Relativity Seminar
of the Institute 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

March 28, 2023
!!! 15:40, Kvasnica's (the small) lecture room (within GR Journal Club) !!!
Efficient trajectory calculations for extreme mass ratio inspirals using near-identity (averaging) transformations
Dr. Philip Lynch
Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Department of Astrophysical and Cosmological Relativity
Future space based gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA) will allow for the detection extreme mass ratio inspirals (EMRIs) which consist of a stellar mass compact object spiralling into a massive black hole (MBH) due to gravitational radiation reaction. These sources are of particular interest for their ability to accurately map the spacetime of the MBH, allowing for unprecedentedly accurate measurements of the MBH's mass and spin, and tests of general relativity in the strong field regime. To reach the science goals of the LISA mission, one requires waveform models that are (i) accurate to within a fraction of a radian, (ii) extensive in the source's parameter space and (iii) fast to compute, ideally in less than a second. We focus on the later criteria by utilising techniques that will speed up inspiral trajectory calculations as well as extending prior models to include the MBH's spin. We develop the first EMRI models that incorporate the spin of the MBH along with all effects of the gravitational self-force (GSF) to first order in the mass ratio. Our models are based on an action angle formulation of the method of osculating geodesics (OG) for generic inspirals in Kerr spacetime. For eccentric equatorial inspirals and spherical inspirals, the forcing terms are provided by an spectral interpolation of the first order GSF. For generic inspirals where sufficient GSF data is not available, we construct a toy model from the previous two models. However, the OG method is slow to evaluate due to the dependence of the equations of motion (EOM) on the orbital phases. Therefore, we apply a near-identity (averaging) transformation (NIT) to eliminate all dependence of EOM on the orbital phases while maintaining all secular effects to post-adiabatic order. This inspiral model can be evaluated in less than a second for any mass-ratio as we no longer have to resolve all ~105 orbit cycles of a typical EMRI.

Zoom link
March 28, 2023
Relativistic positioning systems and automatic differentiation
Dr. Justin C. Feng
Center for Astrophysics and Gravitation @ Instituto Superior Técnico, University of Lisbon
I discuss the framework of relativistic positioning systems for satellite navigation. I then discuss a new approach to the relativistic location problem (i.e. the determination of the position of a user from satellite data) in a generic spacetime geometry. This approach makes use of automatic differentiation. Time permitting, I will discuss in detail the potential applications of this powerful new tool in computational physics.

Zoom link
April 4, 2023
Exact solutions in 2+1 dimensional gravity
Mgr. Matúš Papajčík
In 3D gravity virtually all spacetimes must belong to either the expanding Robinson-Trautman class or the non-expanding Kundt class of geometries. We investigate the exact solutions of the coupled system of Einstein-Maxwell equations for these geometries, allowing also for a non-vanishing cosmological constant. We then discuss the found solutions, namely we identify the special subclass of charged black hole spacetimes in 2+1 gravity. We also make a brief overview of the current method of algebraic classification in three dimensions, and present a new method based on projections of the Cotton tensor onto a suitable null triad. The algebraic classification of some exact solutions is then discussed using this new method.

Zoom link

Jiří Bičák                                                                                                  Oldřich Semerák