# Work

This page highlights some of the things I have worked on.

## Kranc

Kranc is a Mathematica application which turns a tensorial description of a time dependent partial differential equation into a module for the Cactus Computational Toolkit. It is designed to take a simple continuum description of a problem and generate highly efficient and portable code. As such, it can be used both for rapid prototyping of evolution systems and for high performance supercomputing.

Kranc is used at the core of the Einstein Toolkit to generate the McLachlan code for solving the Einstein equations of General Relativity. This code is used by many groups worldwide on many supercomputers to simulate extreme physics from binary black hole and neutron star mergers to supernova explosions.

Kranc was originally written by Sascha Husa, Ian Hinder, Christiane Lechner and is now actively developed by Ian Hinder and Erik Schnetter. It forms part of my PhD Thesis.

## Open binary black hole simulation

On February 11, 2016, the LIGO collaboration announced that we had achieved the first ever direct detection of gravitational waves. The gravitational waves were generated over a billion years ago by the merger of a binary black hole system. The event has been given the name GW150914.

With Barry Wardell and Eloisa Bentivegna, I created an open simulation which shows how to use the Einstein Toolkit to evolve the last 6 orbits and merger of a binary black hole system with parameters that match the GW150914 event. Along with the associated tutorials, it shows how to extract waveforms and other physical properties from the simulated spacetime; how to visualise the 3D data generated by the simulation; and how to produce a numerical relativity waveform of the kind that may be used for the analysis of LIGO signals. Anybody with access to about 100 cores of a modern computer cluster can now simulate this binary black hole merger!

## Eccentric binary black holes

The first gravitational wave detected reported by LIGO, GW150914, is understood to come from the last few orbits of a binary black hole system in a circular orbit. I am interested in the possibility of astrophysical sources in eccentric orbits, which are not currently searched for by LIGO, because the effects of gravitational wave emission tend to damp away orbital eccentricity. However, there are several astrophysical scenarios where eccentric binaries may emit GWs detectable by LIGO, and in order to accurately estimate their parameters, it is important to have waveform models which incorporate eccentricity, and which have been tested against numerical relativity. I constructed an eccentric waveform model incorporating both the inspiral phase, based on existing post-Newtonian expressions, and the merger phase, calibrated to new numerical relativity simulations which I designed and ran. The model accuracy is tested against the simulations. This model may be used in future to measure or constrain eccentricity in gravitational wave observations.

## Adaptive Mesh Refinement

Computational simulations of physical systems often need to describe nature on many different length scales. To efficiently simulate such systems, it is necessary to adapt the location of computational grid points to where it is needed in the simulation domain. I am very interested in different approaches to this problem, with particular application to solving the Einstein equations for binary black hole systems. Recently, I have been working to implement new adaptive mesh refinement methods in the Einstein Toolkit, which should significantly improve the speed of the simulations, and the amount of science which can be done with finite computational resources.