Exact quasinormal modes for a special class of black holes

Analytic exact expressions for the quasinormal modes of scalar and electromagnetic perturbations around a special class of black holes are found in d\ge3 dimensions. It is shown that, the size of the black hole provides a bound for the angular momentum of the perturbation. Quasinormal modes appear when this bound is fulfilled, otherwise the excitations become purely damped.Comment: 8 pages, no figures. Slightly updated version of the plenary talk given at the General Relativity Conference: "50 Years of FaMAF and Workshop on Global Problems in Relativity", hosted during November 2006 at FaMAF, Universidad Nacional de Cordoba, Cordoba, Argentina

Impossibility of large phase shifts via the "giant Kerr effect" with single-photon wavepackets

An approximate analytical solution is presented, along with numerical calculations, for a system of two single-photon wavepackets interacting via an ideal, localized Kerr medium. It is shown that, because of spontaneous emission into the initially unoccupied temporal modes, the cross-phase modulation in the Schrodinger picture is very small as long as the spectral width of the single-photon pulses is well within the medium's bandwidth. In this limit, the Hamiltonian used can be derived from the "giant Kerr effect" for a four-level atom, under conditions of electromagnetically-induced transparency; it is shown explicitly that the linear absorption in this system increases as the pulse's spectral width approaches the medium's transparency bandwidth, and hence, as long as the absorption probability remains small, the maximum cross-phase modulation is limited to essentially useless values. These results are in agreement with the general, causality- and unitarity-based arguments of Shapiro and co-workers.Comment: 8 pages, 2 figures, to be submitted to Physical Review

First record of Megischus brunneus Cresson, 1865 (Hymenoptera: Stephanidae) from Hispaniola, the Antilles

The occurrence of Megischus brunneus Cresson (Hymenoptera: Stephanidae) is recorded for the first time from Hispaniola, the Antilles. The species was previously known from southern Florida and Cuba. This finding further demonstrates the similarities between the Cuban and Hispaniolan biota.Se registra por primera vez la presencia de Megischus brunneus Cresson (Hymenoptera: Stephanidae) para la Hispaniola. Esta especie se conocía anteriormente del sur de la Florida y Cuba. Este hallazgo constituye otro ejemplo de las similaridades de la biota cubana e hispaniolana

Investigating the Zero Lower Bound on the Nominal Interest Rate under Financial Instability

This paper introduces a zero lower bound constraint on the nominal interest rate in a financial accelerator model with nominal and real rigidities. We .rst analyze the implicationsfor aggregate dynamics of binding the zero lower bound for shocks that depress the nominalinterest rate. We include a sudden decrease in the value of the business sector net worth and an increase in its returns volatility, as two financial shocks that originate in the endogenous credit market of the model. We then explore the effects of the central bank management of expectations and a fiscal stimulus in a deep recession scenario, where the interest rate initially binds its zero bound. We find that a commitment by the central bank to keep the interest rate low for more time than prescribed by a typical interest rate rule may indeed reduce the volatility of output and inflation. For government purchases, we find a fiscal multiplier greater than one for at least 5 quarters. This is due to the presence of the zero lower bound and the Fisher (1933)’s debt-deflation channel, which implies that government spending may reduce the business sector risk premium and thus the cost of investment.monetary economics ;