Seminar of Theory of Relativity and Gravitation
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2020-12-04 (Friday)
join us at 11:15 
Dipanjan Dey (CHARUSAT, India)Shadow of naked singularity
Recent observation of the shadow of the Messier 87 (M87) galactic center by the Event Horizon Telescope (EHT) group has triggered a great interest to investigate the causal structure of spacetime around the galactic center. From the recent investigations on gravitational lensing, it is now established that the shadow is not the signature of a black hole alone, it can also be cast by timelike or nulllike naked singularities in the presence of a photon sphere. In my talk, I will show a naked singularity, even without a photon sphere, can cast a shadow. I will show some distinguishable features of the shadow cast by timelike and nulllike naked singularities in the absence of the photon sphere. These novel features of the shadows of nulllike and timelike naked singularities may help us to distinguish between black holes and naked singularities observationally.Zoom Meeting ID: 814 7211 6408Passcode: 795365
2020-11-27 (Friday)
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Michele Grasso (CFT)Light propagation in Numerical Relativity with BiGONLight
The improvement in the astronomical data quality has pushed for a better modelling and prediction of cosmological observables. In this scenario, numerical simulations have covered a crucial role to test models and compare theoretical predictions with observations. During my talk, I will present a new numerical tool to study light propagation in cosmological simulations called BiGONLIght. The mathematical machinery is based on the covariant formulation of the geometric optics using the bilocal geodesic operators (BGO) recently proposed in [PRD, 99(6), 064038]. The analysis of the optical properties of complicated spacetimes using the BGO formalism constitutes an efficient approach to probe the dynamics of the inhomogeneities on the large scale.Zoom meeting ID: 814 7211 6408 Passcode: 795365
2020-11-20 (Friday)
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Piotr Waluk (CFT)Quasi-local mass of weak gravitational field
The widely accepted ADM expression for the energy of an asymptotically flat spacetime satisfies a "natural" consistency test - its second variation is equal to the canonical hamiltonian functional for linearized gravity on a Minkowski background. A viable quasi-local mass candidate should possess a similar property, namely - its second-order approximation should equal the hamiltonian of the well-understood linearized theory.We show that the Hawking quasi-local mass passes this test, provided that certain gauge conditions are fulfilled. These conditions seem to correspond to the shape of the boundary of the examined region and provide a suggestion about the way the problem of quasi-local energy should be posed. Zoom meeting ID: 814 7211 6408Passcode: 795365
2020-11-13 (Friday)
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Jose Luis Jaramillo (Institut Mathematiques de Bourgogne)On the stability of black hole quasi-normal modes
Zoom meeting ID: 814 7211 6408 Passcode: 795365
2020-11-06 (Friday)
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Maciej Dunajski (Cambridge)Roger Penrose's Twistor Theory
Zoom Meeting ID: 814 7211 6408 Passcode: 795365
2020-10-30 (Friday)
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Daniele Pranzetti (Perimeter Institute)Edge modes of gravity
In this seminar I will spell out a new program for quantum gravity, grounded in the notion of corner symmetry algebra and its representations. This requires the understanding of the classical and quantum nature of edge modes and symmetries in gravitational systems. The goal of this analysis is to: i) achieve a clear understanding of how different formulations of gravity provide non-trivial representations of different sectors of the corner symmetry algebra, and ii) set the foundations of a new proposal for states of quantum geometry as representation states of this corner symmetry algebra. In the first part, I will explain how different formulations of gravity, in both metric and tetrad variables, share the same bulk symplectic structure but differ at the corner, and in turn lead to inequivalent representations of the corner symmetry algebra. In the second part, I will perform a detailed analysis of the corner symplectic potential and symmetries of Einstein-Cartan-Holst gravity, use this to provide a new look at the simplicity constraints, and tackle the quantization. Zoom meeting ID: 814 7211 6408 Passcode: 795365
2020-10-23 (Friday)
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Marek Demiański (IFT UW)Nobel Prize in Physics 2020
Zoom meeting ID: 814 7211 6408 Passcode: 795365
2020-10-16 (Friday)
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Reinhard Meinel (University of Jena)Quasi-stationary routes to black holes: exterior and interior perspective
Zoom. Meeting ID: 814 7211 6408 Passcode: 795365