Nuno D. Antunes

Numerical Simulations of String Networks in the Abelian-Higgs Model

We present the results of a field theory simulation of networks of strings in the Abelian-Higgs model. From a random initial configuration the resulting vortex tangle approaches a self-similar regime in which the length density of lines of zeros of reduces as t-2. The network loses energy directly into scalar and gauge radiations supporting a recent claim that particle production, not gravitational radiation, is the dominant energy loss mechanism for cosmic strings. This means that cosmic strings in grand unified theories are severely constrained by high energy cosmic ray fluxes: Either they are ruled out, or an implausibly small fraction of their energy ends up in quarks and leptons.

Kinky Brane worlds

We present a toy model for five-dimensional heterotic M theory where bulk three-branes, originating in 11 dimensions from M five-branes, are modeled as kink solutions of a bulk scalar field theory. It is shown that the vacua of this defect model correspond to a class of topologically distinct M-theory compactifications. Topology change can then be analyzed by studying the time evolution of the defect model. In the context of a four-dimensional effective theory, we study in detail the simplest such process, that is, the time evolution of a kink and its collision with a boundary. We find that the kink is generically absorbed by the boundary thereby changing the boundary charge. This opens up the possibility of exploring the relation between more complicated defect configurations and the topology of brane-world models.

Is domain formation decided before or after the transition

There is increasing evidence that causality provides useful bounds in determining the domain structure after a continuous transition. In devising their scaling laws for domain size after such a transition, Zurek and Kibble presented arguments in which causality is important both before and after the time at which the transition begins to be implemented. Using numerical simulations of kinks in 1+1 dimensions, we explain how the domain structure is determined exclusively by what happens after the transition, even though the correlation length freezes in before the transition.

The Thermodynamics of Cosmic String densities in U(1) Scalar Field Theory

We present a full characterization of the phase transition in U(1) scalar field theory and of the associated vortex string thermodynamics in 3D. We show that phase transitions in the string densities exist and measure their critical exponents, both for the long string and the short loops. Evidence for a natural separation between these two string populations is presented. In particular, our results strongly indicate that an infinite string population will only exist above the critical temperature. Canonical initial conditions for cosmic string evolution are shown to correspond to the infinite temperature limit of the theory.

Decoherence, tunneling, and noise-induced activation in a double-potential well at high and zero temperature.

We study the effects of the environment on tunneling in an open system described by a static double-well potential. We describe the evolution of a quantum state localized in one of the minima of the potential at t = 0, in both the limits of high and zero environment temperature. We show that the evolution of the system can be summarized in terms of three main physical phenomena--namely, decoherence, quantum tunneling, and noise-induced activation--and we obtain analytical estimates for the corresponding time scales. These analytical predictions are confirmed by large-scale numerical simulations, providing a detailed picture of the main stages of the evolution and of the relevant dynamical processes.

Formation of domain wall lattices

We study the formation of domain walls in a phase transition in which an

Kink-boundary collisions in a two-dimensional scalar field theory

{S}_{5}\ifmmode\times\else\texttimes\fi{}{Z}_{2}

Quantum effects after decoherence in a quenched phase transition

symmetry is spontaneously broken to

Spontaneous formation of domain wall lattices in two spatial dimensions

{S}_{3}\ifmmode\times\else\texttimes\fi{}{S}_{2}.

Out of equilibrium dynamics of quench-induced spontaneous symmetry breaking and topological defect formation

In one compact spatial dimension we observe the formation of a stable domain wall lattice. In two spatial dimensions we find that the walls form a network with junctions, there being six walls to every junction. The network of domain walls evolves so that junctions annihilate antijunctions. The final state of the evolution depends on the relative dimensions of the simulation domain. In particular we never observe the formation of a stable lattice of domain walls for the case of a square domain but we do observe a lattice if one dimension is somewhat smaller than the other. During the evolution, the total wall length in the network decays with time as