Weekly Seminars for October 2009 
Monday 12^{th} October 2009  12:00 P.M. Università "La Sapienza" Roma  Sala Lauree (Physics Dept., Old Building  1st floor)
Title: Effects of Interplanetary Dust on LISA DragFree Constellation Abstract: The relative distances between the dragfree test masses of the LISA constellation – the proposed space laser interferometer for gravitational waves – will be measured at the level of picometers over the 5 million km long arms. It is thus relevant to understand the finest details of the gravitational field acting on the test masses in their geodesic 5 years – the nominal mission duration – journey, as they are induced by all the bodies orbiting in the Solar system, even those that are usually disregarded because of their very low masses. One of them is interplanetary dust. We present a preliminary study of the perturbing effects on the orbits of LISA induced by interplanetary dust. We started from models of the distribution of interplanetary dust in Solar system, which can be inferred from the observations of zodiacal light. An analytical expression for the gravitational potential of the dust is derived and introduced in the Lagrange equations to derive the changes in the orbital paramters of the LISA probes. The differential equations have been numerically integrated and the solution compared to the unperturbed heliocentric motion of the LISA satellites. In this way we can infer the effect of interplanetary dust on the breathing motion of the constellation. Tuesday 13^{th} October 2009  12:00 P.M. Università "La Sapienza" Roma  Sala Lauree (Physics Dept., Old Building  1st floor)
Title: Dynamics of extended bodies in general relativity and an application to gravitational waves Abstract: Comportamento di un corpo esteso in relatività generale in una metrica che è soluzione esatta delle equazioni di Einstein. Vengono trattate le equazioni di Dixon in tale metrica e si vede il comportamento del corpo dopo l'interazione con l'onda gravitazionale. Il corpo presenta chiramente variazione di stato, in cui cè variazione di spin quantità di moto etc. Abbiamo associato tale cambiamento di stato ad un fenomeno astrofisico chiamato fenomeno di glitch. Abbiamo anche calcolato approssimativamente anche le ampiezze associate ai due stati di polarizzazione del onda senza tenere conto degli effetti di backreaction. Friday 16^{th} October 2009  4 P.M. Università "La Sapienza" Roma  Sala Persico (Physics Dept., Old Building  1st floor)
Title: On hydrodynamic phase of GRB sources Abstract: Here we study the evolution of an optically thick plasma composed by electrons, positrons, photons and baryons (e−, e+, γ, b). This plasma represents possible explanation for the energy emitted in GRBs and other astrophysical events which emit gamma rays. We analyzed the equations of motion and simulated the evolution for different energy to mass ratios (baryon loading) and we saw how different initial spatial distribution for energy and mass densities can change the early evolution of the plasma. We focus on the application of this work to GRBs and propose that the initial structure of this plasma and its evolution can explain structured short GRBs and can avoid the problem of injection energy which is necessary to explain some afterglows. Thursday 29^{th} October 2009  3 P.M. Università "La Sapienza" Roma  Aula Conversi (Physics Dept., Old Building  1st floor)
Title: Mach's Principle as the Universal Basis of Dynamics Abstract: Mach's principle, the suggestion that inertial motion arises from some physical effect of the masses of the universe, was the main stimulus to the creation of the general theory of relativity. However, Einstein attempted to implement Mach's proposal indirectly, not through an explicitly relational theory of time and motion. This has led to much confusion about the status and even definition of Mach's principle. The key to clarification of this confusion is the realization that Newton introduced his notions of absolute space and time in order to define velocity as the necessary first step in the creation of dynamics. The implementation of Mach's principle therefore requires a relational definition of velocity, which necessarily requires consideration of all the dynamical degrees of freedom in the universe. I shall show that there is a uniquely appropriate manner in which this can be done. It is closely related to gauge theory and leads to a direct relational derivation of general relativity, the universal light cone, and gauge theory. Mach's principle therefore provides the basis of all known fundamental dynamical theories. This insight has considerable historical and philosophical interest in connection with the debate about the absolute or relative status of motion. It is also likely to have significance for the creation of quantum gravity, the putative quantum theory of the universe. Thursday 29^{th} October 2009  4 P.M. Università "La Sapienza" Roma  Aula Conversi (Physics Dept., Old Building  1st floor)
Title: A solution to the cosmological constant problem via the quantization of unimodular gravity Abstract: A quantization of unimodular gravity is described, which results in a quantum effective action which is also unimodular, ie a function of a metric with fixed determinant. A consequence is that contributions to the energy momentum tensor of the form of the metric times a spacetime constant, whether classical or quantum, are not sources of curvature in the equations of motion derived from the quantum effective action. This solves the first cosmological constant problem, which is suppressing the enormous contributions to the cosmological constant coming from quantum corrections. We discuss several forms of unimodular gravity and put two of them, including one proposed by Henneaux and Teitelboim, in constrained Hamiltonian form. The path integral is constructed from the latter. Furthermore, the second cosmological constant problem, which is why the measured value is so small, is also addressed by this theory. We argue that a mechanism first proposed by Ng and van Dam for suppressing the cosmological constant by quantum effects obtains at the semiclassical level. Thursday 29^{th} October 2009  5 P.M. Università "La Sapienza" Roma  Aula Conversi (Physics Dept., Old Building  1st floor)
Title: SU(2) gauge structure in Quantum Gravity and the Immirzi field Abstract: It is outlined the relevance for Quantum Gravity of inferring a kinematical SU(2) gauge structure in a generic Lorentz frame. Then, the analysis of Hamiltonian constraints is performed in vacuum and in presence of matter fields. SU(2) Gauss constraints are shown to arise even though no restriction of the local Lorentz frame takes place. Finally, it is presented the application of the proposed scheme to an external Immirzi scalar field, for which a dynamical relaxation to a fixed vacuum expectation value is proposed.
Thursday 29^{th} October 2009  6 P.M. Università "La Sapienza" Roma  Aula Conversi (Physics Dept., Old Building  1st floor)
Title: Planckscale birefringence and the CMB Abstract: We show that Plackscaleinduced birefringence can produce an observable trace in CMB polarization data.
Thursday 29^{th} October 2009  6:30 P.M. Università "La Sapienza" Roma  Aula Conversi (Physics Dept., Old Building  1st floor)
Title: Probing the QuantumGravity Realm with Cold Atoms Abstract: We show that precision measurements of the recoil of ultracold atoms can provide a window of Planckscale sensitivity on the nonrelativistic limit of the dispersion relation.
