of the Institute of Theoretical Physics

Seminar is held on Tuesdays at 13:10 pm in the lecture room of the Institute

on the 10th floor of the department building at Trója, V Holešovičkách 2, Prague 8

- November 12, 2024
- Domain Walls, Dark Matter and Gravitational Waves
*Dr. Alex Vikman*- CEICO, Czech Academy of Sciences
I will discuss cosmological domain walls with a review of their evolution and how they produce gravitational waves. Particular attention will be devoted to melting domain walls which are described by tension red-shifting with the expansion of the Universe, so that this network eventually fades away completely. These melting domain walls emit gravitational waves with the low-frequency spectral shape favoured by the recent NANOGrav 15 yrs data. This scenario involves a feebly coupled scalar field, which can serve as a promising dark matter candidate. This ultra-light dark matter has mass below 0.01 neV which is accessible through planned observations thanks to the effects of superradiance of rotating black holes. This talk is based on recent works: arXiv:2104.13722, arXiv:2112.12608, arXiv:2307.04582 and arXiv: 2406.17053 and arXiv: 2410.21971.

- November 19, 2024
- TBA
*Dr. Vojta Witzany*- ITP, Charles University
- November 26, 2024
- A New Understanding of Einstein-Rosen Bridges
*Dr. Sravan Kumar*- Institute of Cosmology and Gravitation, University of Portsmouth
In 1935, Einstein and Rosen (ER), in their seminal paper "Particle Problem in General Relativity" proposed that "a particle in the physical Universe has to be described by mathematical bridges connecting two sheets of spacetime" in order to achieve a consistent understanding of quantum fields in curved spacetime. We discuss similar conclusions independently arrived at by Schrodinger in 1956 and 't Hooft in 2016 in the context of cosmological and black hole spacetimes. Quantum effects at gravitational horizons involve inverse harmonic oscillators. We demonstrate that an analogous ER proposal of a mathematical bridge is envisioned by Berry and Keating for the description of a quantum inverse harmonic oscillator that has horizons in its phase space. Recently proposed direct-sum quantum theory reconciles the ER's vision by expressing a single quantum state in the physical space as a direct-sum of two components in the parity conjugate regions with opposite arrows of time. We discuss the implications of this framework for offering a new understanding of the ER bridges, promising a unitarity description of quantum fields in curved spacetime along with observer complementarity. Furthermore, we present compelling evidence for our new understanding of ER bridges in the form of large-scale parity asymmetric features in the cosmic microwave background. We finally discuss the implications of this new understanding for our endeavors in combining gravity and quantum mechanics.

- December 3, 2024
- TBA
*Dr. Francesco Di Filippo*- ITP, Charles University
- December 10, 2024
- No seminar
- (seminar in Czech language)
- December 17, 2024
- TBA
*Viktor Skoupý*

- October 1, 2024
- Remarks on Perturbative Quantum Gravity
*Dr. Luca Buoninfante*- Radboud University, Nijmegen
Is the lack of perturbative renormalizability in Einstein’s general relativity a failure of the perturbative QFT framework to describe quantum aspects of gravity at the fundamental level? My answer is NO. In fact, the addition of quadratic curvature invariants to the Einstein-Hilbert action makes it possible to achieve “strict” renormalizability in four dimensions. In this talk I show that strict renormalizability is still a very powerful criterion for selecting unique and predictive theories in sub-Planckian regimes, even when gravity is taken into account. After describing some aspects of Quadratic Gravity, I will compare it with other approaches and argue that it is the most predictive as it can explain new physics in the sub-Planckian regime, for example, it offers a natural explanation for the inflationary phase in the early Universe. Finally, I will make some comments on the (super-)Planckian regime.

- October 8, 2024
- Conformal Renormalization of anti-de Sitter gravity
*Prof. Rodrigo Olea*- PUCV Valparaiso, Chile
Boundary counterterms are required to cancel the divergences in the bulk action of asymptotically AdS gravity. These boundary terms are prescribed by a systematic procedure known as Holographic Renormalization, developed in the context of AdS/CFT correspondence. In this seminar, we show that, in four and six bulk dimensions, these counterterms can also be obtained from a proper embedding of Einstein in Conformal gravity (Conformal Renormalization).

- October 15, 2024
- ALREADY STARTS AT 13:00
- Classical Double Copy: Solutions, Horizons, and Penrose Limits
*Prof. Cynthia Keeler*- University of Arizona
We will review the classical double copy, which maps exact solutions of classical gauge theories like electromagnetism, to solutions of general relativity. We will cover both the Kerr-Schild and Weyl formulations of this map. Following a survey of known exact solutions, we will review why a position-space classical solution double copy is even feasible. We will briefly discuss some perturbative approaches, and then close by relating several gravitational objects (including horizons and Penrose limits) to their gauge theory analogues.

