Particle Physics, Cosmology, Quantum Gravity and String Theory (Program 1)

An important part of this research program focuses on string theory and quantum aspects of gravity and cosmology. A central theme in these studies is the correspondence between gravity theory and quantum field theories living on the boundary of space. This is called the holographic principle, or simply holography. According to this picture, one of the macroscopic dimensions of space time and one of the forces in the universe, namely gravity, are emergent phenomena in an underlying lower-dimensional microscopic description.

A concrete realization of holography, known as the AdS/CFT correspondence, involves space times with a negative cosmological constant, unlike our real world. The arguments that led to the holographic principle apply more generally, however, and suggest that one should be able to establish a holographic dictionary that applies to our own universe. The study of this question and the implications and applications of holography is the main research direction of the string theory group in Amsterdam, consisting of Jan de Boer, Kostas Skenderis, Marika Taylor and Erik Verlinde.

Jan de Boer and his group have studied an interesting holographic model for Brownian motion, aspects of supersymmetric black holes, and state counting. The research of Kostas Skenderis and his group deals with applications of holography to cosmology, with real time holographic dictionary, and with the pure spinor formulation of string theory. Marika Taylor has studied realizations of holography in non relativistic settings. Erik Verlinde has worked on a description of neutron stars in AdS/CFT and more generally on the emergence of gravity from the holographic principle.

In this same program, the group led by Eric Laenen works on physics directly linked to the LHC, in particular on the top quark sector of the Standard Model and beyond. They have developed novel theoretical methods, involving all orders in perturbation theory, and flexible Monte Carlo tools that increase the precision and variety of interesting observables. In the near future they plan to gain yet more ground there, and confront their results with LHC data.

The research of Jan Pieter van der Schaar (KdV) and his students is concentrated mainly on string cosmology, specifically the consequences of string theory for (eternal) inflation. This ranges from predictions for non-Gaussian contributions in the primordial spectrum of density perturbations to more formal aspects applying holography and the AdS/CFT correspondence in (inflationary) cosmology. Specifically they have derived new non-Gaussian constraints on initial state modifications in inflation. In addition they are working on new types of non-Gaussian analysis that in the future can be applied to the Cosmic Microwave Background temperature anisotropy data of the recently launched PLANCK satellite.

Theo Nieuwenhuizen reconsiders the question of what makes up the dark matter. His analysis of the strong and weak lensing of the galaxy cluster Abell 1689 brings a dark matter particle with mass of 1-2 eV, much below the values tested in dark matter searches. The best case is the neutrino, with mass of ca 1.5 eV, which will be tested in the KATRIN experiment in 2015. The isothermal distribution should be caused by the condensation of the initially free streaming neutrinos on the galaxy cluster, at redshift z ~ 10. Thereby they reionize most of the gas, without need of heavy, early stars. Since neutrinos can not explain explain galactic structure formation, cold dark matter was initially invoked. But hydrodynamics also predicts the often overlooked viscous structure formation. Large scale structure formation appears to go in three steps. At redshift z=5100 the proto-plasma fragments in galaxy clusters and voids; the latter have now the observed typical size of 40 Mpc. After the transition of plasma to neutral gas the Jeans mechanism makes the plasma fragment in clumps of 40,000 solar masses. They again fragment due to viscosity in milli brown dwarfs of earth mass. Thus galactic dark matter consists of dark baryons, locked up in these objects. Many observations are in line with this picture, while most of them cannot be explained with cold dark matter.

The work of Sander Bais and his group is focused on topological aspects of gauge theories. In 4 dimensions the focus is on topological Yang Mills theory and the algebraic structure underlying non-abelian versions of electric magnetic duality. In 3 dimensions, it is the study of topological order and in particular the study of topological symmetry breaking, i.e. the breaking of quantum symmetries. This subject has a significant overlap with the work of Schoutens in the condensed matter group, because of its relevance to topological quantum computation.