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Relativity Seminar

usually takes place the first Thursday of each month at 13.30 IM, rear building, ground floor – please check the current calendar
Chair: Marcello Ortaggio. Deputy chair: Vojtěch Pravda
Old web page with some of the seminar history: http://gravity.math.cas.cz/ .

Black hole scalarization in Einsteinian Cubic Gravity and its compatibility with cosmic Evolution

Cristián Erices
Universidad Central de Chile
Thursday, 24. November 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
rear building, ground floor
 
We address the issue of black hole scalarization and its compatibility with cosmic inflation and big bang cosmology from an effective field theory (EFT) point of view. In practice, using a well-defined and healthy toy model which (in part) has been broadly considered in the literature, we consider how higher-order theories of gravity, up to cubic operators in Riemann curvature, fit within this context. Interestingly enough, we find that already at this minimal level, there is a nontrivial interplay between the Wilson coefficients which are otherwise completely independent, constraining the parameter space where scalarization may actually occur. Conclusively, we claim that the EFT does exhibit black hole scalarization, remaining compatible with the inflationary paradigm, and admitting general relativity as a cosmological attractor.

Graviton corrections to the Newtonian potential using invariant observables

Markus B. Fröb
Institut für Theoretische Physik, Universität Leipzig
Monday, 26. September 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
rear building, ground floor
We consider the effective theory of perturbative quantum gravity coupled to a point particle, quantizing fluctuations of both the gravitational field and the particle's position around flat space. Using a recent relational approach to construct gauge-invariant observables, we compute one-loop graviton corrections to the invariant metric perturbation, whose time-time component gives the Newtonian gravitational potential. The resulting quantum correction consists of two parts: the first stems from graviton loops and agrees with the correction derived by other methods, while the second one is sourced by the quantum fluctuations of the particle's position and energy-momentum, and may be viewed as an analog of a ``Zitterbewegung''. As a check on the computation, we also recover classical corrections which agree with the perturbative expansion of the Schwarzschild metric.

Perturbative higher curvature corrections of black holes and black strings

Marcelo Oyarzo
Departamento de Física, Universidad de Concepción
Thursday, 22. September 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
N.B. **Library**
I will talk about the corrections induced by higher curvature terms inspired in the low-energy effective action of string theory in the graviton-dilaton sector. We consider a suitable field redefinition in order to obtain a ghost free Lagrangian, which is consistent with the perturbative truncation coming from string theory. In this setup, we solve the field equations analytically for a spherically symmetric ansatz and for a slowly rotating black hole in D=4. Comments regarding the corrections to the spectrum of the Gregory-Laflamme instability due to the higher curvature terms in D=5 are also given.

High energy particle collisions near black holes and naked singularities: general explanation and classification of scenarios

Oleg B. Zaslavskii
Department of Physics and Technology, V. N. Kharkiv Karazin National University
Thursday, 16. June 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
If two particles collide near a rotating black hole, their energy in the center of mass frame E_c.m. can become unbounded under certain conditions. This is the Banados-Silk-West (BSW) effect. We give a brief review and general physical explanation from the geometric and kinematic viewpoints. In doing so, the Killing energy E of debris at infinity remains, in general, restricted. If E is also unbounded, this is called the super-Penrose process. We elucidate when such a process is possible and give full classification of corresponding relativistic objects for rotating space-times. In particular, we show that it is possible for rotating wormholes. The key role in consideration is played by the Wald inequalities. We also discuss briefly the case of a pure electric super-Penrose process near black holes and singularities. It is valid even in the flat space-time.

Symmetry operators for the conformal wave equation in rotating black hole spacetimes

Finnian Gray
University of Waterloo and Perimeter Institute for Theoretical Physics
Thursday, 19. May 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
rear building, ground floor
 
We present covariant symmetry operators for the conformal wave equation in the (off-shell) Kerr-NUT-AdS spacetimes. These operators fit within the general theory of conformally invariant operators and all mutually commute for the conformal class of related spacetimes. This guarantees the R-separability of the conformal wave equation therein. Key to constructing this physically motivated example is the principal Killing-Yano tensor and the tower of hidden symmetries that it generates.

The quest for best-case scenarios of energy extraction from black holes

Filip Hejda
CEICO, FZU AV ČR
Thursday, 5. May 2022 - 13:30 to 14:30
Institute of Mathematics of ASCR, Zitna  25, Praha 1
rear building, ground floor
 
Although Penrose process remains a textbook example of a mechanism to extract energy from black holes, it has several practical limitations. A major problem is that it requires a particle to split into two fragments with very high relative velocity. This issue can be circumvented either by considering particles influenced by electromagnetic interaction or by switching to particle collisions instead of decays. Regarding the collisional Penrose process, a lot of attention has been devoted to idealised, best-case scenarios of test particle collisions with arbitrarily high centre-of-mass collision energies in the vicinity of extremal black holes. Bañados, Silk and West (BSW) first described such a possibility for particles with fine-tuned angular momentum orbiting a maximally rotating black hole. However, even in this simplified setup, the energy that can be extracted from the black hole turned out to be subjected to unconditional upper bounds. On the other hand, no such bounds were found... more

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