- October 15, 2024
- FROM 14:00
- Wavization, Gaussian beams and caustic avoidance
*Dr. Nezihe Uzun*- Center for Theoretical Physics, PAS, Warsaw
Geometric optics limit is known to be a good-enough approximation for the calculation of distances and image distortions in the Universe. In cosmology, we usually assume a point source that emits spherical waves and only a small section of the wavefront is accessible to the observer. In the geometric optics limit, this section is represented by a thin bundle of rays. Accordingly, the intensity profile on the transverse, observational screen is homogeneous. In this talk, I will outline the analogies between the paraxial ray optics of the Newtonian theory and the thin bundles in general relativity. I then propose a method adopted from the paraxial wave optics in order to study the wave-like effects of light propagation in general relativity. This method is sometimes referred to as ``wavization'' due to its resemblance to the semi-classical quantization techniques. The idea is to use certain phase space methods and symplectic symmetries to superpose two bundles initiated from a small yet finite size source. With this, we explore the possibility of obtaining inhomogeneous intensity profiles on the transverse plane associated with the fundamental Gaussian mode. The Gaussianity of the intensity profiles of such light beams is sourced by the spacetime curvature and not necessarily by random processes. We observe that their form is preserved throughout the propagation in curved spacetime on account of the symplectic symmetries of the underlying phase space. Finally, we show that the caustics can be avoided with this method. In addition, the extended source effect can be studied on account of Gaussian beam decomposition techniques.

- October 22, 2024
- Symmetry reduction of gravitational Lagrangians: A complete cookbook
*Dr. Ivan Kolář*- ITP, Charles University
The reduction of gravitational Lagrangians by a specified symmetry group can sometimes result in field equations that are fully equivalent to the reduced ones—if this holds for all Lagrangians, it is known as the principle of symmetric criticality (PSC), which is a property of the infinitesimal group action only. In this talk, I will explain the rigorous process of symmetry reduction of Lagrangians and present all infinitesimal group actions that satisfy PSC. I will identify the corresponding invariant metrics (and l-chains), discuss their interrelations, and analyze the simplifications allowed by the residual diffeomorphism group and Noether identities, both before and after the variation of the reduced Lagrangian.

- October 29, 2024
- ALREADY STARTS AT 13:00
- Gravitational radiation at infinity I: the case with vanishing cosmological constant
*Dr. Francisco Fernández-Álvarez*- Department of Physics, University of País Vasco, Bilbao
This is the first of two seminars on gravitational radiation at conformal infinity. In this one, the case with vanishing cosmological constant is addressed. After giving some brief historical context, the framework and tools are presented, namely: conformal completions à la Penrose and the Bel-Robinson tensor. The asymptotic version of the latter is used to characterise gravitational radiation in a new covariant, geometric and invariant way. This result is compared with the traditional condition based on the news tensor and showed to be equivalent to it. The advantages of the new criterium are stressed: in particular, its tidal nature allows one to apply it when the cosmological constant is positive - this matter is treated in the second talk of this series.

- October 29, 2024
- FROM 14:00
- Gravitational radiation at infinity II: the case with positive cosmological constant
*Prof. José M.M. Senovilla*- Department of Physics, University of País Vasco, Bilbao
In this second presentation the case with a positive cosmological constant will be presented and the main differences with the case without cosmological constant will be enumerated. The entire idea is based on a couple of vector fields, defined at infinity, that describe the energy-momentum properties of the tidal gravitational field: the asymptotic super-momentum and the asymptotic super-Poynting vectors. They will be shown to lead to a criterion on the existence/absence of gravitational radiation at infinity. The necessity of going to the tidal level will be explained. The problem of incoming radiation will be analysed under the viewpoint of the so-called asymptotic radiant super-momenta. Application to specific spacetimes -and in particular to the general Einstein-Maxwell solution with type-D black holes- will be given showing the validity of the criterion. Time permitting, some discussion on conserved charges and symmetries will also be presented.

- November 5, 2024
- No seminar (Dean's day)
- (seminar in Czech language)

- 2024: spring
- 2023: spring, fall
- 2022: spring, fall
- 2021: spring, fall
- 2020: spring, fall
- 2019: spring, fall
- 2018: spring, fall
- 2017: spring, fall
- 2016: spring, fall
- 2015: spring, fall
- 2014: spring, fall
- 2013: spring, fall
- 2012: spring, fall
- 2011: spring, fall
- 2010: spring, fall
- 2009: spring, fall
- 2008: spring, fall
- 2007: spring, fall
- 2006: spring, fall
- 2005: spring, fall
- 2004: spring, fall
- 2003: spring, fall
- 2002: spring, fall
- 2001: spring, fall
- 2000: spring, fall
- 1999: spring, fall
- 1998: spring, fall
- 1997: fall

*Oldřich Semerák*

© November 8, 2024; David Kubiznák

© November 10, 2024; generated by application