Relativity Seminar
of the Department 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

Next seminars:

July 17, 2018
!!! at 10:40 !!!
Searching for dark matter candidates with black hole superradiance
David Kubizňák, Ph.D.
Perimeter Institute, Waterloo, Ontario, Canada

The ultralight massive vector fields, such as dark photons, are a feature of many beyond standard model physics scenarios, string theory for example. Together with axion-like scalars such fields provide a compelling candidate for cold dark matter. However, due to their tiny mass and weak coupling, direct searches for such particles are challenging, and alternative methods for detection are sought. One such promising method is to look for the superradiant instabilities of rotating black holes, leading to a formation of the bosonic condensate outside the horizon, and a subsequent gravitational wave emission and black hole spin-down. Consequently, gravitational and electromagnetic wave observations of astrophysical black holes provide direct constraints on this type of dark matter candidates.

July 17, 2018
at 13:10
Dilatonic imprints on exact gravitational wave signatures
David Kubizňák, Ph.D.
Perimeter Institute, Waterloo, Ontario, Canada

By employing the moduli space approximation, we analytically calculate the gravitational wave signatures emitted upon the merger of two extremally charged dilatonic black holes. We probe several values of the dilatonic coupling constant a, and find significant departures from the Einstein-Maxwell (a=0) counterpart studied previously. For (low energy) string theory black holes (a=1) there are no coalescence orbits and only a memory effect is observed, whereas for an intermediate value of the coupling (a=1/sqrt(3)) the late-time merger signature becomes exponentially suppressed, compared to the polynomial decay in the a=0 case without a dilaton. Such an imprint shows a clear difference between the case with and without a scalar field (as for example predicted by string theory) in black hole mergers.

Previous seminars:

March 6, 2018
Extreme light physics frontier
Prof. Johann Rafelski
Department of Physics, University of Arizona, Tuscon

New foundational physics opportunities arising in the context of extreme light technologies being implemented at ELI. I will show how particle dynamics at the extreme acceleration condition relate to the understanding of vacuum structure in the presence of extreme EM fields. I will argue that we will be able to connect theoretical ideas about radiation-reaction improved forces to feasible experiments. I will describe efforts to improve classical relativistic dynamics both in radiation reaction and magnetic moment regime.

March 13, 2018
The Bekenstein bound and an emergent picture of gravity
Dr. Giovanni Acquaviva

The Bekenstein bound states that the entropy of a system contained in a certain volume is bounded from above by the entropy of a black hole with corresponding surface area. We relate such universal bound to the existence of fundamental degrees of freedom and provide model-independent considerations about their features. In particular, both geometry and fields propagating on it are seen as phenomena emergent from the more fundamental dynamics, in analogy with many examples in condensed matter physics. An immediate consequence is that, even though the fundamental evolution is considered unitary, the fields develop an entanglement with the spacetime geometry, hence leading to an effective non-unitary evolution on the emergent level. We exemplify some consequences of this scenario by providing a toy-model of black hole evaporation, in which the entanglement between geometry and fields is interpreted at our low-energy scales as an effective loss of information in Hawking radiation.

March 20, 2018
Gravitational memory effects at the event horizon
Dr. Kepa Sousa
Institute of Theoretical Physics, Autonomous University of Madrid / Spanish National Research Council

We discuss the memory effect at the horizon of a non-extremal black hole arising from a burst of gravitational radiation. We show that, generically, the burst of radiation induces a change in all the mass and angular momentum multipoles of the horizon, but only the changes in the quadrupole and higher-order multipoles are necessary to fully characterise the memory effect: permanent shifts of the velocity of massless particles travelling along the horizon induced by the burst of radiation. We argue that the quadrupole and higher-order horizon multipoles are not intrinsic degrees of freedom of the horizon, instead they should be understood as distortions induced by the presence of matter and radiation external to the black hole. We conclude that, contrary to previous statements in the literature, these memory effects on the horizon cannot "store holographically" the information about the state of particles falling to the black hole.

