Weekly Seminars for May 2007 |
Wednesday 9, 16.00 Speaker: Dr. Li Xin-Li from Max Planck Institut fur Astrophysik - Garching, Germany Title: Gamma-Ray Burst precursors as the remnant of the thermal radiation initially trapped in the fireball Abstract: The thermal radiation trapped in an opaque GRB fireball leaks out when the fireball becomes tranparent, producing a transient event. I willshow that, generally, the transient event corresponds to a thermal precursor of a GRB. Observational aspects of the GRB precursors predicted by the model will be discussed.
Wednesday 16, 17.00 Speaker: Dr. Christian Corda from European Gravitational Obs. (EGO) - Cascina (PI), Italy Title: The production of matter from curvature in the R^{-1} theory of gravity and the longitudinal response function of interferometers Abstract: The strict analogy between scalar-tensor theories of gravity and high order gravity is well known in the literature. We show that, from the particular R^{-1} theory, it is possible to produce, in the linearized approch, particles which can be seen like massive scalar modes of gravitational waves. We also analyse the response of interferometers to this type of particles. The presence of the mass generates a longitudinal force in addition of the transverse one which is proper of the massless gravitational waves and the response of an arm of an interferometer to this longitudinal effect in the frame of a local observer will be shown. This longitudinal response function is directly connected with the function of the Ricci scalar in the particular action of this high order theory. Important conseguences from a theoretical point of view could arise from this approach, because it opens to the possibility of using the signals seen from interferometers to understand which is the correct theory of gravitation. The presence of the mass could also have important applications in cosmology because the fact that gravitational waves can have mass could give a contribution to the dark matter of the Universe.
Tuesday 22, 9.30 Speaker: Dr. Roberto De Pietri from Università di Parma and INFN Title: Dynamic instabilities of rotating relativistic stars Abstract: We present results on dynamical instabilities in rapidly rotating neutron-stars. Using 3D numerical simulations in full General Relativity, we analyse the effect of over-criticality on the development of the bar-mode instability, the effects of the stellar compactness on the position of the threshold for the onset of the dynamical bar-mode instability and the appearance of other dynamical non-axisymmetric instabilities beside the bar-mode one. By using an extrapolation technique we accurately determine the threshold for a wide range of compactnesses. Our calculations of the threshold are in good agreement with the Newtonian prediction and improve the previous post-Newtonian estimates. In addition, we find that for stars with sufficiently large mass and compactness, the m=3 deformation is the fastest growing one. For all of the models considered, the non-axisymmetric instability is suppressed on a dynamical timescale with an m=1 deformation dominating the final stages of the instability. The presented results suggest that an odd-mode deformation represents a general and late-time feature of non-axisymmetric dynamical instabilities both in full General Relativity and in Newtonian gravity.
Tuesday 22, 9.30 Speaker: Dr. Alessandro Nagar from Relativity and Gravitation Group, Politecnico di Torino Title: Coalescing Binary Black Holes: comparison between numerical and analytical approach Abstract: We discuss the transition from quasi-circular inspiral to plunge of a system of two nonrotating black holes of masses m1 and m2 in the extreme mass ratio limit m1*m2\ (m1+m2)^{2}. In the spirit of the Effective One Body (EOB) approach to the general relativistic dynamics of binary systems, the dynamics of the two black hole system is represented in terms of an effective particle of mass mu= m1m2/(m1+m2) moving in a (quasi-)Schwarzschild background of mass M= m1+m2 and submitted to an O(mu) radiation reaction force defined by Pad'e resumming high-order Post-Newtonian results. We then complete this approach by numerically computing, `a la Regge-Wheeler-Zerilli, the gravitational radiation emitted by such a particle. We then present very recent results that compare EOB-based analytical results and numerical relativity ones in the comparable mass case.
Wednesday 23, 15.00 Speaker: Dr. Alessandro Nagar from Relativity and Gravitation Group, Politecnico di Torino Title: Coalescing Binary Black Holes: comparison between analytical and numerical results. Abstract: We discuss the transition from quasi-circular inspiral to plunge of two nonrotating black holes in the extreme mass ratio limit. The dynamics is represented in terms of an effective particle of reduced mass in a (quasi) Schwarzschild background and submitted to an /O/mu radiation reaction force resumming high-order Post-Newtonian results. The gravitational radiation emitted is numerically evaluated.
Thursday 24, 16.00 Speaker: Prof. Richard Manchester from ARC Federation Fellow, Australia Telescope National Facility, CSIRO - Sydney, Australia Title: Pulsars and Gravity Abstract: Pulsars are extraordinarily good clocks. This property has been exploited in a wide range of interesting and important applications ranging from detecting intra-cluster gas in globular clusters to testing theories of gravitation. Many pulsars, especially millisecond pulsars, are in orbit around another star, providing a near-ideal gravitational laboratory. The original binary pulsar, PSR B1913+16, discovered by Hulse and Taylor in 1974 has provided the first observational evidence for the existence of gravitational waves and has verified that Einstein¹s general theory of relativity (GR) is an accurate theory of gravitation. Surveys with the Parkes radio telescope have in the past few years more than doubled the number of known pulsars. Many interesting pulsars have been discovered, including the first-known double pulsar, PSR J0737-3039A/B. This extraordinary system not only gives new insight into magnetospheric and pulse emission physics, it also an unrivalled system for testing gravitational theories. We now have significant measurements of five post-Keplerian parameters for the system, giving four independent tests of GR and verifying it at the 0.05% level. Timing measurements of an array of pulsars widely distributed on the celestial sphere can in principle give a direct detection of gravitational waves. Since mid-2004, the Parkes Pulsar Timing Array (PPTA) project has been making regular timing measurements of 20 millisecond pulsars. Simulations suggest that with a 5-year data span the PPTA will have sufficient sensitivity to detect predicted levels of the stochastic gravitational-wave background at the Earth. Already, significant constraints have been placed on the equation of state in the inflationary era and on the tension of cosmic strings in the early Universe.
Wednesday 30, 16.00 Speaker: Dr. Michele Castellana from ICRA, Phys. Dep. of "Sapienza", Rome Title: Simmetria BRST associata al gruppo di Lorentz locale per l'azione della relatività generale Abstract: La quantizzazione della relatività generale presenta una serie di difficoltà nell'imposizione dei vincoli classici a livello quantistico, per la quale la procedura à la Dirac può portare ad alcune inconsistenze. Applicando il metodo BRST (Becchi, Rouet, Stora e Tyutin) arriveremo ad una condizione di fisicità sugli stati che confronteremo con l'analoga data nella formulazione canonica di Ashtekar |