Marcelo Gleiser

Anisotropic Stars: Exact Solutions

We study the effects of anisotropic pressure on the properties of spherically symmetric, gravitationally bound objects. We consider the full general-relativistic treatment of this problem and obtain exact solutions for various forms of the equation of state connecting the radial and tangential pressures. It is shown that pressure anisotropy can have significant effects on the structure and properties of stellar objects. In particular, the maximum value of 2M / R can approach unity (2M / R < 8/9 for isotropic objects) and the surface redshift can be arbitrarily large.

Strong dissipative behavior in quantum field theory

We study the conditions under which an overdamped regime can be attained in the dynamic evolution of a quantum field configuration. Using a real-time formulation of finite temperature field theory, we compute the effective evolution equation of a scalar field configuration, quadratically interacting with a given set of other scalar fields. We then show that, in the overdamped regime, the dissipative kernel in the field equation of motion is closely related to the shear viscosity coefficient, as computed in scalar field theory at finite temperature. The effective dynamics is equivalent to a time-dependent Ginzburg-Landau description of the approach to equilibrium in phenomenological theories of phase transitions. Applications of our results, including a recently proposed inflationary scenario called {open_quotes}warm inflation,{close_quotes} are discussed. {copyright} {ital 1998} {ital The American Physical Society}

Anisotropic Stars II: Stability

We investigate the stability of self-gravitating spherically symmetric anisotropic spheres under radial perturbations. We consider both the Newtonian and the full general-relativistic perturbation treatment. In the general-relativistic case, we extend the variational formalism for spheres with isotropic pressure developed by Chandrasekhar. We find that, in general, when the tangential pressure is greater than the radial pressure, the stability of the anisotropic sphere is enhanced when compared to isotropic configurations. In particular, anisotropic spheres are found to be stable for smaller values of the adiabatic index γ.

Oscillons: Resonant configurations during bubble collapse

Oscillons are localized, nonsingular, time-dependent, spherically symmetric solutions of nonlinear scalar field theories which, although unstable, are {ital extremely} long lived. We show that they naturally appear during the collapse of subcritical bubbles in models with symmetric and asymmetric double-well potentials. By a combination of analytical and numerical work we explain several of their properties, including the conditions for their existence, their longevity, and their final demise. We discuss several contexts in which we expect oscillons to be relevant. In particular, their nucleation during cosmological phase transitions may have wide-ranging consequences.

Microphysical approach to nonequilibrium dynamics of quantum fields

We examine the nonequilibrium dynamics of a self-interacting [lambda][phi][sup 4] scalar field theory. Using a real time formulation of finite temperature field theory we derive, up to two loops and [ital O]([lambda][sup 2]), the effective equation of motion describing the approach to equilibrium. We present a detailed analysis of the approximations used in order to obtain a Langevin-like equation of motion, in which the noise and dissipation terms associated with quantum fluctuations obey a fluctuation-dissipation relation. We show that, in general, the noise is colored (time dependent) and multiplicative (couples nonlinearly to the field), even though it is still Gaussian distributed. The noise becomes white in the infinite temperature limit. We also address the effect of couplings to other fields, which we assume play the role of the thermal bath, in the effective equation of motion for [phi]. In particular, we obtain the fluctuation and noise terms due to a quadratic coupling to another scalar field.

PSEUDO-STABLE BUBBLES ∗

The evolution of spherically symmetric unstable scalar field configurations (“bubbles”) is examined for both symmetric (SDWP) and asymmetric (ADWP) double-well potentials. Bubbles with initial static energies E0 ∼ Ecrit evolve into time-dependent, localized configurations which are very long-lived compared to characteristic time-scales in the models examined. The stability of these configurations is investigated and possible applications are briefly discussed.

A first principles warm inflation model that solves the cosmological horizon and flatness problems

A quantum field theory warm inflation model is presented that solves the horizon and flatness problems. The model obtains, from the elementary dynamics of particle physics, cosmological scale factor trajectories that begin in a radiation dominated regime, enter an inflationary regime, and then smoothly exit back into a radiation dominated regime, with non-negligible radiation throughout the evolution. {copyright} {ital 1999} {ital The American Physical Society}

Gravitational Stability of Scalar Matter

The question of the stability of matter against gravitational collapse is of general interest to astrophysics. In this work we investigate the stability against small radial oscillations of equilibrium configurations of cold, gravitationally bound states of complex scalar fields, known as boson stars. These equilibrium configurations exhibit a mass profile against central density which is very similar to that of ordinary neutron stars, with a pronounced maximum mass at Mc = 0.633MPl2/m, where MPl is the Planck mass, for a certain value of the central density σc(0). We give analytical and numerical proof that configurations with central densities greater than σc(0) are unstable against radial perturbations by studying the behavior of the eigenfrequencies of the perturbations for different values of σ(0).

ANISTROPIC STARS: EXACT SOLUTIONS AND STABILITY

I report on recent work concerning the existence and stability of self-gravitating spheres with anisotropic pressure. After presenting new exact solutions, Chandrasekhars variational formalism for radial perturbations is generalized to anisotropic objects and applied to investigate their stability. It is shown that anisotropy can not only support stars of mass M and radius R with 2M/R≥8/9 and arbitrarily large surface redshifts, but that stable configurations exist for values of the adiabatic index γ smaller than the corresponding isotropic value.

Entropic measure for localized energy configurations: Kinks, bounces, and bubbles

Abstract We construct a configurational entropy measure in functional space. We apply it to several nonlinear scalar field models featuring solutions with spatially-localized energy, including solitons and bounces in one spatial dimension, and critical bubbles in three spatial dimensions, typical of first-order phase transitions. Such field models are of widespread interest in many areas of physics, from high energy and cosmology to condensed matter. Using a variational approach, we show that the higher the energy of a trial function that approximates the actual solution, the higher its relative configurational entropy, defined as the absolute difference between the configurational entropy of the actual solution and of the trial function. Furthermore, we show that when different trial functions have degenerate energies, the configurational entropy can be used to select the best fit to the actual solution. The configurational entropy relates the dynamical and informational content of physical models with localized energy configurations.