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H. J. de Vega

Publications and source records attributed to H. J. de Vega.

At least 127 records · Page 7Linked to original sources
arXiv1998

Out of Equilibrium Fields in Inflationary Dynamics. Density Fluctuations

The energy and time scales during the inflationary stage of the universe calls for an out of equilibrium quantum field treatment. Moreover, the high energy densities involved make necessary the use of non-perturbative approaches as large N and Hartree methods. We start these lectures by introducing such non-perturbative out of equilibrium methods in cosmological universes. We discuss the renormalization procedure and the choice of initial conditions. We then study the nonlinear dynamics of quantum fields in matter and radiation dominated FRW and de Sitter universes. For a variety of initial conditions, we compute the evolution of the inflaton,its quantum fluctuations and the equation of state. We investigate the explosive particle production due to spinodal unstabilities and parametric amplification in FRW and de Sitter universes with and without symmetry breaking.We find that the particle production is sensitive to the expansion of the universe.For symmetry breaking scenarios, we determine generic late time fields behavior for FRW and deSitter cosmologies.We find that quantum fluctuations damp in FRW as the square of the scale factor while the order parameter approaches a minimum of the potential at the same rate.We con- sider an O(N) inflaton model coupled self-consistently to gravity in the semi- classical approximation for a `new inflation' scenario. We find that spinodal instabilities drive the growth of non-perturbatively large quantum fluctuations which shut off inflation. These fluctuations assemble with the inflaton zero mode yielding a new effective field that actually rolls down behaving classi- cally. We compute the amplitude and index for scalar density and tensor perturbations. In all models of this type the spinodal instabilities produce a `red' spectrum of primordial scalar density perturbations.

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arXiv1998

The fractal structure of the universe : a new field theory approach

While the universe becomes more and more homogeneous at large scales, statistical analysis of galaxy catalogs have revealed a fractal structure at small-scales (λ< 100 h^{-1} Mpc), with a fractal dimension D=1.5-2 (Sylos Labini et al 1996). We study the thermodynamics of a self-gravitating system with the theory of critical phenomena and finite-size scaling and show that gravity provides a dynamical mechanism to produce this fractal structure. We develop a field theoretical approach to compute the galaxy distribution, assuming them to be in quasi-isothermal equilibrium. Only a limited, (although large), range of scales is involved, between a short-distance cut-off below which other physics intervene, and a large-distance cut-off, where the thermo- dynamic equilibrium is not satisfied. The galaxy ensemble can be considered at critical conditions, with large density fluctuations developping at any scale. From the theory of critical phenomena, we derive the two independent critical exponents nu and eta and predict the fractal dimension D = 1/nu to be either 1.585 or 2, depending on whether the long-range behaviour is governed by the Ising or the mean field fixed points, respectively. Both set of values are compatible with present observations. In addition, we predict the scaling behaviour of the gravitational potential to be r^{-(1 + eta)/2}. That is, r^{-0.5} for mean field or r^{- 0.519} for the Ising fixed point. The theory allows to compute the three and higher density correlators without any assumption or Ansatz. We find that the N-points density scales as r_1^{(N-1)(D-3)}, when r_1 >> r_i, 2 leq i leq N . There are no free parameters in this theory.

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arXiv1997

Asymptotic Dynamics in Scalar Field Theory: Anomalous Relaxation

We analyze the dynamics of dissipation and relaxation in the unbroken and broken symmetry phases of scalar theory in the nonlinear regime for large initial energy densities, and after linear unstabilities (parametric or spinodal) are shut-off by the quantum backreaction. A new time scale emerges that separates the linear from the non-linear regimes. This scale is non- perturbative in the coupling and initial amplitude. The non-perturbative evolution is studied in the large the N limit for the O(N) vector model. A combination of numerical analysis and the multitime scale analysis reveals the presence of unstable bands in the nonlinear regime. These are associated with power law growth of quantum fluctuations, that result in power law relaxation and dissipation with non-universal and non-perturbative dynamical anomalous exponents.We find substantial particle production during this non-linear evolu- tion which is of the same order as that in the linear regime. The expectation value of the scalar field vanishes asymptotically transferring all of the initial energy into produced particles via the non-linear resonances in the unbroken symmetry phase.The effective mass for the quantum modes tends asympto- tically to a constant plus oscillating O(1/t) terms. This slow decay causes the power behaviour in the modes which become free for t=infty. We derive a simple expression for the equation of state for the fluid of produced particles that interpolates between radiation-type and dust-type equations according to the initial value of the order parameter for unbroken symmetry. For broken symmetry the produced particles are massless Goldstone bosons with a radiation-type equation of state. We find the onset of a novel form of dynamical Bose condensation in the collisionless regime in the absence of thermalization.

