Talks

Graphity models are characterized by configuration spaces in which states correspond to graphs and Hamiltonians that depend on local properties of graphs such as degrees of vertices and numbers of shortcycles. It has been argued that such models can be useful in studying how an extended geometry might emerge from a background independent dynamical system. As statistical systems, graphity models can be studied analytically by estimating their partition functions or numerically by Monte Carlo simulations. In this talk I will present recent results obtained using both of these approaches. In particular, I will describe the transition between the high and low temperature regimes and arguethat matter degrees of freedom must play an important role in order for the graph states dominating in the low temperature regime to resemble interesting extended geometries.

It is well known that the derivation of the Bell Inequality rests on two major assumptions, usually called outcome independence and parameter independence. Parameter independence seems to have a straightforward motivation: it expresses a non-signalling requirement between space-like separated sites and is thus motivated by locality. The status of outcome independence is much les clear. Many authors have argued that this assumption too expresses a locality requirement, in the form of a \'screening off\' condition. I will argue that the assumption also admits of an entirely different interpretation, suggested by the concept of sufficiency in the general theory of statistical inference. In this view, the assumption of outcome independence can be explained as expressing the idea that the specification of the hidden variable is sufficient, i.e. it exhausts all the relevant statistical information about the measurement outcomes. In this view, the assumption has no roots in locality at all. Rather, I would claim, it stems from the assumption that there exists such an exhaustive state description in our putative hidden variable theories.

In this talk I will analyse the stochastic background of gravitational waves coming from a first order phase transition in the early universe. The signal is potentially detectable by the space interferometer LISA. I will present a detailed analytical model of the gravitational wave production by the collision of broken phase bubbles, together with analytical results for the gravitational wave power spectrum. Gravitational wave production by turbulence and magnetic fields will also be briefly discussed.

We discuss recent developments in the study of black holes and similar compact objects in string theory. The focus is on how these solutions are effected by higher-derivative terms in an effective action. The setting of this investigation is an off-shell formulation of five-dimensional supergravity, including terms of order four-derivatives whose precise form are determined by embedding this theory in M-theory. We find that certain singular solutions are fully regularized by the higher-derivative terms and that generic solutions receive calculable corrections to the entropy, or other relevant quantities such as the dual central charge. A particular solution studied corresponds to the geometry sourced by a fundamental string and may set the stage for a new and exciting example of holography.

Many numerical studies show that dark matter halos have a plethora of substructure, down to the smallest resolved scales. However, the very bottom of the Cold Dark Matter (CDM) hierarchy at a few earth masses, where the spectral index n approaches -3 and structure begins to form simultaneously on a variety of scales, remains relatively unexplored. It is possible that the subhalo mass distribution, which appears to be described by a simple power-law down to mass scales 10^6 solar masses, remains unchanged and independent of scale and n. A few studies have indicated that this appears to be the case, which is surprising considering all other statistical indicators, such as the halo mass function, as well as the internal properties of halos, such as concentration, show a dependence on n. To explore the effect of the spectral index on the subhalo mass function we ran two large, scale-free simulations, P(k)=Ak^n with n=-1 and -2.5. We find that the subhalo mass function does depend on the spectral index, with the power-law becoming shallower as n->-3.