Tiling with almost-BPS states

In the light of recent studies of BPS triple junctions in the Wess-Zumino model we describe techniques to construct infinite lattices using similar junctions. It is shown that whilst these states are only approximately locally BPS they are nevertheless stable to small perturbations, giving a metastable tiling of the plane.Comment: replaced with verrsion accepted for PRL,4 pages, 7 figures, late

The covariance of multi-field perturbations, pseudo-susy and f_NL

We reconsider cosmological perturbation theory for multi-component scalars, enforcing covariance in field-space, and ensuring that phyical observations are independent of field re-definitions. We use the formalism to clarify some issues in the literature, and use pseudo-supersymmetry to derive exact expressions for terms of interest.Comment: 22 pages, 2 figures. Replaced with Journal versio

The fermion spectrum in braneworld collisions

In braneworld collisions fermions originally localised on one brane can be transferred to another brane, or to a space-time boundary. By modelling branes as scalar field kinks we bounce them off boundaries and study resulting effects according to a braneworld observer. Extending on our previous work, we numerically compute the spectrum of excitations of fermion modes localised on the brane and boundary, in terms of the momentum $k$ along the brane dimensions. We find that the spectrum depends strongly on collision velocity and fermion-scalar coupling. Also, high-momentum modes tend to ``fall off'' the kinks and become delocalised radiation.Comment: 13 pages, 9 figure

Book review: borderless economics: Chinese sea turtles, Indian fridges and the new fruits of global capitalism by Robert Guest

Robert Guest, Business Editor of The Economist, travels the world to make the case for the positive effects of migration and international connections, in a readable, wide-ranging new book on globalisation. Amusing, intelligent, and full of statistics, Borderless Economics is the perfect starting point for exploring new diasporas and international networks, finds Kate Saffin

Book review: the liberty of servants: Berlusconi’s Italy by Maurizio Viroli

Silvio Berlusconi turned Italy’s government into a medieval court and its people into servants, argues Maurizio Viroli. His latest book details the anti-democratic effects of the now ex-Prime Minister’s regime, and makes a passionate call for a civic rebirth in his homeland. Kate Saffin feels that Viroli’s ideas about what went so wrong in Italy need to be heeded if the country is to succeed in its present struggle and fulfil its potential

Colliding branes and big crunches

We examine the global structure of colliding domain walls in AdS spacetime and come to the conclusion that singularities forming from such collisions are of the big-crunch type rather than that of a black brane.Comment: 5 pages, 6 figure

Particle transfer in braneworld collisions

We study the behaviour of fermions localized on moving kinks as these collide with either antikinks or spacetime boundaries. We numerically solve for the evolution of the scalar kinks and the bound (i.e. localized) fermion modes, and calculate the number of fermions transfered to the antikink and boundary in terms of Bogoliubov coefficients. Interpreting the boundary as the brane on which we live, this models the ability of fermions on branes incoming from the bulk to ``stick'' on the world brane, even when the incoming branes bounce back into the bulk.Comment: 17 pages, 15 figures. New version has clearer discussion of boundary conditions, and corrects a typ