Seminar of Theory of Relativity and Gravitation
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2024-03-01 (Friday)
join us at 11:15 
Karim Mosani (Tuebingen University)Geometry and topology of trapped photon region in a class of stationary spacetimes
In Einstein’s general relativity, extremely strong gravity can trap light. In a spacetime admitting a singularity, we say that light (or a“photon”) is trapped if it neither escapes to spatial infinity nor falls into the singularity. Null geodesics govern the trajectories oflight. In the Schwarzschild spacetime with positive mass M, there exist (unstable) circular orbits of trapped photons at theSchwarzschild radius r = 3M, outside the black hole horizon at r=2M. These orbits fill a three-dimensional submanifold of topologyS^2\times R called the photon sphere of the Schwarzschild spacetime. In general, a region in spacetime that is a union of all trapped nullgeodesics is called the Trapped Photon Region (TPR) of spacetime. In this talk, we will consider a particular stationary space-time classconstructed by the Newman-Jenis algorithm. We will see that, unlike the TPR of Schwarzschild spacetime, the TPR in such spacetimes is nota submanifold of the spacetime in general. However, the lift of TPR in the phase space is a five-dimensional submanifold. This result hasapplications in various problems in mathematical relativity (This work is an extension of a similar result but in sub-extremal Kerr spacetimeby Cederbaum and Jahns- 2019). This is a joint work with Carla Cederbaum.
2024-01-26 (Friday)
room 1.40, Pasteura 5 at 11:15
Sumit Dey (IFT UW)A classical perspective into Emergent paradigm of Gravity
I will provide a survey into the notion of the emergent paradigm of gravity. It was shown by Jacobson, that one can derive the Einstein field equations from an equilibrium thermodynamic relationship implemented for local causal horizons estab-lished at a given point in spacetime. Later, it was shown by Padmanabhan that the Einstein field equations projected on a general null surface assume a thermodynamic interpretation analogous to the first law of thermodynamics for a certain physical process.Damour also showed that the field equations of general relativity on a null surface as-sume the dynamics which look structurally equivalent to Navier-Stokes equations. These strong interconnections between the gravitational field equations and the laws of thermodynamics and fluid flow form the basis of the emergent nature of gravity.
2024-01-19 (Friday)
join us at 11:15
Pankaj Joshi (Ahmedabad University)Changing Paradigms in Blackhole Physics
The collapse of massive matter clouds under their own gravity was studied by Oppenheimer and Snyder, and Datt, in the late 1930s within the framework of Einstein gravity. As a result of this gravitational collapse, a region developed in space-time from which no particles or even light could escape, which came to be known later as a `Blackhole'. Subsequently, such blackholes came in intense focus again in the 1960s, further to observations of highly energetic phenomena in the universe such as Quasars. Then, assuming that all massive stars must end up as blackholes only when they undergo gravitational collapse at the end of their lives, much development took place in blackhole physics. Further investigation of gravitational collapse phenomena however revealed that under physically realistic conditions, the collapse could result in the delay of event horizons which define a blackhole. Then the singularity developing as the collapse endstate would be visible to faraway observers. We discuss these intriguing perspectives and emerging trends in this very active arena of research today. Various findings such as quantum effects near visible or naked singularities, observational effects that may distinguish blackholes from naked singularities, the recent possibilities on regular blackholes, and others are pointed out. We also allude to the likely insights that mayemerge on quantum gravity, and implications for observational astrophysics, from these developments.
2024-01-12 (Friday)
room 1.40, Pasteura 5 at 11:15
Maciej Dunajski (University of Cambridge)Quasi Einstein Metrics on Surfaces
We prove that the intrinsic Riemannian geometry of compact cross-sections of any Einstein extremal horizon must admit a Killing vector field. This extremal horizon is a special case of a quasi-Einstein structure. We shall discuss another global example of such structures corresponding to projective metrizability.
