectrum of tensor and scalar metric perturbations are discussed.