12:50 PM - 01:20 PM (30 min)

Short Lighning talk session I

1. Black hole induced spins from hyperbolic encounters in dense clusters

12:50 PM – 12:59 PM

Santiago Jaraba (Instituto de Física Teórica (UAM-CSIC)) (slides) (recording)

The black holes that have been detected via gravitational waves (GW) can have either astrophysical or primordial origin. Some GW events show significant spin for one of the components and have been assumed to be astrophysical, since primordial black holes are generated with very low spins. However, it is worth studying if they can increase their spin throughout the evolution of the universe. Possible mechanisms that have already been explored are multiple black hole mergers and gas accretion. We propose here a new mechanism that can occur in dense clusters of black holes: the spin-up of primordial black holes when they are involved in close hyperbolic encounters. We explore this effect numerically with the Einstein Toolkit for different initial conditions, including variable mass ratios. For equal masses, there is a maximum spin that can be induced on the black holes, χ=a/m≤0.2. We find however that for large mass ratios one can attain spins up to at least χ≃0.5, where the highest spin is induced on the most massive black hole. For small induced spins we provide simple analytical expressions that depend on the relative velocity and impact parameter.

2. NRPyElliptic: A User-Friendly Hyperbolic Relaxation Solver for Numerical Relativity

12:59 PM – 01:08 PM

Thiago Assumpção (West Virginia University) (recording)

I will introduce NRPyElliptic, an elliptic solver built on the NRPy+ infrastructure. As its first application, it sets up conformally flat, binary puncture initial data on prolate-spheroidal-like grids. The code employs a hyperbolic relaxation scheme, whereby an elliptic equation is transformed into a hyperbolic equation. Our implementation of this scheme implements new performance optimizations that speed up the solver by orders of magnitude over the original approach. NRPyElliptic is easily extensible to other nonlinear elliptic PDEs and supports other coordinate systems as well. It has been developed as an Einstein Toolkit thorn and as a stand-alone code, both of which are documented in pedagogical Jupyter notebooks.

3. FLRWSolver: realistic cosmological initial conditions for the Einstein Toolkit

01:08 PM – 01:17 PM

Hayley Macpherson (University of Cambridge) (recording)

We are well and truly entering the era of precision cosmology, with upcoming surveys expected to map hundreds of thousands of supernovae to percent-level precision. The majority of our cosmological modelling relies on the assumption that Newtonian dynamics atop an exact homogeneous and isotropic expanding spacetime is sufficient to describe the late Universe. Investigations into general-relativistic effects on cosmological data have shown they could be important for near-future surveys. I will describe our publicly-available thorn FLRWSolver, written to provide realistic cosmological initial conditions for the Einstein Toolkit. This thorn has been shown to be reliable in studying late-Universe dynamics in numerical relativity, and is a useful tool to address some important questions regarding the validity of current assumptions in cosmology.