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
I will show how to formulate consistent thermodynamics of the Lorentzian Taub-NUT spacetimes, maintaining (as recently shown relatively harmless) Misner strings. The obtained first law is of full cohomogeneity and allows for asymmetric distributions of Misner strings as well as their potential variable strengths -- encoded in the gravitational "Misner charges". Notably, the angular momentum is no longer given by the Noether charge over the sphere at infinity and picks up non-trivial contributions from Misner strings.
I consider light propagation in a plasma on a general relativistic spacetime. In the first part of my talk I review a Hamiltonian formalism for light rays in a pressure-less non-magnetised plasma and I present, on the basis of this formalism, the resulting equations for the deformation of light bundles (Sachs equations). In the second part I discuss the shadows of black holes in the presence of a plasma. The latter is of relevance in view of the forthcoming observations by the Event Horizon Telescope.
An existing canonical formulation of dynamics for a discrete system will be briefly reviewed and used to define a version of quantum field theory on a fixed simplicial lattice. Such a model may serve as an intermediate step for incorporating matter or gauge fields into lattice-based quantum gravity theories. For start, we shall consider Euclidean lattice and real scalar field. We will discuss the relation to the algebraic approach used in quantum field theory in curved spacetime and to path integral.
The fate of Cauchy horizons, such as those found inside charged black holes, is intrinsically connected to the decay of small perturbations exterior to the event horizon. As such, the validity of the Strong Cosmic Censorship conjecture is tied to how effectively the exterior damps fluctuations. By studying scalar and fermionic fields in the exterior of Reissner-Nordstrom-de Sitter black holes we identify three families of modes: one directly linked to the photon sphere, another family whose existence and timescale is closely related to the de Sitter horizon and, finally, a third family which dominates for near-extremally-charged black holes. We give a detailed description of scalar and fermionic perturbations of such black holes, and conjecture that Strong Cosmic Censorship is violated in the near extremal regime.
I will review the recent idea that, within the formalism of Cartan geometry, a spontaneously-broken gauge theory of the Lorentz group contains Ashtekar's chiral formulation of general relativity accompanied by dust, which could play the role of dark matter. The model is "pre-geometric" in the sense that the spacetime metric may be constructed only in the symmetry-broken regime; however, in principle the two phases can be smoothly connected, and spacetime be realised dynamically as a symmetry-breaking process.
We present several classes of static, spherically symmetric vacuum solutions to quadratic gravity. Notably, besides the Schwarzschild black hole, these solutions also contain a static, spherically symmetric black hole with non-trivial Ricci tensor.
The standard cosmological model has been constrained with unprecedented accuracy. Nevertheless, we are facing off new challenges. The lack of detection of Dark Matter and Dark Energy have opened to new paths. On one side, we are entering the "no-WIMP" era. On the other side, explaining the accelerated expansion of the Universe may require an extension of General Relativity. I will review the state of art of the standard cosmology while introducing new tests of both Dark Matter and Modified Gravity. I will introduce a relatively new paradigm for Dark Matter, named Ultra-light axions, and explain how to probe it with the current and forthcoming dataset. Then, I will also show some tests of the standard cosmological model and of modified gravity that near in future may be helpful to constrain/rule out models.
We define multipole moments for an arbitrary theory of gravity in terms of canonical Noether charges associated with specific residual transformations in canonical harmonic gauge, which we call multipole symmetries. We show that our definition exactly matches Thorne's mass and current multipole moments in Einstein gravity. For radiative configurations, the total multipole charges -- including the contributions from the source and the radiation -- are given by surface charges at spatial infinity. The conservation of total multipole charges is used to derive the variation of source multipole moments in terms of the radiative multipole fluxes.
Exactly 7x7 days before the centennial celebration of the Eddington-Dyson light-bending measurement, the Event Horizon Telescope Collaboration published the image resolving, for the first time, a photon sphere around a black hole. It is a supermassive black hole in the nucleus of the M87 galaxy. This galaxy is active and very interesting, and the measurement is a technological breakthrough. We will try to cover both these aspects in this discussion. In the meantime, light will continue orbiting that black hole -- just once in 7 days.
We present the so-called almost universal spacetimes. Key feature of this class of metrics is that the field equations of any generalized theory of gravity with Lagrangian constructed from the metric, the Riemann tensor and its covariant derivatives of arbitrary order reduce to only one algebraic and one differential equation (in any dimension). We prove that all Kundt spacetimes of Weyl type III and traceless Ricci type N or more special are almost universal. Explicit examples of Weyl type II almost universal metrics are also given. The considerable simplification of the field equations of higher-order gravities for almost universal spacetimes is then employed to obtain new Weyl type II, III, and N vacuum solutions to quadratic gravity and 6D conformal gravity. Necessary conditions for almost universal metrics are also studied.
Using the Sparling form and a geometric construction adapted to spacetimes with a 2-dimensional isometry group, we analyse a quasi-local measure of gravitational energy. We then study the gravitational radiation through spacetime junctions in cylindrically symmetric models of gravitational collapse to singularities. The models result from the matching of collapsing dust fluids interiors with gravitational wave exteriors, given by the Einstein-Rosen type solutions. For a given choice of a frame adapted to the symmetry of the matching hypersurface, we are able to compute the total gravitational energy radiated during the collapse and state whether the gravitational radiation is incoming or outgoing, in each case. This also enables us to distinguish whether a gravitational collapse is being enhanced by the gravitational radiation.
Work done in collaboration with Dr. Filipe Mena (University of Lisbon).
Black holes are fundamental objects in equilibrium predicted by General Relativity. However, in reality, black holes form, evolve and eventually evaporate, thus they are dynamical. Do they have a boundary? If so, where it is? For dynamical black holes, the usual Event Horizon is global and teleological, thus not well defined. One can then resort to using the local concept of closed trapped surface to try and define the surface of black holes, leading to the concepts of dynamical and trapping horizons. We will show the fundamental problems inherent to dynamical or trapping horizons. The trapped region and its boundary will then be introduced, and the difficulties in finding them highlighted. Finally, the concept of core of a black hole will be briefly discussed.
Jiří Bičák Oldřich Semerák