March 27, 2018
Cosmic strings in axisymmetric black hole spacetimes; the C-metric "engines"
Dr. David Kofroň

The interpretation of so-called cosmic string in black hole spacetimes has settled down to an unsatisfactory state. I try to provide a different model for these cosmic strings by explicit construction of these spacetimes from the Bonnor rocket solution. It is shown that the correct stress-energy tensor is that of null dust with a rather strange energy density - first derivative of Dirac delta distribution. We will discuss the Schwarzschild solution and the C-metric. In the latter case we will show that there is a momentum flux through the cosmic string, causing the acceleration of the black hole.

April 3, 2018
On the Hamiltonian formalism for spinning bodies in curved space-time
Mgr. Vojtěch Witzany
ZARM, University of Bremen

When a spinning, stellar-mass black hole is orbiting a super-massive black hole, it will be influenced both by gravitational back-reaction and by a spin-curvature coupling to the background. This leads to a so-called extreme mass ratio inspiral, the gravitational-wave signature of which is hoped to be detected by the space-based detector LISA. In this talk, I will present Hamiltonians that describe the motion of the spinning body in a given space-time under various supplementary conditions. Additionally, I will discuss the motivation of the corresponding non-canonical Poisson structure, its extensions to quadrupole order, and the transformation to canonical coordinates on the phase space.

April 10, 2018
No seminar (shifted to Friday)
(seminar in Czech language)
April 13, 2018
!!! ON FRIDAY (at 13:10) !!!
Neutral hydrogen as a cosmological probe
Dr. Aurélie Pénin
University of the Western Cape, South Africa

The distribution of neutral hydrogen (HI) will be mapped over unprecedented volumes of Universe thanks to the intensity mapping technique with several forthcoming experiments such as the Square Kilometre Array and its South African pathfinder, MeerKAT. Unveiling the cosmological information over a wide scale range requires the understanding of the spatial distribution of HI, a biased tracer of dark matter. Most current approaches of the modelling of clustering are constructed to predict a linearly biased power spectrum on large scales. By doing so they ignore the coupling between small and large scale modes during the build-up of the large scale structure: density fluctuations in denser environments are enhanced as compared to those in less dense ones. Meanwhile the relation between neutral hydrogen and dark matter is barely known and often assumed to be similar to that of optical galaxies which might not be the case. Using a perturbative approach I will show how non-linearities have a significant contribution on linear scales: they modify the expected signature of baryonic acoustic oscillations and these modifications depend on the location of HI in the cosmic web. I will also discuss the observed lack of clustering in the cross-correlation HI x galaxies.

April 17, 2018
An extended f(R) theory of gravity and its application in astrophysical systems
Dr. Raíla André
Manipal Centre for Natural Sciences, Manipal University, Karnataka, India

General relativity is a highly successful theory, surviving multiple tests. There are still several indications that it may be incomplete, most natably the discrepancy between the predicted and observed values of the density of matter in the Universe, the observed dynamics of the galaxies and the current accelerated expansion of the Universe. Faced with this situation, two different approaches try to solve these issues: the introduction of unknown components (dark matter and dark energy) into GR and the generalization of this theory, where the geometry of spacetime is modified and such entities are not considered. In this context, an introduction to an alternative theory of gravity known as f(R) gravity will be presented in this seminar. Also in this scope, the results of an application of this alternative theory to a stellar structure model will be presented. The interest of this model is manifested in the fact that, due to the expressive gravitational field, the interior of stars can be seen as appropriate places to test such alternative theories of gravity. In these regions, high curvature regimes emerge and modify the stellar structure. The f(R) gravity model has proved to be necessary for the description of stars with strong fields.

April 24, 2018
Accreting black holes via X-ray polarimetry
Bc. Romana Mikušincová
ITP / Astronomical Institute of the Czech Academy of Sciences

In this talk I will summarize the results of my Master's Thesis for which I performed polarization simulations of the X-ray binary GRS 1915+105. The aim of this work is to put independent constraints on a black hole spin and inclination of the system via X-ray polarimetry. To simulate polarization spectra, we used a multicolor blackbody emission model accounting for thermal radiation from the disk accretion. Finally, we fit these data to estimate the precision of constraints on black hole spin and inclination.