2023-12-22 (Friday)
room 1.40, Pasteura 5 at 11:15
Maciej Kolanowski (UC, Santa Barbara)Strong Cosmic Censorship: A Quantum Update
Strong Cosmic Censorship is a (more than 50 years old) conjecture that generic solutions to the Einstein equations (with or without matter) should not contain any Cauchy horizons. We will remind the audience of the motivations behind this conjecture (and the shortcomings of these physical arguments). We will review various (non-equivalent) formulations of the conjecture. We will discuss known counterexamples and show that almost all of them are removed once one treats the perturbations as the quantum (rather than the classical) fields. Then, the only remaining exceptions are highly-spinning BTZ black holes. However, we show that (at least for certain matter contents) the inclusion of the back-reaction replaces the Cauchy horizon with the singularity, thus allowing us to proclaim that the strong cosmic censorship is in 2023 at its strongest (so far).
2023-12-15 (Friday)
room 1.40, Pasteura 5 at 11:15
Wojciech Kamiński (IFT UW)The Fefferman-Graham obstruction tensor and conformal Einstein's equations
The Fefferman-Graham obstruction tensor is an interesting conformal invariant, which vanishes on solutions of Einstein's equations. It is a complicated object and it depends on high order derivatives of the metric. However, it shares many nice properties of the Einstein tensor. In particular, it has a well-posed initial value problem ( it is hyperbolic in a suitable gauge). Due to conformal properties, vanishing of the obstruction tensor can serve as a useful tool for analyzing asymptotic behaviour of solutions of the Einstein field equations.
2023-12-08 (Friday)
room 1.40, Pasteura 5 at 11:15
Jerzy Lewandowski (IFT UW)Intrinsic uniqueness of extreme Horizons
Killing horizon extremality condition, combined with Einstein's vacuum equations, induces equations that must be satisfied by the 2-dimensional geometry of the horizon section and the rotation vector field defined on it. The study of these equations leads to the theory of the intrinsic uniqueness of extreme black holes: the spherical topology of the global section (in the rotating case), rigidity, no-hair - all those properties are proved in turn. Extreme horizons in higher dimensional spacetimes also satisfy a similar equation and many results are still valid. In addition to a review of that intrinsic theory of extremal horizon new results will be presented: local rigidity of non-rotating extremal horizon and extremal horizons of the topology of three dimensional sphere.
2023-12-01 (Friday)
join us at 11:15
Alexander Kamenshchik (Universita di Bologna, Italy)Newman-Janis algorithm’s application to regular black hole models. Regular rotating black hole: to Kerr or not to Kerr?
We examine the Newman-Janis algorithm's application to an exact regular static solution sustained by a minimally coupled scalar field with a non-standard kinetic term. Although coordinate complexification leads to a regular Kerr-like black hole, we are facing discrepancies in Einstein's equations in a fairly small domain, for which the regularizing parameter is responsible. We also present some regular cosmological solutions free of singularities, undergoing the crossing of the phantom divide line. Some general questions concerning the search for solutions of the Einstein equations and the singularities treatment are briefly discussed.
2023-11-24 (Friday)
room 1.40, Pasteura 5 at 11:15
Orest Hrycyna (NCBJ)A generic cosmological model without initial singularity and fundamental symmetry of space-time
Dynamical systems methods are used to investigate a cosmological model with non-minimally coupled scalar field and asymptotically monomial potential function. We found that for values of the non-minimal coupling constant parameter $\frac{3}{16}
2023-11-17 (Friday)
room 1.40, Pasteura 5 at 11:15
David Matejov (Charles University, Prague)Uniqueness of extremal isolated horizons and their identification with horizons of type D black holes
We investigate axisymmetric extremal isolated horizons (EIHs). They are described by an induced metric characterized by a single function. This function is subject to particular constraints following from Ricci identities in the Einstein-Maxwell theory. At first, we restrict our attention to asymptotically flat spacetimes. In the extremal case the solution is unique and depends on 5 independent parameters with direct geometrical or physical interpretation. The solution is possible to identify with extremal horizons in exact electrovacuum Plebański-Demiański spacetimes, that are all (double aligned) black holes of algebraic type D without a cosmological constant. Several interesting subclasses include accelerating extremely charged Reissner-Nordström black hole (C-metric) or extremal accelerating Kerr-Newman. As a natural extension, I will generalize the previous results to asymptotically (anti-) de Sitter spacetimes with non-zero cosmological constant and identify the EIHs geometry with the corresponding horizon geometry of a black hole in a special case of the famous Kerr-Newman-NUT-(A)dS metric.