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Seminarium Teorii Względności i Grawitacji

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Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 11:15

Jerzy Kijowski (CFT)

"Gravitation". What is this?

The concept of an "inertial frame" will be analyzed in detail. The gravitational field can be understood as a field of local inertial frames. From this point of view, the (pseudo) metric tensor describes only a small sector of the whole theory, which also includes electromagnetism and yet another kind of matter. Mathematically, such a complete theory can be understood as realizing the old ideas of Einstein, Weyl, and Born-Infeld. The talk will be also broadcasted via zoom

Zapraszamy na spotkanie o godzinie 11:15

Tomasz Smołka (KMMF FUW)

Hamiltonian charges in spacetimes with a positive cosmological constant

Hamiltonian charges, and their fluxes, of weak gravitational waves on a de Sitter background will be discussed. We compare the results with their counterparts for Maxwell theory and scalar field respectively. Used asymptotic conditions on the linearized metric have been modeled on the asymptotic behavior of the full solutions of the Einstein equations with positive cosmological constant. Considered space of solutions is greater than the solutions which fulfill so called Bondi asymptotic conditions. This is joint work with P. T. Chruściel.

Zapraszamy na spotkanie o godzinie 11:15

Klaus Liegener (TUM, Munich)

Towards semiclassical cosmology in Loop Quantum Gravity

Unravelling the quantum modifications to the early cosmos is of the grand challenges for Quantum Gravity. With this goal in mind several techniques were developed to prepared the necessary mathematical backbone. In this talk we will expand on two of them: Classical symmetry restriction and quantum speed limit for coherent states:Symmetry restriction enables to determine in the Hamiltonian formulation whether a symmetric configuration will keep its symmetry during evolution. Will explain the theory behind it and apply it to classical General Relativity which can be restricted to cosmological configurations of Robertson-Walker type.Meanwhile, on the quantum level coherent states can be used to mimic such symmetric configurations and merging the program of expectation values in coherent states with the Quantum Speed Limit yields necessary consistency checks for any proposal of stable families of states. To showcase the strength of the developed tools, they are applied to a prominent model: the Euclidean part of the quantum scalar constraint in cosmology. There will be broadcast in room 1.40

Zapraszamy na spotkanie o godzinie 11:15

Włodzimierz Piechocki (NCBJ)

Quantum Schwarzschild spacetime

We quantize the Schwarzschild spacetime with naked singularity using the affinecoherent states quantization method. The novelty of our approach is quantization of bothtemporal and spatial coordinates. Quantization smears the gravitational singularity indicatedby the Kretschmann scalar avoiding its localization in the configuration space. This way weresolve, to some extent, the singularity problem of considered spacetime at quantum level. The talk will be also broadcasted in room 1.40

Zapraszamy na spotkanie o godzinie 11:15

Piotr Chruściel (U. Vienna)

Asymptotically hyperbolic initial data sets with higher genus infinity and negative mass

I will describe a construction of asymptotically locally hyperbolic metrics with negative scalar curvature (i.e., vacuum zero-extrinsic-curvature general relativistic initial data sets) with negative total mass. Based on joint work with Raphaela Wutte and Erwann Delay.

Zapraszamy na spotkanie o godzinie 11:15

Sebastian Szybka (UJ)

Toy models of standing gravitational waves

For most wave phenomena occurring in nature, there is a special subclass of solutions called standing or stationary waves. This class plays an important role in physics, leading to many interesting effects. Surprisingly, as pointed out twenty years ago by Hermann Bondi and Hans Stephani, not much is known about standing gravitational waves. I will present our recent results on this topic. On Zoom.

Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 11:15

Andrzej Okołów (IFT UW)

Hilbert spaces built over metrics of fixed signature

I will present two Hilbert spaces constructed over the set of all metrics of arbitrary but fixed signature, defined on a manifold. Every state in one of the Hilbert spaces is built of an uncountable number of wave functions representing some elementary quantum degrees of freedom, while every state in the other space is built of a countable numberof them. Each Hilbert space is unique up to natural isomorphisms and carries a unitary representation of the diffeomorphism group of the underlying manifold. The spaces obtained for signature (3,0) may possibly serve as a kinematical Hilbert space for canonical quantization of the ADM formalism.

Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 11:15

Andrzej Krasiński (CAMK)

Apparent horizons in Lemaitre - Tolman models

The locus of zero expansion for bundles of light rays emitted at noncentralpoints is investigated for Lemaitre - Tolman (LT) models. The three loci thatcoincide for a central emission point: (1) maxima of R along the rays, (2)expansion = 0, (3) R = 2M are all different for a noncentral emitter. Theintersection of (1) with the equatorial hypersurface (EHS) is numericallydetermined for an exemplary toy model (ETM), for two typical emitter locations.The equation of (2) is derived for a general LT model, and its intersection withthe EHS in the ETM is numerically determined for the same two emitter locations.Typically, the expansion scalar has no zeros or two zeros along a ray, andbecomes +\infty at the Big Crunch (BC) (the +\infty signifies an infinitedivergence of the ray bundle at the BC). The only rays on which \theta becomes-\infty (i.e. the rays converge) at the BC are the radial ones. For noncentralemitters in a collapsing LT model, R = 2M is still the ultimate barrier behindwhich events become invisible from outside; loci (1) and (2) are not suchbarriers. The limiting trasition from an LT model to the correspondingFriedmann model is discontinuous.

Zapraszamy do sali 1.40, ul. Pasteura 5 o godzinie 11:15

Maciej Kolanowski (IFT UW)

A gentle introduction to the extremal horizons

We will discuss the topic of the extremal horizons (it means such that their Hawking temperature vanishes). A (hopefully!) pedagogical introduction will start with the basic properties of null surfaces and then non-expanding horizons. After that, we will derive the near horizon geometry equation and present a (highly non-complete) list of the results regarding the uniqueness of its solutions, mainly in the Einstein and Einstein-Maxwell theories. Having finished that, we will present (much less known) equations that constraint connections admissible by the extremal Killing horizons. Finally, we will use them to prove certain uniqueness theorem on the extremal Reissner-Nordstrom-(A)dS black holes. Quite surprisingly, the presented result is known not to hold when the cosmological constant is vanishing. Talk partially based on arXiv:2111.00806. The talk will be also transmitted via zoom

Zapraszamy na spotkanie o godzinie 11:15

Lennart Brocki (UWr)

On symmetries and charges at spatial infinity

Following the recent work of Henneaux and Troessaert, which revisits the problem of spacetime symmetries at spatial infinity, we analyze this problem using a Bondi-type metric without determinant condition as our starting point. We find that allowing only such spacetimes that radiate only a finite amount of energy provides additional damping factors such that no parity conditions have to be imposed in order to cancel divergences in the asymptotic symplectic structure. It turns out that in this case the symmetries at spatial infinity form the BMS symmetry appended with an additional infinite set of abelian symmetries. We furthermore find that imposing the determinant condition would result in a drastic reduction of symmetries, with no spatial (super) translations present. The talk will be also broadcasted in the room 1.40.

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