Seminar of Theory of Relativity and Gravitation
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2021-03-31 (Wednesday)
join us at 11:15 
Iwo Białynicki-Birula, Marek Demiański, Jerzy LewandowskiRemembering Ted Newman
Ted Newman passed away on March 24-th. We will present recollections of our time spend with Ted and his influence on our scientific work. Zoom Meeting ID: 814 7211 6408 Passcode: 795365
2021-03-26 (Friday)
join us at 16:00
Jorge Pullin (LSU)Conditional probabilities with evolving observables and the problem of time in quantum gravity
We show that combining Rovelli's evolving constants of the motion and the Page and Wootters conditional probabilities solves problems associated with both approaches and provides correct propagators in model systems. The resulting picture leads to a fundamental loss of quantum coherence.(Notice exceptional time)
2021-03-19 (Friday)
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Shu Nakamura (Gakushuin, Tokyo)Essential self-adjointness of real principal type operators
We consider the essential self-adjointness for formally self-adjoint Schrödinger-like operators with real principal type, but not necessarily elliptic, symbols. In order to show the essential self-adjointness, we need geometric conditions even for operators on the Euclidean space with asymptotically constant coefficients. We explain how the essential self-adjointness can be proved under the null non-trapping condition. This is a joint work with Kouichi Taira, and the preprint is available at https://arxiv.org/abs/1912.05711.Zoom Meeting ID: 814 7211 6408 Passcode: 795365
2021-03-12 (Friday)
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Eryk Buk (IFT UW)Axisymmetric, extremal horizons at the presence of cosmological constant
Firstly, we will review so-called quasi-local description of black hole horizon and present geometrical constraints (called near-horizon geometry equations) induced on it by Einstein equations. Next we will present solutions to these constraints on the axisymmetric, extremal horizon, in spacetime with a cosmological constant. We will account for proper boundary conditions, introduced in order to eliminate conical singularity. Lastly we will embed this solution in Kerr-de Sitter spacetime, with special attention kept on a doubly-extremal horizon. Zoom Meeting ID: 814 7211 6408 Passcode: 795365
2021-03-05 (Friday)
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Jarosław Kopiński (CFT)A new spinorial approach to mass inequalities for black holes
In this talk I will discuss a new spinorial strategy for the construction of geometric inequalities involving the ADM mass of black hole systems in General Relativity. This approach is based on a second order elliptic equation (the approximate twistor equation) for a valence 1 Weyl spinor. In particular, I will show that the mass is bounded from below by an integral functional over a marginally outer trapped surface (MOTS) which depends on a freely specifiable valence 1 spinor. From this main inequality, by choosing the free data in an appropriate way, one obtains a new nontrivial bounds of the mass in terms of the inner expansion of the MOTS. The analysis makes use of a new formalism for the $1+1+2$ decomposition of spinorial equations. Zoom Meeting ID: 814 7211 6408 Passcode: 795365
2021-01-29 (Friday)
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Paweł Nurowski (CFT)Simple models in Penrose's Conformal Cyclic Cosmology
Zoom Meeting ID: 814 7211 6408 Passcode: 795365
2021-01-22 (Friday)
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Claus Kiefer (University of Cologne)On the Quantum Fate of Black Hole Singularities
Under general conditions, gravitational collapse in general relativity predicts the occurrence of singularities. This is the content of the singularity theorems proven by Penrose, Hawking, and others. It is generally expected that a quantum theory of gravity will avoid the occurrence of singularities. Such a theory is not yet available in complete form, but the question of singularity avoidance can be addressed in existing approaches. I shall choose the framework of quantum geometrodynamics - the direct quantization of general relativity. Attention is restricted to spherically-symmetric cases. I shall show how the classical singularity can be avoided in two simple models - the collapse of a dust shell and the collapse of a dust cloud. Zoom: 814 7211 6408 Passcode: 795365
2021-01-15 (Friday)
join us at 11:15
Bartłomiej Bąk (KMMF)The variational principles in the general relativity theory
I want to present few results from my Master Thesis concerning equvalence between the three variational principles ("pictures") used in general relativity: the metric picture (the most popular one, where the metric tensor plays role of the the primary object), the Palatni picture (here both the metric tensor and the connection are treated as a priori independent objects) and the affine picture (the least known, where the spacetime connection is primary). Such construction shows quite interesting fact: the non-metricity of the connection arises as a natural consequence of the properties of the matter field. As an example I will present the model of gravity with cosmological constant and its natural generalization which unifies the gravity with electromagnetism. The model realizes few ideas which strongly influenced XX-century theoretical physics:1) Weyl's concept of a non-metric connection, where "non-metricity" is described by the electromagnetic field, 2) Einstein's idea of unification: gravity and electromagnetism should correspond to the symmetric and the antisymmeric part, respectively, of the same geometric object. In case of our model this unifying object is the Ricci tensor of a non-metric connection, 3) Born-Infeld generalisation of electrodynamics, where the standard, linear Maxwell theory arises as an approximation of a certain non-linear, but very natural from geometric point of view, theory. At the end I shortly describe some problems of the so called "Weyl conformal gravity theories" and an interesting perturbation of the Proca theory. Zoom: 814 7211 6408Passcode: 795365
2020-12-18 (Friday)
join us at 11:15
Tomasz Trześniewski (UJ)On the spectral dimensionality of quantum space(time)s
The spectral dimension is one of definitions of the effective dimensionality of spacetime that is commonly applied to characterize quantum gravity models. A quite universal prediction is the dimensional reduction to 2 in the UV regime. The notion of spectral dimension can be seen as arising from properties of either a (fictitious) diffusion process or spectral geometry. In the latter context, there also exists the related notion of dimension spectrum. The application of both concepts may lead to various pitfalls; they are actually associated with the heat trace expansion, which is an important tool in quantum field theory. Furthermore, quantum spacetime is often described in terms of broadly understood noncommutative geometry, which requires even more care. It turns out that the spectral dimension and dimension spectrum complement each other, as can be illustrated with the use of two contrasting examples: the quantum sphere and kappa-Minkowski spacetime, with different possible choices of Laplacians that determine their geometries. In the former case, we also observe curious oscillations of dimension in the UV regime (which could leave an imprint on the cosmic microwave background).Zoom Meeting ID: 814 7211 6408Passcode: 795365
2020-12-11 (Friday)
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José Senovilla (Universidad del Pais Vasco UPV EHU, Bilbao, Spain)Singularity theorems: A critical appraisal
The 2020 Nobel prize in Physics has revived the interest in the singularity theoremsand, in particular, in the Penrose theorem published in 1965. In this talk I will brieflyreview the main ideas behind the theorems and I will proceed to an evaluation of theirhypotheses and implications. I will try to dispel some common misconceptions aboutthe theorems and their conclusions, as well as to convey some of their rarely mentionedconsequences. Several examples will be used for illustrative purposes. Zoom meeting ID: 814 7211 6408 Passcode: 795365