Long distance modifications of gravity in four dimensions

We discuss some general characteristics of modifications of the 4D Einstein-Hilbert action that become important for low space-time curvatures. In particular we focus on the chameleon-like behaviour of the massive gravitational degrees of freedom. Generically there is at least one extra scalar that is light on cosmic scales, but for certain models it becomes heavy close to any mass source.Comment: 4 pages, contribution to the proceedings of the Rencontres de Moriond: Contents and Structures of the Universe, March 18-25, 2006, La Thuil

Dark energy, MOND and sub-millimeter tests of gravity

We consider modifications of General Relativity obtained by adding the logarithm of some curvature invariants to the Einstein-Hilbert action. These non-linear actions can explain the late-time acceleration of the universe giving an expansion history that differs from that of a pure cosmological constant. We show that they also modify the Newtonian potential below a fixed acceleration scale given by the late-time Hubble constant times the speed of light. This is exactly what is required in MOND, a phenomenological modification of the Newtonian potential that is capable of explaining galactic rotation curves without the need to introduce dark matter. We show that this kind of modification also predicts short distance deviations of Newton's law at the sub-mm scale and an anomalous shift in the precession of the Moon's orbit around the Earth, both effects of a size that is less than an order of magnitude below current bounds.Comment: 6 pages, to appear in proceedings of the XLIrst Rencontres de Morion

Chip-to-chip optical wireless link feasibility using optical phased arrays on silicon-on-insulator

One- and two-dimensional integrated optical phased arrays (OPAs) on silicon-on-insulator have been fabricated and measured having directivities of more than 40dBi and steering ranges up to 10 degrees . These OPAs would allow data rates of 100Mbps at distances up to 0.5m

Hamiltonian simulation of the Schwinger model at finite temperature

Using Matrix Product Operators (MPO) the Schwinger model is simulated in thermal equilibrium. The variational manifold of gauge invariant MPO is constructed to represent Gibbs states. As a first application the chiral condensate in thermal equilibrium is computed and agreement with earlier studies is found. Furthermore, as a new application the Schwinger model is probed with a fractional charged static quark-antiquark pair separated infinitely far from each other. A critical temperature beyond which the string tension is exponentially suppressed is found, which is in qualitative agreement with analytical studies in the strong coupling limit. Finally, the CT symmetry breaking is investigated and our results strongly suggest that the symmetry is restored at any nonzero temperature.Comment: Updated manuscript matching its published version: more detailed continuum extrapolation of chiral condensate in section II

Dynamical mass generation by source inversion: calculating the mass gap of the chiral Gross-Neveu model

We probe the U(N) chiral Gross-Neveu model with a source-term J\l{\Psi}\Psi. We find an expression for the renormalization scheme and scale invariant source $\hat{J}$, as a function of the generated mass gap. The expansion of this function is organized in such a way that all scheme and scale dependence is reduced to one single parameter $d$. We obtain a non-perturbative mass gap as the solution of $\hat{J}=0$. A physical choice for $d$ gives good results for $N>2$. The self-consistent minimal sensitivity condition gives a slight improvement.Comment: 14 pages, 2 figure