May 15, 2018
Vizualizácia čiernodierových priestoročasov / Dynamika geodetik kolem porušených černých dír: Melnikovova metoda
Bc. Michal Maixner / Bc. Lukáš Polcar
(seminar in Czech language)
May 22, 2018
Transgressions, boundary terms and Katz-like vectors in Lovelock gravity
Nelson Merino, PhD
APC Laboratory, Paris 7

The divergence of the Katz vector provides a non-equivalent alternative to the Gibbons-Hawking-York (GHY) term, allowing to have a well-posed variational principle in General Relativity. It is constructed in terms of the metric and the difference of the Christoffel symbols of the dynamical and background manifolds. Unlike the GHY term, which was generalized by Myers to the case of Lovelock gravity, the Katz formulation is performed only in tensorial language and, up to now, it was not known how to generalize it to more general theories. In this work we provide a way to write the Katz vector by means of a local Lorentz-covariant construction. In particular, we show that the Katz vector in Einstein-Hilbert gravity can be obtained as a dimensional continuation of a transgression form, which is a function of two suitable Lorentz connections. Then we generalize this result to the case of Einstein-Gauss-Bonnet gravity and show that the corresponding generalized Katz vector solves the Dirichlet problem and gives the correct mass for the Boulware-Deser black hole. After discussing and contrasting this result with the ones already known in the literature, we show how a Katz-like vector can be constructed for a generic Lovelock theory.

May 29, 2018
Horizonty kolem prstencových zdrojů
Bc. Milan Pešta
(seminar in Czech language)
June 5, 2018
Physics of gravitational waves in the presence of positive cosmological constant
Sk Jahanur Hoque, PhD
The Institute of Mathematical Sciences, Taramani, Chennai, India

Cosmological observations have established that our universe has positive cosmological constant. A positive cosmological constant profoundly alters the asymptotic structure of space-time. In this talk, we discuss the linearized gravitational field produced by compact sources in the background with positive cosmological constant -- de Sitter space. Using the covariant phase space formalism, we obtain the quadrupole formula in such a setting. We also show that the energy flux of gravitational waves measured at future null infinity is the same as that measured across the cosmological horizon of the compact source. To get an order of estimate we also discuss power radiated by a binary system in de Sitter background.

June 12, 2018
Linear local gauge-invariant observables on spacetimes of sub-maximal symmetry
Igor Khavkine, PhD
Institute of Mathematics, Czech Academy of Sciences

The Killing operator $K_{ab}[v]=\nabla_a v_b + \nabla_b v_a$ is the generator of gauge symmetries (linearized diffeomorphisms) $h_{ab}\mapsto h_{ab} + K_{ab}[v]$ in linearized gravity. A linear local gauge-invariant observable is a differential operator $I[h]$ such that $I[K[v]] = 0$ for any gauge parameter field $v_a$. A set $\{I_i[h]\}$ of such observables is complete if the simultaneous conditions $I_i[h] = 0$ are sufficient to conclude that the argument is a pure gauge mode, $h_{ab} = K_{ab}[v]$. The explicit knowledge of a complete set of local gauge invariant observables has multiple applications from the points of view of both physics and geometry, whenever a precise separation of physical and gauge degrees of freedom is required. Surprisingly, until very recently, such complete sets have been known explicitly only on spacetimes of maximal symmetry (Minkowski or (anti-)de Sitter). I will discuss recent progress that has allowed an explicit construction of complete sets of local gauge invariant observables on backgrounds of sub-maximal symmetry, most notably on cosmological (FLRW) and black hole (Schwarzschild and Kerr) spacetimes.

Previous semesters:

Jiří Bičák                                                                                                  Oldřich Semerák

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