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arXiv1997

Non-Perturbative Quantum Dynamics of a New Inflation Model

We consider an O(N) model coupled self-consistently to gravity in the semiclassical approximation, where the field is subject to `new inflation' type initial conditions. We study the dynamics self-consistently and non-perturbatively with non-equilibrium field theory methods in the large N limit. We find that spinodal instabilities drive the growth of non-perturbatively large quantum fluctuations which shut off the inflationary growth of the scale factor. We find that a very specific combination of these large fluctuations plus the inflaton zero mode assemble into a new effective field. This new field behaves classically and it is the object which actually rolls down. We show how this reinterpretation saves the standard picture of how metric perturbations are generated during inflation and that the spinodal growth of fluctuations dominates the time dependence of the Bardeen variable for superhorizon modes during inflation. We compute the amplitude and index for the spectrum of scalar density and tensor perturbations and argue that in all models of this type the spinodal instabilities are responsible for a `red' spectrum of primordial scalar density perturbations. A criterion for the validity of these models is provided and contact with the reconstruction program is established validating some of the results within a non-perturbative framework. The decoherence aspects and the quantum to classical transition through inflation are studied in detail by following the full evolution of the density matrix and relating the classicality of cosmological perturbations to that of long-wavelength matter fluctuations.

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arXiv1997

Self-Consistent Dynamics of Inflationary Phase Transitions

The physics of the inflationary universe requires the study of the out of equilibrium evolution of quantum fields in curved spacetime. We present the evolution for both the geometry and the matter (described by the quantum inflaton field) by means of the non-perturbative large N limit combined with semi-classical gravitational dynamics including the back-reaction of quantum fluctuations self-consistently for a new inflation scenario. We provide a criterion for the validity of the classical approximation and a full analysis of the case in which spinodal quantum fluctuations drive the evolution of the scale factor. Under carefully determined conditions, we show that the full field equations may be well approximated by those of a single composite field which obeys the classical equation of motion in all cases. The de Sitter stage is found to be followed by a matter dominated phase. We compute the spectrum of scalar density perturbations and argue that the spinodal instabilities are responsible for a `red' spectrum with more power at longer wavelengths. A criterion for the validity of these models is provided and contact with the reconstruction program is established.

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arXiv1997

Non-Equilibrium Production of Photons via π^0\to 2γin DCC's

We study production of photons via the non-equilibrium relaxation of a Disoriented Chiral Condensate with the chiral order parameter having a large initial amplitude along the π^0 direction. Assuming the validity of the low energy coupling of the neutral pion to photons via the U_A(1) anomalous vertex, we find that for large initial amplitudes along the π^0 direction, photon production is enhanced by parametric amplification. These processes are non-perturbative with a large contribution during the non-equilibrium stages of the evolution and result in a distinct distribution of the produced photons and a polarization asymmetry. For initial amplitudes of the π^0 component of the order parameter between 200-400 MeV, corresponding to energy densities between 1-12 GeV/fm^3 we find a peak in the photon distribution at energies between \approx 300 -600 MeV. We also find polarization asymmetries typically between 5-10%. We discuss the potential experimental impact of these results as well as the problems associated with its detection.

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arXiv1997

Non linear integral equation and excited--states scaling functions in the sine-Gordon model

The NLIE (the non-linear integral equation equivalent to the Bethe Ansatz equations for finite size) is generalized to excited states, that is states with holes and complex roots over the antiferromagnetic ground state. We consider the sine-Gordon/massive Thirring model (sG/mT) in a periodic box of length $L$ using the light-cone approach, in which the sG/mT model is obtained as the continuum limit of an inhomogeneous six vertex model. This NLIE is an useful starting point to compute the spectrum of excited states both analytically in the large $L$ (perturbative) and small $L$ (conformal) regimes as well as numerically.

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arXiv1997

Out of Equilibrium Dynamics of an Inflationary Phase Transition

We study the non-linear dynamics of an inflationary phase transition in a quartically self coupled inflaton model within the framework of a de Sitter background. Large N and Hartree non-perturbative approximations combined with non-equilibrium field theory methods are used to study the self-consistent time evolution including backreaction effects. We find that when the system cools down from an initial temperature T_i > T_c to below T_c with the initial value of the zero mode of the inflaton phi(0) << m lambda^{-1/4}, the dynamics is determined by the growth of long-wavelength quantum fluctuations. For phi(0) >> m lambda^{-1/4} the dynamics is determined by the evolution of the classical zero mode. In the regime where spinodal quantum fluctuations give the most important contribution to the non-equilibrium dynamics, we find that they modify the equation of state providing a graceful exit from the inflationary stage. Inflation ends through this new mechanism at a time scale t_s >= [H/m^2]ln[lambda^{-1}] which for H >= m and very weak coupling allows over one hundred e-folds during the de Sitter phase. Spatially correlated domains grow to be of horizon-size and quantum fluctuations ``freeze-out'' for times t> t_s.

