BMS Group at Spatial Infinity: the Hamiltonian (ADM) approach

New boundary conditions for asymptotically flat spacetimes are given at spatial infinity. These boundary conditions are invariant under the BMS group, which acts non trivially. The boundary conditions fulfill all standard consistency requirements: (i) they make the symplectic form finite; (ii) they contain the Schwarzchild solution, the Kerr solution and their Poincar\'e transforms, (iii) they make the Hamiltonian generators of the asymptotic symmetries integrable and well-defined (finite). The boundary conditions differ from the ones given earlier in the literature in the choice of the parity conditions. It is this different choice of parity conditions that makes the action of the BMS group non trivial. Our approach is purely Hamiltonian and off-shell throughout.Comment: 26 page

On the mathematical description of combined PMD PDL effects in optical communications and how their induced impairments can be minimized

In this paper it is shown that the correct mathematical framework of combined polarization mode dispersion and polarization dependent losses (combined PMD-PDL effects or impairments) in optical fibers is the irreducible spinor representation of the extended Lorentz Group. Combined PMD-PDL effects are shown to be formally identical to Lorentz Transformations acting on spin 1/2 zero mass particles. Since there are two different irreducible spinor representations of the restricted Lorentz Group, there must also exist two kinds of states of polarizations (SOPs) that are relevant in the description of PMD-PDL effects. The optical process that allows to convert one kind into the other is identified as optical phase conjugation. Optical phase conjugation plays the same role as the time inversion operator in the Lorentz Group representation theory. A practical and extremely important example of utility of these ideas, a technique that significantly reduces the PMD-PDL induced impairments, is presented. This technique allows to cancel the PDL part of the combined PMD-PDL impairments in a very simple and straightforward way

Celestial holography: An asymptotic symmetry perspective

We review the role that infinite-dimensional symmetries arising at the boundary of asymptotically flat spacetimes play in the context of the celestial holography program. Once recast into the language of conformal field theory, asymptotic symmetries provide key constraints on the sought-for celestial dual to quantum gravity in flat spacetimes.Comment: Invited review for Physics Reports (preprint), 79 pages, 7 figure

Exotic renormalisation group flows from black holes

In this thesis we construct a variety of black hole solutions that have planar horizons and are asymptotically Anti-de Sitter, thus relevant in the context of the gauge/gravity correspondence. We use the correspondence to investigate the renormalisation group flows of the corresponding dual field theories. Our solutions break translations along one or more spatial directions of the dual field theory, thus making them suitable for describing lattices in strongly coupled matter. After a brief introduction to the gauge/gravity duality, three different set-ups are considered. First, in the context of type IIB supergravity, our solutions are dual to anisotropic plasmas that arise from deforming an infinite family of CFTs. Second, we construct black holes in D = 11 supergravity, making our solutions relevant for ABJM theory. And finally, we take a bottom-up approach, designing a gravity model for which the black hole solutions allow us to model interesting phase transitions that are triggered by the strong breaking of translational invariance. In each scenario, we observe boomerang-like RG flows, in which the UV fixed point reappears as the IR fixed point. Similarly, all of our constructions reveal one or more intermediate scaling regimes, and we show how this can affect the scaling of some transport coefficients. For the phenomenological boomerang flows, we show that the entropic c-function is not monotonic. Furthermore, this model will reveal a novel thermal insulating ground state that has non-power-law scaling. The relation between the thermal diffusivity and butterfly velocity of these novel ground states is also studied