Mohammadtaher Safarzadeh (UCSC)
Title: Probing the binary black holes merger rate history with stochastic gravitational wave background searches
Abstract: What is the merger rate of binary black holes (BBHs) across cosmic time?
Although the local merger rate of binary black holes is about 20-100 Gpc^-3 year^-1, their merger rate evolution with redshift is unknown. What determines the merger rate of BBHs is a convolution of the (rather known) star formation rate history of the universe with their (unknown) delay time distribution (DTD). The DTD of a BBH population determines how long it would take for BBHs to merge due to emission of gravitational waves (GWs) from when they are born. As the horizon redshift of the current GW detectors is limited, BBH mergers at high redshifts are not individually detectable while they contribute to GW background. I will show how the lack of stochastic GW background detection would rule out the combination of high local merger rates of the BBHs with a short underlying DTD.
Achilleas Porfyriadis (BHI, Harvard)
Title: Leaky boundary conditions and the backreaction in AdS2
Abstract: In the past, it was often said that AdS2 does not have any dynamics because backreaction in AdS2 is so strong that it destroys its boundary. Today, it is often said that AdS2 only has boundary dynamics which are studied in simple models of 2D gravity. On the other hand, all black holes near extremality universally feature a near-horizon geometry that is AdS2-like and this includes the Reissner-Nordstrom and Kerr black holes in 4D asymptotically flat gravity. In this talk I will discuss a calculation that features leaky boundary conditions in the near-horizon AdS2xS2 geometry of an extreme Reissner-Nordstrom. In this calculation AdS2 reacts to a perturbation by a scalar field without destroying the AdS2 boundary, maintaining the connection to the far asymptotically flat region of the black hole.