Seminar of Theory of Relativity and Gravitation
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2025-02-28 (Friday)
room 1.40, Pasteura 5 at 11:15 
Maciej Ossowski (IFT UW)Axis and conicity in the presence of the NUT parameter
In a spacetime where the Misner string is present - due to the NUT parameter or a time-shift - the axis of rotation is singular and cannot be a part of the spacetime manifold. I will discuss a geometric definition of the axis, the corresponding axial symmetry and the associated measure of the conical singularity - namely the conicity. It turns out that in the generic case, the conicity is necessarily observer-dependent and in particular in the spacetime with a NUT parameter there exists an observer measuring no difference of conicities for both parts of the axis. Consequently, any calculation concerning the Misner string - e.g. black holes thermodynamics or acceleration - should take the observer-dependency into account. The definition of conicity correctly reproduces the previously known results for Plebański-Demiański spacetimes.Based on a joint work with Ivan Kolář and Pavel Krtouš.
2025-01-24 (Friday)
join us at 11:15
Antony Valentini (Imperial College London)Beyond the Born rule in quantum gravity
We have recently developed a new understanding of probability in quantum gravity. In this talk we provide an overview of this new approach and its implications. Adopting the pilot-wave formulation of quantum physics, we argue that there is no Born rule at the fundamental level of quantum gravity with a non-normalisable Wheeler-DeWitt wave function. Instead, the universe is in a perpetual state of ‘quantum nonequilibrium’. Dynamical relaxation to the Born rule can occur only after the early universe has emerged into a semiclassical or Schrödinger approximation, with a time-dependent and normalisable wave function. We also show that quantum-gravitational corrections to the Schrödinger approximation can generate tiny deviations from the Born rule. The possibility of observing these effects is discussed. [Reference: A. Valentini, Beyond the Born rule in quantum gravity, Found. Phys. 53, 6 (2023).] Broadcast in room 1.40
2025-01-17 (Friday)
room 1.40, Pasteura 5 at 11:15
Maciej Dunajski (DAMTP, University of Cambridge)Integrability of quantum dots
2025-01-10 (Friday)
room 1.40, Pasteura 5 at 11:15
Luca Cafaro (IFT UW)Stellar collapse with pressure in effective loop quantum gravity
Recently, a substantial amount of work has been conducted on the spherically symmetric dust collapse within the framework of effective loop quantum gravity. I will outline those results as well as explore more general collapse scenarios with pressure. Numerical investigations show that the shell-focusing singularity characterizing the end state of any classical stellar collapse is here resolved by quantum gravitational effects and replaced by a bounce of the star. However, they also show that shell-crossing singularities remain a general feature of these models and that the inclusion of pressure does not alter the qualitative picture emerging from semiclassical models of inhomogeneous dust collapse.
2024-12-20 (Friday)
room 1.40, Pasteura 5 at 11:15
Anna Liu (The Chinese University of Hong Kong)Exploring the Universe with Gravitational Waves: From Lensing to Testing General Relativity
Since the first detection of gravitational waves in 2015, the LIGO-Virgo-KAGRA detectors observed over 90 signals, a number expected to triple by the end of the current observing run. These detections have opened a new window into the universe, enabling groundbreaking tests of gravity in the strong-field regime and offering insights into the distribution of matter in the universe. Yet, extracting the full physics potential from gravitational wave signals remains a challenge.Gravitational lensing is emerging as a powerful tool in gravitational-wave astronomy, but detecting and interpreting lensed gravitational waves presents unique challenges due to weaker signals, limited detectors, and the complex nature of gravitational-wave sources. In this talk, I will outline recent progress in identifying and analyzing lensed gravitational waves, highlight how lensing can mimic or obscure key physical signatures, and discuss the implications for astrophysics and cosmology. I will also touch on how unaccounted-for lensing effects can bias tests of general relativity, and provide a brief overview of recent advancements in performing these tests with gravitational waves. These developments underscore the exciting potential of gravitational-wave astronomy to uncover new physics and deepen our understanding of the universe.
2024-12-13 (Friday)
join us at 11:15
Koushiki Bhattacharyya (Ahmedabad University, India)Gravitational Collapse of Real Scalar Fields: Fundamentals and Applications
Gravitational collapse is a physical process that begins when an object's outward pressure—whether that of a star, galaxy, or a dust cloud—fails to counteract its own self-gravity. Without gravitational collapse in an ever-expanding Universe, there would be no stable dense objects in the sky: no stars, nebulas, galaxies, or black holes. Among the various collapse scenarios studied, the gravitational collapse of real scalar fields holds particular interest due to its dual appeal: its mathematical simplicity and its ability to model realistic matter fields in Cosmological and Astrophysical phenomena. This talk will provide a comprehensive overview of the possible end-states, both singular and regular, arising from such collapses. In the first part, I will discuss the unhindered collapse of scalar fields, exploring the nature of the resulting singularities and their physical viability, including whether they are visible or hidden from external observers. In the second part, I will explain how introducing specific non-zero potentials can halt the collapse, leading to the formation of regular end-states. Finally, I will highlight the relevance of these regular end-states in a Cosmological context.
2024-11-29 (Friday)
room 1.40, Pasteura 5 at 11:15
Sebastian Szybka (UJ)Linearized Halilsoy and Chandrasekhar standing gravitational waves
The Halilsoy and Chandrasekhar cylindrical gravitational waves are two different classes of exact solutions to the vacuum Einstein equations that describe standing gravitational waves. Both families satisfy the definition of standing gravitational waves proposed by Stephani, but only the latter class satisfies Chandrasekhar's stricter definition. The aim of our research is to compare both classes of solutions in the linearized regime.
2024-11-22 (Friday)
room 1.40, Pasteura 5 at 11:15
Wojciech Kaminski (IFT UW)Extreme isolated horizons
Based on joint work with Jurek Lewandowski arxiv:2406.20068
2024-11-15 (Friday)
room 1.40, Pasteura 5 at 11:15
Alice Boldrin (NCBJ)Time problem in perturbation theory
I will discuss the time problem in the framework of quantum fields on quantum spacetimes, considering the specific example of primordial gravitational waves propagating through a bouncing quantum Friedman universe. We will see that the dynamical variables, such as the scale factor or the amplitude of a gravitational wave, obtained from different internal clocks, evolve differently. These expectation values (background evolution) and mode functions of operators (perturbations), irrespective of the clock chosen, converge to a unique evolution for large classically-behaving universes. This is the phase space domain in which unambiguous predictions can be made.
2024-11-08 (Friday)
room 1.40, Pasteura 5 at 11:15
Prof. Jerzy Lewandowski memorial seminar
This seminar will be devoted to the life and scientific achievements of professor Jerzy (Jurek) Lewandowski, who passed away on October 8-th. Speakers: Abhay Ashtekar, Hanno Sahlmann, Paweł Nurowski, Yongge Ma, Wojciech Kamiński and Denis Dobkowski-Ryłko.