Seminar of Theory of Relativity and Gravitation
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2023-06-16 (Friday)
room 1.40, Pasteura 5 at 11:15 
Andrea Bevilacqua (NCBJ)κ-deformed boost for a two-particle system
In this talk I will first introduce the κ-Minkowski spacetime, whose coordinates satisfy the AN(3) algebra. By building the group elements associated to the AN(3) algebra I will then introduce the momentum-space group manifold, and I will emphasize its relation with the κ-Poincaré algebra, which describes the symmetries of κ-Minkowski spacetime. I will then turn to the study of how particles and antiparticles behave in such spacetime, and to do so I will briefly introduce the action of a complex scalar field and show the translation charges. Starting from these results, I will then consider a particle/antiparticle pair resulting from the decay of some initial particle, both in its center of mass and in a boosted reference frame. For the latter case, I will first obtain the finite two-particle boost. The behaviour of particles and antiparticles in a boosted frame will then be highlighted, especially in comparison to the non-deformed case.
2023-05-26 (Friday)
room 1.40, Pasteura 5 at 11:15
Rafał Ćwiek (KMMF)Injectivity of Lorentzian EPRL map in spin foam models
We introduce Lorentzian EPRL map assigning invariants of the SL(2,C) group to invariants of the SU(2) group with the lowest spins. We prove that if the EPRL map is injective for the tensor product of three arbitrary irreducible representations of the SL(2,C) group from the main series of representations it is also injective for the tensor product of four representations with non-equal spins and for the tensor product of five or more representations with arbitrary spins.
2023-05-19 (Friday)
room 1.40, Pasteura 5 at 11:15
Mehdi Assanioussi (NCBJ)Graph coherent states for loop quantum gravity
I will present the construction of the graph coherent states in loop quantum gravity. The realization of these coherent states is based on the notion of graph change, as for instance the one induced by the loop quantum dynamics for Yang-Mills and gravity theories. Using a Fock-like canonical structure, these states take the form of an infinite superposition of basis network states with different graphs. I will further discuss some properties of the graph coherent states and conclude with comments on certain extensions of the proposed construction and other avenues to explore. The talk is also broadcasted via zoom
2023-05-05 (Friday)
join us at 11:15
Marek Biesiada (NCBJ)Measuring the speed of light on extragalactic objects
Speed of light is one of the most fundamental constants of Nature and its constancy lies at the foundations of theory of relativity. The first measurements of the speed of light in human history were made by astronomers: Ole Roemer and James Bradley. Since then laboratory experiments provided the most accurate measurements of this quantity. It has been only quite recently when the astronomical measurements of c reentered the scene - this time using extragalactic objects.In may talk I will review current ideas of how to measure the speed of light at cosmological scales. In particular, our results obtained so far will be presented. I will also digress on a broader context of such studies, which is the empirical support of the paradigm that the laws of physics, and fundamental constants in particular, are the same on the Earth now as they were in a distant universe across its history.
2023-04-28 (Friday)
room 1.40, Pasteura 5 at 11:15
Jerzy Kowalski-Glikman (NCBJ and UWr)Why there is (almost) nothing rather than something? The cosmological constant problem
In the talk, I will present a new understanding of the cosmological constant problem, built upon the realization that the vacuum energy density can be expressed in terms of a phase space volume. To this end, a UV-IR regularization is introduced, which implies a relationship between the vacuum energy and entropy. Combining this insight with the holographic bound on entropy then yields a bound on the cosmological constant consistent with observations. It follows that the universe is large, and the cosmological constant is naturally small, because the universe is filled with a large number of degrees of freedom.
2023-04-21 (Friday)
join us at 11:15
Jakub Gizbert-Studnicki (Institute of Theoretical Physics, Jagiellonian University)Four-dimensional Causal Dynamical Triangulations - the status report
Causal Dynamical Triangulations (CDT) is a lattice regularization of the theory of quantum gravity, based on the formalism of Regge Calculus and Feynman path integrals. Due to mathematical complexity, analytical solutions exists only in two dimensions, and the four-dimensional model is analyzed by numerical simulations. In my talk I will briefly introduce the four-dimensional CDT and present its most important results, including the dynamically emerging semiclassical universe, the non-trivial phase structure, the impact of space-time topology and the search for a continuum limit. If time permits, I will also discuss extensions of the pure gravity model to include scalar fields and prospects for further developments of the CDT approach. Broadcast in room 1.40
2023-04-14 (Friday)
room 1.40, Pasteura 5 at 11:15
Maciej Kolanowski (IFT UW)Smooth extremal horizons and other tales
We argue that an extremal limit ($T\to0$) of a generic black hole in AdS$_4$ has a singular horizon. That leaves imprints on black hole thermodynamics. Moreover, an infalling observer would notice large tidal forces, even at finite temperatures. Similar, albeit weaker, observations also hold for a positive cosmological constant but not when it vanishes. However, this fine-tuning is removed by quantum effects. Thus, even quantum-corrected extremal Kerr is singular, at least for the matter content of the standard model. This result should remain valid for any approach to quantum gravity.
2023-03-31 (Friday)
room 1.40, Pasteura 5 at 11:15
Piotr Chruściel (University of Vienna)Gluing variations
I will review various gluing/extension/embedding constructions for Einstein equations.
2023-03-24 (Friday)
room 1.40, Pasteura 5 at 11:15
Ilkka Mäkinen (NCBJ)Loop quantum gravity in diagonal gauge
We apply the master constraint method on the kinematical Hilbert space of loop quantum gravity to implement a set of gauge conditions fixing the densitized triad to be diagonal. We establish a set of solutions of the constraint operator and show that the action of the basic operators of loop quantum gravity on the resulting states takes on a greatly simplified form. This work represents a systematic derivation of the model of quantum-reduced loop gravity, proposed by Alesci and Cianfrani as an approach to probe the physical content of loop quantum gravity, from the framework of full loop quantum gravity.
2023-03-17 (Friday)
room 1.40, Pasteura 5 at 11:15
Jan Ostrowski (DBP, NCBJ)Relativistic structure formation beyond the standard perturbation theory
Cosmological structure formation is usually modelled with the standard perturbation theory which has restricted domain of applicability, the main restrictions being a small density contrast and a non-relativistic velocities. Alternatively, exact solutions of Einstein's field equations can be employed, however their integrability usually requires some high degree of symmetry which makes them unrealistic. In my talk I will present two methods, the silent universe model (a dust model with no gravitational waves) and the relativistic Zel'dovich approximation (1st order solution to the Lagrangian perturbation scheme), that partially circumvent these problems and are well suited to describe relativistic dynamics with initial conditions defined only in a statistical sense. Talk is also broadcasted via zoom