Seminar of Theory of Relativity and Gravitation
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2022-06-10 (Friday)
room 1.40, Pasteura 5 at 11:15 
Naresh Dadhich (IUCAA)On the Kerr-NUT spacetime and its duality
It turns out that the Kerr-NUT metric is the most general electrovacuum solution of axially symmetric spacetime admitting separable of variables for the Hamilton-Jacobi and the Klein-Gordon equations. The metric admits an interesting invariance when mass and NUT parameters and radial and angle coordinates are interchanged. Note that mass is gravoelectric charge while NUT parameter is gravomagnetic charge, and the duality transformation is therefore between gravoelectric and gravomagnetic charges along with the corresponding coordinates, radial and angle. In particular, Kerr and massless rotating NUT solutions are dual to each-other.
The seminar will also be broadcasted via Zoom, for the link contact Maciej Ossowski.
2022-05-27 (Friday)
room 1.40, Pasteura 5 at 11:15
Mikołaj Korzyński (CFT)Weighing spacetime along the line of sight using times of arrival of electromagnetic signals
I will discuss a new method of determining the spacetime curvature and matter density along the line of sight using variations of times of arrival (TOA) of electromagnetic signals, measured in the vicinity of two given points. We measure the variations of the TOA's up to quadratic order in the displacements of the source and the receiver with the help two groups of synchronised clocks, equipped with transmitters and receivers. The TOA's are affected by the gravitational field along the line of sight and it turns out that it is possible to determine from this data the first two moments of the matter density profile along the line of sight. The measurement is insensitive to the states of motion of the two groups of clocks, their angular positions or the influence of masses off the line of sight. Potential applications include precise binary pulsar timing and dark matter search.
2022-05-20 (Friday)
room 1.40, Pasteura 5 at 11:15
Maciej Dunajski (Cambridge)Quantum state reduction, and Newtonian twistor theory
The seminar will be also broadcasted via zoom.
2022-05-13 (Friday)
room 1.40, Pasteura 5 at 11:15
Cong Zhang (IFT UW)Fermion coupling to loop quantum gravity
In the model of fermion field coupled to loop quantum gravity, we consider the Gauss and the Hamiltonian constraints. According to the explicit solutions to the Gauss constraint, the fermion spins and the gravitational spin networks intertwine with each other so that the fermion spins contribute to the volume of the spin network vertices. For the Hamiltonian constraint, the regularization and quantization procedures are presented in detail. By introducing an adapted vertex Hilbert space to remove the regulator, we propose a diffeomorphism covariant graph-changing Hamiltonian constraint operator. This operator shows how fermions move in the loop quantum gravity spacetime and simultaneously influences the background quantum geometry. The seminar is also broadcasted via zoom
2022-04-29 (Friday)
join us at 11:15
Saurya Das (University of Lethbridge, Canada)Quantum Raychaudhuri Equation: Implications for spacetime singularities and the quantum origin of Lambda
The Raychaudhuri equation predicts the convergence of geodesics and gives rise to the singularity theorems. The quantum Raychaudhuri equation (QRE), on the other hand, shows that quantal trajectories, the quantum equivalent of geodesics, do not converge and are not associated with any singularity theorems. Furthermore, the QRE gives rise to the quantum corrected Friedmann equation. The quantum correction is dependent on the wavefunction of the perfect fluid whose pressure and density enter the Friedmann equation. We show that for a suitable choice of the wavefunction this term can give rise to a small positive cosmological constant, just as observed in nature. We discuss implications. Seminar is also broadcasted in room 1.40
2022-04-22 (Friday)
join us at 11:15
Konstantin Eder (FAU, Erlangen)Super Cartan geometry, loop quantum supergravity and applications
