Strains and Jets in Black Hole Fields

We study the behaviour of an initially spherical bunch of particles emitted along trajectories parallel to the symmetry axis of a Kerr black hole. We show that, under suitable conditions, curvature and inertial strains compete to generate jet-like structures.Comment: To appear in the Proceedings of the Spanish Relativity Meeting 2007 held in Tenerife (Spain) 3 Figure

Geometric transport along circular orbits in stationary axisymmetric spacetimes

Parallel transport along circular orbits in orthogonally transitive stationary axisymmetric spacetimes is described explicitly relative to Lie transport in terms of the electric and magnetic parts of the induced connection. The influence of both the gravitoelectromagnetic fields associated with the zero angular momentum observers and of the Frenet-Serret parameters of these orbits as a function of their angular velocity is seen on the behavior of parallel transport through its representation as a parameter-dependent Lorentz transformation between these two inner-product preserving transports which is generated by the induced connection. This extends the analysis of parallel transport in the equatorial plane of the Kerr spacetime to the entire spacetime outside the black hole horizon, and helps give an intuitive picture of how competing "central attraction forces" and centripetal accelerations contribute with gravitomagnetic effects to explain the behavior of the 4-acceleration of circular orbits in that spacetime.Comment: 33 pages ijmpd latex article with 24 eps figure

Euler characteristics of moduli spaces of curves

Let Mgn be the moduli space of n-pointed Riemann surfaces of genus g. Denote by Mgn the Deligne-Mumford compactification of Mgn. In the present paper, we calculate the orbifold and the ordinary Euler characteristic of Mgn for any g and n such that n > 2-2g

Analytical determination of the two-body gravitational interaction potential at the 4th post-Newtonian approximation

We complete the analytical determination, at the 4th post-Newtonian approximation, of the main radial potential describing the gravitational interaction of two bodies within the effective one-body formalism. The (non logarithmic) coefficient a_5 (nu) measuring this 4th post-Newtonian interaction potential is found to be linear in the symmetric mass ratio nu. Its nu-independent part a_5 (0) is obtained by an analytical gravitational self-force calculation that unambiguously resolves the formal infrared divergencies which currently impede its direct post-Newtonian calculation. Its nu-linear part a_5 (nu) - a_5 (0) is deduced from recent results of Jaranowski and Sch\"afer, and is found to be significantly negative.Comment: 6 pages, no figure

Is the ECB sufficiently accountable and transparent? CEPS Working Document No. 169, July 2001

More than two years after its inception, the ECB is still perceived as lacking transparency by many academics and market participants.1 Our analysis, based on a series of indicators, suggests instead that the ECB is, at least on paper, one of the most transparent and accountable central banks. The discrepancy between theory and public perception suggests that much remains to be done within the given institutional framework to improve the transparency of the ECB. What is the best way to achieve this goal? Several suggestions have been put forward, such as publishing the detailed minutes of the ECB Governing Council meetings. This would result in shifting the true debate to informal meetings of the Governing Council, while formal meetings would only record pre-packaged consensus with no or little discussion. In our view, the best way to make the ECB more accountable is to engage it in substantive discussions about its policy. The ECB should provide more information about the background analysis that leads to policy decisions. For example, the ECB should transform its ‘staff projections’ into true inflation forecasts and it should be more open about the arguments that shape the internal debates, which precedes decisions. Accountability cannot be ensured by the ECB alone. An important role has to be played by its counterparts, such as the European Parliament, the Council of EU Finance Ministers and the public at large