Comment on ``Perturbative Method to solve fourth-order Gravity Field Equations"

We reconsider the cosmic string perturbative solution to the classical fourth-order gravity field equations, obtained in Ref.\cite{CLA94}, and we obtain that static, cylindricaly symmetric gauge cosmic strings, with constant energy density, can contain only $\beta$-terms in the first order corrections to the interior gravitational field, while the exact exterior solution is a conical spacetime with deficit angle $D=8\pi\mu$.Comment: 6 pages, Revte

European accelerator-based neutrino projects

Future neutrino projects in Europe will follow two distinct time lines. On the medium term, they will be dominated by the CERN-Gran Sasso long-baseline project, with two experiments OPERA and ICARUS, mainly concentrated on $\tau$ appearance. On the longer term, several projects are under discussion. A new proton driver at CERN that accelerates a 4 MW beam to 2.2 GeV of energy would open the possibility of a low-energy super-beam, possibly sent to the French laboratory under the Frejus. A new radioactive heavy ion facility could produce a pure $\nu_e$ beam, to be used independently or simultaneously with the super-beam. In the framework of R&D for Super-Beam and Neutrino Factory, the HARP experiment is studying hadron production at low energies on various targets.Comment: talk given at NOON'0

Black hole collisions: how far can perturbation theory go?

The computation of gravitational radiation generated by the coalescence of inspiralling binary black holes is nowdays one of the main goals of numerical relativity. Perturbation theory has emerged as an ubiquitous tool for all those dynamical evolutions where the two black holes start close enough to each other, to be treated as single distorted black hole (close limit approximation), providing at the same time useful benchmarks for full numerical simulations. Here we summarize the most recent developments to study evolutions of perturbations around rotating (Kerr) black holes. The final aim is to generalize the close limit approximation to the most general case of two rotating black holes in orbit around each other, and thus provide reliable templates for the gravitational waveforms in this regime. For this reason it has become very important to know if these predictions can actually be trusted to larger separation parameters (even in the region where the holes have distinct event horizons). The only way to extend the range of validity of the linear approximation is to develop the theory of second order perturbations around a Kerr hole, by generalizing the Teukolsky formalism.Comment: 6 pages, Latex, uses moriond.sty, proceedings of the talk given at the Moriond 99' euroconferenc

Evolution of primordial magnetic fields in mean-field approximation

We study the evolution of phase-transition-generated cosmic magnetic fields coupled to the primeval cosmic plasma in turbulent and viscous free-streaming regimes. The evolution laws for the magnetic energy density and correlation length, both in helical and non-helical cases, are found by solving the autoinduction and Navier-Stokes equations in mean-field approximation. Analytical results are derived in Minkowski spacetime and then extended to the case of a Friedmann universe with zero spatial curvature, both in radiation and matter dominated eras. The three possible viscous free-streaming phases are characterized by a drag term in the Navier-Stokes equation which depends on the free-streaming properties of neutrinos, photons, or hydrogen atoms, respectively. In the case of non-helical magnetic fields, the magnetic intensity $B$ and the magnetic correlation length $\xi_B$ evolve asymptotically with the temperature $T$ as $B(T) \simeq \kappa_B (N_i v_i)^{\varrho_1} (T/T_i)^{\varrho_2}$ and $\xi_B(T) \simeq \kappa_\xi (N_i v_i)^{\varrho_3} (T/T_i)^{\varrho_4}$. Here, $T_i$, $N_i$, and $v_i$ are, respectively, the temperature, the number of magnetic domains per horizon length, and the bulk velocity at the onset of the particular regime. The coefficients $\kappa_B$, $\kappa_\xi$, $\varrho_1$, $\varrho_2$, $\varrho_3$, and $\varrho_4$, depend on the index of the assumed initial power-law magnetic spectrum, $p$, and on the particular regime, with the order-one constants $\kappa_B$ and $\kappa_\xi$ depending also on the cut-off adopted for the initial magnetic spectrum. In the helical case, the quasi-conservation of the magnetic helicity implies, apart from logarithmic corrections and a factor proportional to the initial fractional helicity, power-like evolution laws equal to those in the non-helical case, but with $p$ equal to zero.Comment: 38 pages, 4 figures, 2 tables, references added, paraghraph added, minor changes, results unchanged, to appear in Eur. Phys. J.