Seminar of Theory of Relativity and Gravitation
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2020-06-05 (Friday)
join us at 11:15 
Włodzimierz Piechocki (NCBJ)Quantum BKL scenario
I will present the quantum model of the asymptotic dynamics underlyingthe Belinski-Khalatnikov-Lifshitz (BKL) scenario. The classical BKL scenarioconcerns generic singularity of general relativity. The quantum BKL scenarioshows that gravitational singularity can be replaced by quantum bounce thatpresents a unitary evolution of considered gravitational system. Our resultssuggest that quantum general relativity, to be constructed, has a goodchance to be free from singularities. [for link ask wkaminsk at fuw.edu.pl]
2020-05-29 (Friday)
join us at 11:15
Cong Zhang (IFT UW)Loop quantum gravity inspired blackhole remnant
The final fate of black holes is a fascinating topic with a lot of debates. The current work addresses this issue within the framework of loop quantum gravity. By developing a numerical method to diagonalize the Hamiltonian operator of the loop quantized model of Schwarzschild interior coupled to a massless scalar field, we compute its quantum dynamics whose the semiclassical limit is proven to be compatible with the effective one. With a further constraint to get rid of the scalar field, the physical states to describe the quantum Schwarzschild black hole is obtained. The particular interesting achievement is that the spectrum of the operator, whose classical limit is proportional to the black hole mass, takes discrete value and the corresponding ground state possesses a non-vanishing eigenvalue, which supports the existence of a stable remnant of the black hole evaporation. Google Meet meeting: https://meet.google.com/kmy-agoo-juc
2020-05-22 (Friday)
join us at 11:15
Piotr Jaranowski (Faculty of Physics, University of Białystok)Applications of the ADM Formalism to the Relativistic Two-Body Problem
Recent applications of the ADM canonical formalism to the relativistic two-body problem will be presented. They concern derivation of the conservative equations of motion of compact binary systems up to at least 4th post-Newtonian (4PN) approximation of general relativity. The derivation employs Dirac delta distributions to model the compact bodies,what leads to ultra-violet divergencies which are regularized by a combination of Riesz-implemented Hadamard's partie finie approach and dimensional regularization. The infra-red ambiguity which plagues the derivation of the near-zone part of 4PN-accurate dynamics is resolved by taking into account the 4PN breakdown of the usual near-zone expansiondue to tail-transported nonlocal-in-time interaction between the bodies.The achieved results play a crucial role in the successful detections, by LIGO/Virgo detectors, of gravitational-wave signals coming from coalescing compact binaries, and for the estimation of the signals' parameters. [zoom for link ask: wkaminsk at fuw.edu.pl]
2020-05-15 (Friday)
join us at 11:15
Maciej Kolanowski (IFT UW)Penrose limits, quantum states and semiclassical Einstein equations
We will show that the Penrose limit enables one to distinguish a quantum state for any massless field on a background of plane pp-waves. Having introduced necessary geometrical tools, we shall investigate properties of the obtained state (including an energy--momentum tensor and particle production) finding that it can really be seen as the bona fide vacuum. Our results are independent of the dimension, coupling between field and curvature and, to some extent, its spin. We also comment on the limitations one encounters when working with a quantum field theory in a non-globally hyperbolic spacetime. Those states can be seen as a generalization of states previously considered by Gibbons for massive theories. Finally, we can use our vacua to find exact solutions to the semiclassical Einstein equations with quantized scalar field. In four dimensions, those are the first explicit non-Ricci-flat solutions with less than ten Killing vectors. We will discuss backreaction due to the presence of non-classical correlations and to the source being a mixed state. Joint work with W. Kamiński. [Zoom link ask wkaminsk at fuw.edu.pl]
2020-05-04 (Monday)
join us at 11:15
Eryk Buk (IFT UW)Symmetries of near-horizon geometry equations
Main point of our talk is presenting Lie point symmetries of near-horizon geometry equations with cosmological constant and source-free electromagnetic field. Before presenting these results we will recall some basic definitions of quasi-local description of black hole's horizon and constraints - aforementioned near-horizon geometry equations - put on it by Einstein-Maxwell equations. We will also explain what we mean by 'Lie point symmetries' and how one could calculate them. The talk is based on our Master Thesis. Google Meet: https://meet.google.com/sgq-kiwt-dhj
2020-04-24 (Friday)
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Naresh Dadhich (IUCAA, Pune)Buchdahl compactness limit and gravitational field energy
Meeting at zoom. For the link ask wkaminsk at fuw.edu.pl.
2020-04-17 (Friday)
join us at 11:15
Andrzej Okołów (IFT UW)Does time always slow down as gravity increases?: Some (pedagogical) remarks on gravitational time dilation
I will consider gravitational time dilation between stationary observers and will present examples which contradict the statement that ‘time slows down as gravity increases’. I will furthermore show that this statement cannot be true in general, if gravity strength is defined in an observer independent manner. I will also discuss aspects of this phenomenon, which are often omitted in textbooks on general relativity. (in order to participate ask wkaminsk at fuw.edu.pl for a link)
2020-04-08 (Wednesday)
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Tomasz Pawłowski (UWr)Loop Quantum Cosmology - a health check
Over two decades of its existence Loop Quantum Cosmology have steadily grown in both the amount of provided results and firmness of its foundations. In this talk I will review the status of both (paying special attention to the latter), focusing on those perceived as potential 'weak points' -- the elements where critical scrutiny and careful reasoning is the most crucial.
2020-04-03 (Friday)
join us at 11:30
Seminar cancelled
2020-03-27 (Friday)
join us at 11:15
Piotr Chruściel (U. Vienna)Energy of weak gravitational waves with a positive cosmological constant
I will present the derivation of a formula for the energy of weak gravitational waves, and its flux, on a de Sitter background. Based on joint work with Jahanur Hoque and Tomasz Smołka. At ZOOM.