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:

No seminars are planned for the near future.


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
ITP

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
ÚTF

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.


Previous semesters:


Jiří Bičák                                                                                                  Oldřich Semerák


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