ANNUAL REPORT 2006

Annual report 2006 28
called dephasing (or decoherence). The same mechanisms that cause decay of the
polarization also cause dephasing, but there exist also mechanisms that cause only
dephasing — without spin decay. The model for spin decay that we have developed
can account for dephasing as well — because it is a fully quantum mechanical model.
Earlier models for spin decay and dephasing were mostly aimed at electrons in a small
confined region in thermal equilibrium (a so-called quantum dot). Our model applies
to an open system out of equilibrium, through which an electrical current can flow.
The focus on nonequilibrium systems is a central theme of this thesis.
Nobbenhuis, S.J.B. (UU)
thesis title: The cosmological constant problem; an inspiration for new physics
advisor: prof. dr. G. ’t Hooft
date: 15 June 2006
present position: merchant banking associate programme, Corporate & Investment
Banking, Fortis, Utrecht, the Netherlands
We have critically compared different approaches to the cosmological constant problem,
which is at the overlap of elementary particle physics and cosmology. This
problem is deeply connected with the difficulties formulating a theory of quantum
gravity. After the 1998 discovery that our universe’s expansion is accelerating, the
cosmological constant problem has obtained a new dimension. We are mainly interested
in the question why the cosmological constant is so small.
We have identified four different classes of solutions: a symmetry, a back-reaction
mechanism, a violation of (some of) the building blocks of general relativity, and
statistical approaches. We conclude that so far none of the approaches gives a satisfactory
solution. A symmetry would be the most elegant solution and we study a
new symmetry under transformation to imaginary spacetime.
Ostojic, S. (UvA)
thesis title: Statistical mechanics of static granular matter
advisor: prof. dr. B. Nienhuis
date: 28 September 2006
present position: postdoctoral fellow, Department of Biology,
Supérieure, Paris, France
École Normale
Grains of sugar in a jar form a peculiar physical system which seems solid as long
as the jar is at rest, but which flows as soon as the jar is sufficiently tilted. Each
of the grains is in itself a classical solid body, the physics of which is extremely well
understood, yet the conglomeration of many of them leads to novel collective behavior.
Such assemblies of large numbers of macroscopic particles are called granular
materials. The most ubiquitous example is sand, but the definition encompasses a
variety of other systems ranging from stacks of books to pills in a conveyor belt, and
from piles of pears on a market stall to rocks in the rings of Saturn.
These assemblies of grains are most commonly found in a static state in many aspects
reminiscent of a solid. Their solid-like behavior is mainly due to the intricate network
formed by repulsive forces between particles in contact. The discrete nature and