Editorial note to "The beginning of the world from the point of view of quantum theory"

This is an editorial note to accompany reprinting as a Golden Oldie in the Journal of General Relativity and Gravitation of the famous note by Georges Lemaitre on the quantum birth of the universe, published in Nature in 1931. We explain why this short (457 words) article can be considered to be the true "Charter" of the modern Big Bang theory.Comment: This is an editorial comment to accompany reprinting of a classical paper in the Journal of General Relativity and Gravitation. 16 pages, 2 figure

Is nonrelativistic gravity possible?

We study nonrelativistic gravity using the Hamiltonian formalism. For the dynamics of general relativity (relativistic gravity) the formalism is well known and called the Arnowitt-Deser-Misner (ADM) formalism. We show that if the lapse function is constrained correctly, then nonrelativistic gravity is described by a consistent Hamiltonian system. Surprisingly, nonrelativistic gravity can have solutions identical to relativistic gravity ones. In particular, (anti-)de Sitter black holes of Einstein gravity and IR limit of Horava gravity are locally identical.Comment: 4 pages, v2, typos corrected, published in Physical Review

Comment on ``Deterministic equations of motion and phase ordering dynamics''

Zheng [Phys. Rev. E {\bf 61}, 153 (2000), cond-mat/9909324] claims that phase ordering dynamics in the microcanonical $\phi^4$ model displays unusual scaling laws. We show here, performing more careful numerical investigations, that Zheng only observed transient dynamics mostly due to the corrections to scaling introduced by lattice effects, and that Ising-like (model A) phase ordering actually takes place at late times. Moreover, we argue that energy conservation manifests itself in different corrections to scaling.Comment: 5 pages, 4 figure

Long-term evolution of space debris under the J2 effect, the solar radiation pressure and the solar and lunar perturbations

The aim of this paper is the development of a model to propagate space debris in the geostationary ring considering the J2 effect due to the Earth oblateness, the Sun and Moon perturbations, and the solar radiation pressure. We justify the importance of considering the J2 effect when propagating space debris independently of the ratio A / m for short and long-term propagation. We study the role of the Sun and the Moon in the period and amplitude of the inclination for different values of A / m. Thanks to the Hamiltonian formulation of the problem and the use of Poincaré’s variables it is possible to express the evolution of the space debris through a simplified dynamical system. We test and validate our obtained analytical solutions with the numerical ones, computed with a powerful integrator named NIMASTEP. We analyse the improvements obtained when we include the J2 effect and the third body perturbations by a rigorous comparison with a previous model, which only considers the solar radiation pressure. Finally, we study the effect of the area-to-mass ratio on short and long-term propagation

Cavity polariton optomechanics: Polariton path to fully resonant dispersive coupling in optomechanical resonators

Resonant photoelastic coupling in semiconductor nanostructures opens new perspectives for strongly enhanced light-sound interaction in optomechanical resonators. One potential problem, however, is the reduction of the cavity Q-factor induced by dissipation when the resonance is approached. We show in this letter that cavity-polariton mediation in the light-matter process overcomes this limitation allowing for a strongly enhanced photon-phonon coupling without significant lifetime reduction in the strongly-coupled regime. Huge optomechanical coupling factors in the PetaHz/nm range are envisaged, three orders of magnitude larger than the backaction produced by the mechanical displacement of the cavity mirrors.Comment: 6 pages, 4 figure

Solidarity economy in Brazil: movement, discourse and practice

Solidarity economy initiatives constitute a worldwide phenomenon that is today at the very heart of numerous economic and social debates. They are active in a very diverse number of economic sectors, aiming for example to creating employment and income for poor and low-qualified workers, excluded from the conventional, State or private, labour markets. In the paper, we begin with presenting a Polanyian framework for the analysis of such economic activities, which enables us to develop a plural and integral conception of a productive organization, and study all these dimensions together. In a second part, we draw upon the thesis of Polanyi that economy is a political and institutionalized process and present an historic overview of the construction of the solidarity economy ”sector” in Brazil. We will put forward the hypothesis that the solidarity economy today, in Brazil as well as in Latin America in general, represents a social movement. In the third and last part of the paper, we ask ourselves if the solidarity economy movement led to a change in grassroots economic initiatives, such as the “people’s cooperatives”. We present the results of an exploratory research undertaken of 15 people’s cooperatives in the State of Rio de Janeiro in Brazil

Can a charged dust ball be sent through the Reissner--Nordstr\"{o}m wormhole?

In a previous paper we formulated a set of necessary conditions for the spherically symmetric weakly charged dust to avoid Big Bang/Big Crunch, shell crossing and permanent central singularities. However, we did not discuss the properties of the energy density, some of which are surprising and seem not to have been known up to now. A singularity of infinite energy density does exist -- it is a point singularity situated on the world line of the center of symmetry. The condition that no mass shell collapses to $R = 0$ if it had $R > 0$ initially thus turns out to be still insufficient for avoiding a singularity. Moreover, at the singularity the energy density $\epsilon$ is direction-dependent: $\epsilon \to - \infty$ when we approach the singular point along a $t =$ const hypersurface and $\epsilon \to + \infty$ when we approach that point along the center of symmetry. The appearance of negative-energy-density regions turns out to be inevitable. We discuss various aspects of this property of our configuration. We also show that a permanently pulsating configuration, with the period of pulsation independent of mass, is possible only if there exists a permanent central singularity.Comment: 30 pages, 21 figures; several corrections after referee's comments, 4 figures modifie

Velocity dominated singularities in the cheese slice universe

We investigate the properties of spacetimes resulting from matching together exact solutions using the Darmois matching conditions. In particular we focus on the asymptotically velocity term dominated property (AVTD). We propose a criterion that can be used to test if a spacetime constructed from a matching can be considered AVTD. Using the Cheese Slice universe as an example, we show that a spacetime constructed from a such a matching can inherit the AVTD property from the original spacetimes. Furthermore the singularity resulting from this particular matching is an AVTD singularity.Comment: 11 pages, 3 figures, accepted for publication in the International Journal of Modern Physics

Optical alignment and polarization conversion of neutral exciton spin in individual InAs/GaAs quantum dots

We investigate exciton spin memory in individual InAs/GaAs self-assembled quantum dots via optical alignment and conversion of exciton polarization in a magnetic field. Quasiresonant phonon-assisted excitation is successfully employed to define the initial spin polarization of neutral excitons. The conservation of the linear polarization generated along the bright exciton eigenaxes of up to 90% and the conversion from circular- to linear polarization of up to 47% both demonstrate a very long spin relaxation time with respect to the radiative lifetime. Results are quantitatively compared with a model of pseudo-spin 1/2 including heavy-to-light hole mixing.Comment: 5 pages, 3 figure