This is the third of our featured project series, where we hear from Dr Katy Clough, STFC Ernest Rutherford Research Fellow at Queen Mary University of London and her team on their work using numerical solutions to understand the consequences of Einstein’s theory of general relativity, in particular concerning black hole environments and inhomogeneities in the early universe.
I use numerical simulations to understand how systems behave in strong gravity. The key example most people have heard of is a black hole (BH), but the theory of strong gravity – General Relativity – also describes the Universe we live in as a whole, especially at times close to the Big Bang.
My work on BH aims to discover signatures of new physics in gravitational wave signals. Black holes can exist in binary pairs, and over time they inspiral and merge. When this happens they emit gravitational waves, which we can detect here on Earth. These signals bring information from near the BH event horizon, which in terms of the strength of gravity is beyond anything we can probe on Earth. It is possible that we may see features offering hints that gravity changes at higher energies, or information about the environments of black holes. I’m particularly interested in using BH to detect dark matter – a component of galaxies that we understand very little about.
We recently developed a new formalism for measuring dynamical friction in a strong gravity setting, and used it to justify effects simulated in binary BH mergers, where we saw unexpectedly large changes in the orbits. This might help explain why supermassive black holes can merge more quickly than they ought to, and if so that would be a really exciting result.
The first successful simulation of a binary black hole merger that we ran with our code, GRChombo, back in 2014. In 2015 we had the first detection of an event just like this in gravitational wave detectors at LIGO. (Credit: Clough)
General Relativity tells us how space and time stretch in all directions, and because we live in 3 space and 1 time direction, there are 20 numbers (if you do the maths!) needed to describe spacetime. We have to work out these numbers at every point in the spacetime region we want to understand. This is computationally expensive, and we need many processors to work together and exchange data in order to figure it out.
Our numerical tools were originally developed to allow the LIGO-Virgo-KAGRA network of gravitational wave detectors to interpret signals from merging BH/neutron stars. We’ll thereby learn a huge amount about astrophysical populations, the main goal of the detectors. Applying the same techniques to aspects of theoretical physics is relatively novel. The UK is one of the world leaders in such studies, in no small part due to the code that we have developed, and made publicly available.
The thing I am probably most proud of is our GRTL code collaboration, which now has around 25 members in the UK, Europe and the US. When I started my PhD 10 years ago we were just a few crazy people trying to make a new code for numerical relativity for fundamental physics, and we’ve built it up into a major internationally recognised resource for solving such problems. It was used to benchmark the new DiRAC3 resources, and we are now adapting it to exploit GPUs in the future. Most importantly, we have a really good team spirit – we’re really friendly and help new joiners to learn the code. I always look forward to our regular collaboration meetings!
Although I love BH, my biggest passion is understanding more about how the Universe began using general relativity simulations. Simulations are a kind of experiment to explore the behaviour of our universe at the earliest times, when we think it should have been very inhomogeneous. I want to see if strong gravity effects could act as a natural smoothing agent, yielding the homogeneous Universe we see now. Within ten years, our ground based detectors will be upgraded and the LISA space-based mission will launch and we will see gravitational wave data from the entire visible universe. In 20 years we should have this data from frequencies varying over many orders of magnitude. This is a discovery era for gravitational wave physics, and it will be great to play a role.
I hope everyone feels their curiosity stirred by these kind of questions – what the Universe is and where it came from – if you don’t then I don’t know how to convince you! The things that make me happiest are just understanding why something is happening, in particular when you think that perhaps no one before has really understood this before you. Often they aren’t big things, but just small technical details, but it is still very satisfying. When I tell people what I do I am quite honest about the fact that it isn’t obviously useful for everyday life, but I rarely encounter anyone who doesn’t find it exciting and worth exploring.
Formation of overdensities called oscillons in the inflaton field after inflation, which may lead to the existence of primordial black holes.” (Credit: Aurrekoetxea/Clough/Muia)
Our team at QMUL is around 4 PhD students, 1 postdoc and 2 faculty (myself and Pau Figueras). We are part of a wider collaboration of ~25 researchers, primarily from King’s College London, Oxford and Cambridge, who both develop and use the code.
Effect of Wave Dark Matter on Equal Mass Black Hole Mergers
Josu C. Aurrekoetxea, Katy Clough, Jamie Bamber, and Pedro G. Ferreira
Phys. Rev. Lett. 132, 211401
Relativistic drag forces on black holes from scalar dark matter clouds of all sizes
Dina Traykova, Rodrigo Vicente, Katy Clough, Thomas Helfer, Emanuele Berti, Pedro G. Ferreira, and Lam Hui
Phys. Rev. D 108, L121502
Oscillon formation during inflationary preheating with general relativity
Josu C. Aurrekoetxea, Katy Clough, and Francesco Muia
Phys. Rev. D 108, 023501
Solving the initial conditions problem for modified gravity theories
Sam E. Brady, Llibert Aresté Saló, Katy Clough, Pau Figueras, and Annamalai P. S.
Phys. Rev. D 108, 104022