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arXiv1997

Scalar Field Dynamics in Friedman Robertson Walker Spacetimes

We study the non-linear dynamics of quantum fields in matter and radiation dominated universes, using the non-equilibrium field theory approach combined with the non-perturbative Hartree and the large N approximations. We examine the phenomenon of explosive particle production due to spinodal instabilities and parametric amplification in expanding universes with and without symmetry breaking. For a variety of initial conditions, we compute the evolution of the inflaton, its quantum fluctuations, and the equation of state. We find explosive growth of quantum fluctuations, although particle production is somewhat sensitive to the expansion of the universe. In the large N limit for symmetry breaking scenarios, we determine generic late time solutions for any flat Friedman-Robertson-Walker cosmology. We also present a complete and numerically implementable renormalization scheme for the equation of motion and the energy momentum tensor in flat FRW cosmologies. In this scheme the renormalization constants are independent of time and of the initial conditions.

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arXiv1997

Erice Lectures on Inflationary Reheating

At the end of the inflationary stage of the early universe, profuse particle production leads to the reheating of the universe. Such explosive particle production is due to parametric amplification of quantum fluctuations for the unbroken symmetry case (appropriate for chaotic inflation), or spinodal instabilities in the broken symmetry phase(which is the case in new inflation). The self-consistent methods presented in these lectures are the only approaches, so far, that lead to reliable quantitative results on the reheating mechanism in the inflationary universe. They fully use the field theoretical Schwinger-Keldysh out of equilibrium method in a non-perturbative way. These approaches take into account the non-linear interaction between the quantum modes and exactly conserve energy (covariantly). Simplified analysis that do not include the full backreaction and do not conserve energy, result in unbound particle production and lead to quantitatively erroneous results. For spontaneously broken theories the issue of whether the symmetry may be restored or not by the quantum fluctuations is analyzed. The precise criterion for symmetry restoration is presented. The field dynamics is symmetric when the energy density in the initial state is larger than the top of the tree level potential. When the initial energy density is below the top of the tree level potential, the symmetry is broken.

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arXiv1997

Photoproduction Enhancement from Non Equilibrium Disoriented Chiral Condensates

We study photoproduction during the non-equilibrium stages of the formation of chiral condensates within the ``quench'' scenario of the chiral phase transition. The dynamics is modeled with a gauged linear sigma model. A novel quantum kinetic approach to the description of photoproduction far off equilibrium is developed. We find that non-equilibrium spinodal instabilities of long wavelength pion fluctuations are responsible for an enhanced photoproduction rate for energies $\leq 80$ MeV at order $α$. These non-equilibrium effects lead to a larger contribution than the typical processes in the medium, including that of the anomalous neutral pion decay $π^0 \rightarrow 2 γ$ (which is of order $α^2$). We follow the evolution of the dynamics throughout the phase transition, which in this scenario occurs on a time scale of about $2.5-3$ fm/c and integrate the photon yield through its evolution. The spectrum of photons produced throughout the phase transition is a non- equilibrium one. For thermal initial conditions at the time of the quench it interpolates between a thermal distribution about 6% above the initial temperature (at the time of the quench) for low energy $\leq 80$ MeV photons, and a high energy tail in thermal equilibrium at the initial temperature, with a smooth crossover at 100 MeV. The rate displays a peak at $\sim 35$ MeV which receives a larger enhancement the closer the initial temperature at the time of the quench is to the critical temperature. It is found that the enhancement of photoproduction at low energies is not an artifact caused by the initial distribution of the photons, but is due to the pionic instabilities. We suggest that these strong out of equilibrium effects may provide experimental signatures for the formation and relaxation of DCC's in heavy ion collisions.

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arXiv1997

Order parameter evolution in scalar QFT: renormalization group resummation of secular terms

The quantum evolution equations for the field expectation value are analytically solved to cubic order in the field amplitude and to one-loop level in the lambda phi-fourth model. We adapt and use the renormalization group (RG) method for such non-linear and non-local equations. The time dependence of the field expectation value is explicitly derived integrating the RG equations. It is shown that the field amplitude for late times approaches a finite limit as the time to the power -3/2. This limiting value is expressed as a function of the initial field amplitude.

