The Two-Dimensional Analogue of General Relativity

General Relativity in three or more dimensions can be obtained by taking the limit $\omega\rightarrow\infty$ in the Brans-Dicke theory. In two dimensions General Relativity is an unacceptable theory. We show that the two-dimensional closest analogue of General Relativity is a theory that also arises in the limit $\omega\rightarrow\infty$ of the two-dimensional Brans-Dicke theory.Comment: 8 pages, LaTeX, preprint DF/IST-17.9

Sub-femtosecond electron bunches created by direct laser acceleration in a laser wakefield accelerator with ionization injection

In this work, we will show through three-dimensional particle-in-cell simulations that direct laser acceleration in laser a wakefield accelerator can generate sub-femtosecond electron bunches. Two simulations were done with two laser pulse durations, such that the shortest laser pulse occupies only a fraction of the first bubble, whereas the longer pulse fills the entire first bubble. In the latter case, as the trapped electrons moved forward and interacted with the high intensity region of the laser pulse, micro-bunching occurred naturally, producing 0.5 fs electron bunches. This is not observed in the short pulse simulation.Comment: AAC 201

Thin-shell wormholes in d-dimensional general relativity: Solutions, properties, and stability

We construct thin-shell electrically charged wormholes in d-dimensional general relativity with a cosmological constant. The wormholes constructed can have different throat geometries, namely, spherical, planar and hyperbolic. Unlike the spherical geometry, the planar and hyperbolic geometries allow for different topologies and in addition can be interpreted as higher-dimensional domain walls or branes connecting two universes. In the construction we use the cut-and-paste procedure by joining together two identical vacuum spacetime solutions. Properties such as the null energy condition and geodesics are studied. A linear stability analysis around the static solutions is carried out. A general result for stability is obtained from which previous results are recovered.Comment: 16 pages, 1 figur

Collapsing shells of radiation in anti-de Sitter spacetimes and the hoop and cosmic censorship conjectures

Gravitational collapse of radiation in an anti-de Sitter background is studied. For the spherical case, the collapse proceeds in much the same way as in the Minkowski background, i.e., massless naked singularities may form for a highly inhomogeneous collapse, violating the cosmic censorship, but not the hoop conjecture. The toroidal, cylindrical and planar collapses can be treated together. In these cases no naked singularity ever forms, in accordance with the cosmic censorship. However, since the collapse proceeds to form toroidal, cylindrical or planar black holes, the hoop conjecture in an anti-de Sitter spacetime is violated.Comment: 4 pages, Revtex Journal: to appear in Physical Review

International Demand Shaping Governance Mechanisms in Brazilian Beef Agri-systems: The Case of the Three Main Processors

Drawing inspiration from international institutions and how they influence American organizations and their internal arrangements, this paper discusses the emergence of coordination between beef cattle producers and processors as a transition of neoclassic market transactions to a contractual form to participate in international trade. Three case studies from the largest Brazilian processors show the rise of strictly coordinated sub-systems to address new demands that rose from export requirements. The key finding from the case studies is that institutional shocks can drive new architectures in the system because it can generate new ones, described as strictly coordinated sub-systems that demand different organizational and technical support. In addition, the cases provide findings about organizational tolerance and its relationship with different formal institutions, based on an analysis of national and international supply in quality systems

Topological dilaton black holes

In four-dimensional spacetime, when the two-sphere of black hole event horizons is replaced by a two-dimensional hypersurface with zero or negative constant curvature, the black hole is referred to as a topological black hole. In this paper we present some exact topological black hole solutions in the Einstein-Maxwell-dilaton theory with a Liouville-type dilaton potential.Comment: 8 pages, Revtex, no figure

Holographic superfluids as duals of rotating black strings

We study the breaking of an Abelian symmetry close to the horizon of an uncharged rotating Anti-de Sitter black string in 3+1 dimensions. The boundary theory living on R^2 x S^1 has no rotation, but a magnetic field that is aligned with the axis of the black string. This boundary theory decribes non-rotating (2+1)-dimensional holographic superfluids with non-vanishing superfluid velocity. We study these superfluids in the grand canonical ensemble and show that for sufficiently small angular momentum of the dual black string and sufficiently small superfluid velocity the phase transition is 2nd order, while it becomes 1st order for larger superfluid velocity. Moreover, we observe that the phase transition is always 1st order above a critical value of the angular momentum independent of the choice of the superfluid velocity.Comment: 9 pages including 5 figures: v2: 12 pages including 7 figures; 2 figures added, discussion on free energy added; accepted for publication in JHE

Software architecture for the cloud - a roadmap towards control-theoretic, model-based cloud architecture

The cloud is a distributed architecture providing resources as tiered services. Through the principles of service-orientation and generally provided using virtualisation, the deployment and provisioning of applications can be managed dynamically, resulting in cloud platforms and applications as interdependent adaptive systems. Dynamically adaptive systems require a representation of requirements as dynamically manageable models, enacted through a controller implementing a feedback look based on a control-theoretic framework. We argue that a control-theoretic, model-based architectural framework for the cloud is needed. While some critical aspects such as uncertainty have already been taken into account, what has not been accounted for are challenges resulting from the cloud architecture as a multi-tiered, distributed environment. We identify challenges and define a framework that aims at a better understanding and a roadmap towards control-theoretic, model-based cloud architecture - driven by software architecture concerns