Milan Mijić

An analysis of the decoherence of quantum fluctuations shows that production of classical adiabatic density perturbations may not take place in models of power-law inflation, a(t)~t^p, with 1<p≤ 3. Some consequences for models of extended inflation are pointed out. In general, the condition for decoherence places new constraint on inflationary models, which does not depend on often complicated subsequent evolution.

It is shown how coarse graining of quantum field theory in de Sitter space leads to the emergence of a classical stochastic description as an effective theory in the infrared regime. The quantum state of the coarse-grained scalar field is found to be a highly squeezed coherent state, whose center performs a random walk on a bundle of classical trajectories.

We review stochastic inflation and its consequences for the interpretation of wave functions in quantum cosmology.

It has normally been assumed that the evolution of a cosmic-string network is independent of whether or not the strings are superconducting. We show here, in a simple model, how the evolution of a superconducting-cosmic-string network may be altered, principally by the enhanced radiation losses. We analyze the evolution of the network as a function of the loop self-intersection probability, and find that a scaling solution may be possible for smaller loop self-intersection probabilities than allowed for normal strings. A possible consequence of this is a significant modification of the loop number density.

The evolution of a scalar field interacting with an environment in the de Sitter phase of an inflationary Universe is studied. The environment is taken to be a second scalar field. It is shown that the coherence length of the quantum fluctuations rapidly decreases after the wavelength of the perturbation crosses the Hubble radius. Hence, the fluctuations can be interpreted as classical. This lends support to the usual derivation of the spectrum of density perturbations in inflationary Universe models.

The dynamics of inflation is that of a relaxation random process. We examine boundary conditions for this process and give a simple proof for the existence of eternal inflation that takes into account the field dependence of the effective cosmological constant and the finite duration of the inflationary phase. Next, natural initial conditions are formulated that lead to a specific interpretation of the wave function in quantum cosmology. We demonstrate that the Hartle-Hawking wave function describes the equilibrium regime for the stochastic process (with the correct quantum-field-theory limit), but only if the cosmological constant is sufficiently large or if it decays sufficiently slowly. We show in which sense inflation is certain even with the Hartle-Hawking wave function, and propose a new framework for the ‘‘tunneling’’ wave function. On the basis of boundary conditions, we argue that the dynamics of the stochastic phase and, hence, the main features of the present Universe, are independent of the physics above the Planck scale.