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arXiv1996

Fractal Dimensions and Scaling Laws in the Interstellar Medium: a new Field Theory approach

We develop a field theoretical approach to the cold interstellar medium (ISM). We show that a non-relativistic self-gravitating gas in thermal equilibrium with variable number of atoms or fragments is exactly equivalent to a field theory of a single scalar field ϕ({\vec x}) with exponential self interaction.We analyze this field theory perturbatively and non-perturbatively through the renormalization group approach.We show scaling behaviour(critical) for a continuous range of the temperature and of the other physical parameters. We derive in this framework the scaling relation Delta M(R) \sim R^{d_H} for the mass on a region of size R, and Delta v \sim R^q for the velocity dispersion where q = (d_H -1)/2. For the density-density correlations we find a power-law behaviour for large distances \sim|{\vec r_1} -{\vec r_2}|^{2 d_H -6}. The fractal dimension d_H turns to be related with the critical exponent nu of the correlation length by d_H = 1/nu. The renormalization group approach for a single component scalar field in three dimensions states that the long- distance critical behaviour is governed by the (non-perturbative) Ising fixed point. The corresponding values of the scaling exponents are nu = 0.631..., d_H = 1.585... and q = 0.293.... Mean field theory yields for the scaling exponents nu =1/2, d_H = 2 and q = 1/2. Both the Ising and the mean field values are compatible with the present ISM observational data: 1.4 \leq d_H \leq 2, 0.3 \leq q \leq 0.6.As typical in critical phenomena, the scaling behaviour and critical exponents of the ISM can be obtained without dwelling into the dynamical (time-dependent) behaviour. The relevant rôle of self- gravity is stressed by the authors in a Letter to Nature, September 5, 1996.

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arXiv1996

Analytic and Numerical Study of Preheating Dynamics

We analyze the phenomenon of preheating,i.e. explosive particle production due to parametric amplification of quantum fluctuations in the unbroken case, or spinodal instabilities in the broken phase, using the Minkowski space $O(N)$ vector model in the large $N$ limit to study the non-perturbative issues involved. We give analytic results for weak couplings and times short compared to the time at which the fluctuations become of the same order as the tree level,as well as numerical results including the full backreaction.In the case where the symmetry is unbroken, the analytic results agree spectacularly well with the numerical ones in their common domain of validity. In the broken symmetry case, slow roll initial conditions from the unstable minimum at the origin, give rise to a new and unexpected phenomenon: the dynamical relaxation of the vacuum energy.That is, particles are abundantly produced at the expense of the quantum vacuum energy while the zero mode comes back to almost its initial value.In both cases we obtain analytically and numerically the equation of state which turns to be written in terms of an effective polytropic index that interpolates between vacuum and radiation-like domination. We find that simplified analysis based on harmonic behavior of the zero mode, giving rise to a Mathieu equation forthe non-zero modes miss important physics. Furthermore, analysis that do not include the full backreaction do not conserve energy, resulting in unbound particle production. Our results do not support the recent claim of symmetry restoration by non-equilibrium fluctuations.Finally estimates of the reheating temperature are given,as well as a discussion of the inconsistency of a kinetic approach to thermalization when a non-perturbatively large number of particles is created.

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arXiv1996

Preheating in FRW Universes

The nonlinear time evolution of the quantum fields is studied in the O(N) model for large N in a radiation dominated FRW universe, with a view towards the phenomenon of explosive particle production due to either spinodal instabilities or parametric amplification, i.e. preheating. Quantum backreaction effects due to the produced particles are included consistently within the large N approximation. We find that preheating persists when the expansion is included, although the amount of particle production is reduced compared to the values found in Minkowski space. We also see that the behavior of the evolving zero mode is very different from that in Minkowski space, though the late time behavior in all cases is determined by a sum rule that implies the existence of Goldstone bosons in the final state.

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arXiv1996

Preheating and Reheating in Inflationary Cosmology: a pedagogical survey

Recent progress in the preheating phenomena for inflationary cosmology is reviewed. We first discuss estimates of the preheating time scale and particle production at the early stages of parametric amplification within the Mathieu and Lam'e approximations and we analyze their precision and limitations. The necessity of self-consistent calculations including the non-linearity of the field theory equations in an energy conserving scheme is stressed. The large N calculations including the field back-reaction are reviewed. For spontaneously broken theories the issue of symmetry restoration is analyzed. A discussion of the possibility and criterion for symmetry restoration is presented.

astro-ph↗
arXiv1996

Self-gravity as an explanation of the fractal structure of the interstellar medium

The gas clouds of the interstellar medium have a fractal structure, the origin of which has generally been thought to lie in turbulence. The energy of turbulence could come from galactic rotation at large-scale, then cascade down to be dissipated on small-scales by viscosity; it has been suggested that such turbulence helps to prevent massive molecular clouds from collapsing in response to their own gravity. Here we show that, on the contrary, self-gravity itself may be the dominant factor in making clouds fractal. We develop a field-theory approach to the structure of clouds, assuming them to be isothermal, and with only gravitational interactions; we find that the observed fractal dimension of the clouds arise naturally from this approach. Although this result does not imply that turbulence is not important, it does demonstrate that the fractal structure can be understood without it.

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