08:25 AM - 08:55 AM (30 min)
08:25 AM – 08:34 AM
Alex Vano-Vinuales (CENTRA, IST, University of Lisbon) (recording)
Reaching future null infinity in numerical relativity simulations is of utmost importance, as it is the only location in spacetime where gravitational ratiation is unambiguously defined. It can be achieved by evolving on hyperboloidal slices, which are smooth spacelike slices that asymptote to null rays and reach null infinity. In the present approach, conformal compactification methods are used to tackle the problem and a stable hyperboloidal code that solves the Einstein equations in spherical symmetry for regular and strong field initial data has been successfully developped. On the way towards extending these results to three spatial dimensions, first the massless scalar field is considered as a toy model. I will describe the current results of evolving the hyperboloidal wave equation in 3D in the NRPy+ code, as well as report on the status of the equivalent implementation within the Einstein Toolkit framework.
08:34 AM – 08:43 AM
Syed Naqvi (Astronomical Observatory, Jagiellonian University) (recording)
The phenomena of standing waves is well known in mechanical and electromagnetic setting where the wave has the maximum and minimum amplitude at the antinodes and nodes, respectively. In context of exact solution to Einstein Field equations, we analyze a spacetime which represents standing gravitational waves in an expanding Universe. The study the motion of free masses subject to the influence of standing gravitational waves in the polarized Gowdy cosmology with a three-torus topology. We show that antinodes attract freely falling particles and we trace the velocity memory effect.
08:43 AM – 08:52 AM
Shailesh Kumar (Indian Institute of Information Technology, Allahabad)
The direct detection of gravitational wave (GW) from binary black hole (BBH) mergers has set a strong evidence for general theory of relativity. These observations have enabled researchers to look for various aspects of black hole spacetimes; Gravitational wave memory effect (GW-memory) is one of such physical effects which has not been detected yet. The GW-memory manifests a permanent displacement in the spacetime which is a relative change in the position of freely falling LIGO test masses. It has been shown that the memory effect is related to the asymptotic symmetries of spacetimes originally discovered by Bondi-van der Berg-Metzner-Sachs (BMS). From theoretical perspectives, recovering asymptotic symmetries near the horizon of black holes has become a matter of interest to the researchers as Hawking, Perry and Strominger conjectured that the charges corresponding to BMS symmetries would help to retrieve the information in the Hawking information paradox. Therefore, memory effect must be well studied in the context of BMS symmetries from both theoretical and experimental perspectives. In this direction, my current research focuses on to investigate some of these aspects by estimating the measurable effects on the detectors after the passage of GWs. My aim would be to discuss some theoretical features of displacement memory effect near the horizon of black holes and its possible connection with near-horizon BMS symmetries.
Reference: S. Bhattacharjee, S. Kumar, A. Bhattacharyya, Displacement memory near the horizon of black holes, J. High Energ. Phys. 2021, 134 (2021).