This talk is devoted to the geometric approach to supergravity and applications in the framework of loop quantum gravity (LQG). Among other things, this approach leads to a reformulation of the theory in which (part of) supersymmetry manifests itself in terms of a gauge symmetry. Using the interpretation of supergravity in terms of a super Cartan geometry, we will derive the Holst variant of the MacDowell-Mansouri action for N-extended AdS supergravity in D=4 for arbitrary values of the Barbero-Immirzi parameter - a free parameter of the theory. We will show that these actions provide unique boundary terms that ensure local supersymmetry invariance at boundaries. The chiral case is special: The action is invariant under an enlarged gauge symmetry, and the boundary theory is a super Chern-Simons theory. The action also implies boundary conditions that link the super electric flux through, and the super curvature on, the boundary. Making use of the enlarged gauge symmetry, we propose a quantization of the theory generalizing standard tools of LQG. Finally, we will give an outlook on applications of these results in the context of supersymmetric black holes and quantum cosmology. There, the enhanced gauge symmetry proves to be a promising tool which in the future may shed a lot of insights on how to relate results from LQG and superstring theory. The seminar is also broadcasted in room 1.40
2022-04-08 (Friday)
join us at 11:15
Claus Kiefer (University of Cologne)Time in quantum cosmology
Time is absolute in standard quantum theory and dynamical in general relativity. The combination of the two theories into a theory of quantum gravity thus leads to a `problem of time'. In my talk, I shall investigate those consequences for the concept of time that can be drawn without a detailed knowledge of quantum gravity.The only assumptions are the experimentally supported universality of the superposition principle and the recovery of general relativity in the classical limit. Attention is focused on quantum cosmology - the application of quantum theory to the Universe as a whole. Among the consequences are the fundamental timelessness of quantum cosmology, the approximate nature of a semiclassical time, and the correlation of entropy with the size of the Universe.The last consequence gives rise to the irreversibility of our world.Ref.: C. Kiefer and P. Peter, Universe 2022, 8, 36 [arXiv:2112.05788] and references therein. The talk will be broadcasted in the room 1.40
2022-04-01 (Friday)
room 1.40, Pasteura 5 at 11:15
Jerzy Kowalski-Glikman (UWr & NCBJ)Charges of Supergravity and Positive Energy Theorem
In my talk I will recall the proof of Positive Energy Theorem for asymptotically flat spacetime, which takes as a starting point the supersymmetry algebra. Then I will describe a new research program, aimed at better understanding of energy (and other conserved charges) associated with finite regions in (super)gravity. The talk wil be also broadcasted via zoom
2022-03-25 (Friday)
room 1.40, Pasteura 5 at 11:15
Przemysław Małkiewicz (NCBJ)Hamiltonian formalism for anisotropic cosmological perturbation theory
I will apply the Dirac procedure for constrained systems to the Arnowitt–Deser–Misner formalism for cosmological perturbation theory in the Bianchi I universe. I will carefully discuss the issue of gauge-fixing, gauge transformations and spacetime reconstruction. I will find the so called Kuchar parametrization to be well suited for this purpose. Then I will shift my attention to the peculiarities of cosmological perturbations in anisotropic backgrounds such as the evolution of the plane wave wavefronts and the couplings between gravitational waves and matter perturbations. I will show that there exist coordinate systems in which (a polarization mode of) a gravitational wave is represented by the density metric perturbation.Seminar will be also broadcasted via zoom
2022-03-18 (Friday)
room 1.40, Pasteura 5 at 11:15
Plebański as he was
Programme:1. Piotr Kielanowski - Introduction, handing over of Plebański's lecture notes to the Library, 15 minutes.2. Mielnik's movie about Jerzy Plebański, 10 minutes.3. Katarzyna Nurowska and Paweł Nurowski - Inauguration of the Jerzy Plebański's web page, 15 minutes.4. Maciej Przanowski - Scientific heritage of Jerzy Plebański, 15 minutes.5. Iwo Białynicki-Birula - My collaboration with Jerzy Plebański, 15 minutes.6. Andrzej Krasiński - My collaboration with Jerzy Plebański, 15 minutes.7. Marek Demiański - My collaboration with Jerzy Plebański, 15 minutes.The seminar will be also broadcasted via zoom.