Title: Gravitational Lensing in Plebanski-Demianski Class Black Hole Spacetimes: The Geometric Optics Limit

Speaker: Torben Frost (KIAA, Peking University)

Time: 2:00 pm, Apr.11th，2024

Venue: C302, New Science Building

 Abstract: Einstein's theory of general relativity admits several exact black hole solutions. One of these solutions is the so-called Plebanski-Demianski spacetime. It is an exact solution to Einstein's electrovacuum field equations with cosmological constant and axisymmetric and stationary. As limiting cases it contains the Schwarzschild metric and the Kerr metric, however, it also contains more exotic spacetimes such as the NUT metric and the C-metric which describe a black hole with gravitomagnetic charge and an accelerating black hole, respectively. Considering that all these spacetimes are exact solutions to Einstein's electrovacuum field equations with cosmological constant now there are two questions. How can we detect effects from the different parameters and in how far can we expect these black holes to represent real astrophysical black holes? In my talk I will address this question using gravitational lensing of light in the geometric optics limit. For this purpose I will first provide an overview on the Plabanski-Demianski spacetime and its physical parameters. Then I will illustrate how we can analytically solve the equations of motion for lightlike geodesics. In the second part of my talk I will then introduce a stationary observer in the domain of outer communication outside the photon region. I will parameterise the lightlike geodesics using latitude-longitude coordinates on the observer's celestial sphere and use this parameterisation and the derived analytic solutions to the equations of motion to derive the angular radius of the shadow of the black hole, write down a lens equation, calculate the redshift and the travel time. Finally, I will discuss the effects of the physical parameters on the observable lensing patterns.

 Bio: Torben did his Ph.D. at ZARM, University of Bremen in 2022. After a short-term postdoc at the same place he started his postdoc at the Kavli Institute for Astronomy and Astrophysics at Peking University in March 2023.