Colloquium

Up to 90% of matter in the Universe could be composed of heavy particles, which were non-relativistic, or 'cold', when they froze-out from the primordial soup. I will review current searches for these hypothetical particles, both via elastic scattering from nuclei in deep underground detectors, and via the observation of their annihilation products in the Sun, galactic halo and galactic center. The emphasis will be on most recent results, and on comparison with reaches of future particle colliders, such as the LHC and ILC.

The origin of the chemical elements that make up our world is one of the oldest most fundamental scientific questions. The universe after the Big Bang consisted only of hydrogen and helium with traces of lithium. All the other elements, including the carbon in our bodies, the iron, silicon, and oxygen that makes up most of our earth, have been created later by nuclear reactions in stars. However, the origin of many elements beyond iron, including gold and uranium, is still a mystery. These elements are attributed to a process called the r-process (rapid neutron capture process) which is of fundamental importance in explaining the origin of stable nuclei and isotopes beyond the iron group (A>90-100). The site of the r process is not known but supernova explosions and/or colliding neutron stars are prime suspects. The problem is that none of the models (related to these sites) can produce r-process elements in the correct proportions as we find them, for example, in the solar system or in certain very old stars. I will discuss an exciting alternative related to quark stars, a new class of compact stars that contain matter at the highest densities. Proposed observational features of quark stars, the probability of their detection, as well as some interesting connections to r-process nucleosynthesis will be presented. I will focus on an alternative based on a dynamical picture of decompressing neutron matter from the surface of quark stars in the scenario termed the Quark-Nova, which is particularly effective for producing the r-process pattern of heavy elements.

In the standard cosmological model, galaxies and large-scale structure grew by a process of gravitational instability from initial perturbations which were of the simplest statistical form imaginable: a statistically homogeneous and isotropic Gaussian random field. One of the properties of such a field is that its Fourier transform has real and imaginary parts which are independently Gaussian and consequently the phases are uniformly random. The same thing applies to the phases of the spherical harmonic coefficients involved when observed fluctuations over the celestial sphere, such as in the cosmic microwave background. Defining anything other than random phases as "weird", I discuss various aspects of cosmic weirdness and the non-randomness they produce in harmonic space. I introduce some novel methods for visualizing weirdness in CMB data and elsewhere, and discuss their relationship to more conventional statistical analyses. If I have time I will also discuss a few other interest things to do with CMB fluctuations.

We show that the origin of the dark matter and dark energy problems originates in the assumption of standard Einstein gravity that Newton's constant is fundamental. We discuss an alternate, conformal invariant, metric theory of gravity in which Newton's constant is induced dynamically, with the global induced one which is effective for cosmology being altogether weaker than the local induced one needed for the solar system. We find that in the theory dark matter is no longer needed, and that the accelerating universe data can be fitted without fine-tuning using a cosmological constant as large as particle physics suggests. In the conformal theory then it is not the cosmological constant which is quenched but rather the amount of gravity that it produces.

Richard Feynman is said to have said that philosophy of science is of no more use to scientists than ornithology is to birds. I will describe how a sociologist looks at the search for gravitational waves. Is it ornithology to birds?

Preon models enjoyed considerable popularity during the early 1980s, but have seen little progress since then. I will describe a correspondence between one of the more successful preon models and a simple game involving the twisting and braiding of ribbons, subject to straightforward topological conditions. This reproduces the fermions and gauge bosons of the standard model, as well as the electromagnetic, weak and colour interactions. The prospect that such structures may occur naturally within Loop Quantum Gravity will be discussed

The problem of vacuum energy is reviewed. The observational evidence in favor of a non-zero cosmological constant is described. I then discuss several possible explanations for how a theoretically natural huge value of vacuum energy could be adjusted down to the unnaturally tiny but observed value.

Synchronization phenomena are abundant in nature, science, engineering and social life. Synchronization was first recognized by Christiaan Huygens in 1665 for coupled pendulum clocks; this was the beginning of nonlinear sciences. First, several examples of synchronization in complex systems are presented, such as in organ pipes, fireflies, epilepsy and even in the (in)stability of large mechanical systems as bridges. These examples illustrate that, literally speaking, subsystems are able to synchronize due to interaction if they are able to communicate. Second, general physical mechanisms for synchronization and de-synchronization phenomena in coupled complex systems are presented and conditions for synchronizability are discussed. It is explained that diffusion properties give a crucial insight into this problem. I will show that the general concepts of curvature and recurrence are helpful to uncover complex synchronization. Third, applications of these new techniques are given. They range from El Nino – Monsoon interactions via electrochemical oscillators and lasers to cognitive processes during reading and to neuroscience.

Many systems take the form of networks: the Internet, the World Wide Web, social networks, distribution networks, citation networks, food webs, and neural networks are just a few examples. I will show some recent empirical results on the structure of these and other networks, particularly emphasizing degree sequences, clustering, and vertex-vertex correlations. I will also discuss some graph theoretical models of networks that incorporate these features, and give examples of how both empirical measurements and models can lead to interesting and useful predictions about the real world.

Noncommutative geometry is a more general formulation of geometry that does not require coordinates to commute. As such it unifies quantum theory and geometry and should appear in any effective theory of quantum gravity. In this general talk we present quantum groups as a microcosm of this unification in the same way that Lie groups are a microcosm of usual geometry, and give a flavour of some of the deeper insights they provide. One of them is the ability to interchange the roles of quantum theory and gravity by `arrow reversal'. Another is that noncommutative spaces typically carry a canonical 1-parameter evolution or intrinsic time created from the fundamental conflict between noncommuting coordinates and differential calculus. In physical terms one could say that quantising space typically has an anomaly for the spatial translation group and this forces the system to evolve. We give an example where we derive Schroedinger's equation in this way.

The causal set -- mathematically a finitary partial order -- is a candidate discrete substratum for spacetime. I will introduce this idea and describe some aspects of causal set kinematics, dynamics, and phenomenology, including, as time permits, a notion of fractal dimension, a (classical) dynamics of stochastic growth, and an idea for explaining some of the puzzling large numbers of cosmology. I will also mention some general insights that have emerged from the study of causal sets, the most recent one concerning the role of intermediate length-scales in discrete spacetime theories.