Accretion disc physics: breaking the symmetries

My research is concerned with astronomical systems called accretion discs. In these systems, a disc of gas orbits in the gravity field of a central star or black hole, balancing the gravitational pull of the star by the centrifugal effect of its rotation. Our Solar System formed out of a disc like this: first some of the rotating gas near the centre made the Sun itself, and then some of the gas further out formed planets, and these continue to revolve in the same orbits today.

Accretion discs are the essential ingredient for a vast range of other astrophysical phenomena as well. Gas falling in the gravity of a black hole forms a disc for the same reason that planets orbit our Sun. The gas in the disc spirals slowly on to a black hole from a disc, heating up and emitting light as it does. As it falls (or "accretes") on to the black hole it emits up to 40% of its rest mass energy as light which we can observe. This makes accreting black holes the most luminous objects in the Universe. We can observe their accretion discs at distances approaching the entire extent of the Universe. Their light has taken almost the entire age of the Universe to reach us, so they show us how things were when the first stars and galaxies were forming. Understanding accretion discs is essential to understanding the Universe as a whole. But the way we currently try to picture them is too simple.

Even today almost every discussion of discs assumes - essentially for convenience - that they have the simplest possible geometry, with gas orbiting the central object in a single plane, and on smooth circular orbits. This approach prevents us considering real physical effects which underly observed phenomena.

My research aims to remove these symmetries by relaxing assumptions made about initial conditions and input physics. This approach has already revealed new disc processes in the last couple of years, and these offer cogent explanations of previously mysterious observed phenomena. I propose to simulate more complete systems with this first principles approach. Using state-of-the-art numerical methods, this work will have a wide impact, and constitute a big change to the standard picture of the field. It will connect to data from a vast range of new observing facilities, both space and ground based. I anticipate that it will significantly change the way we understand various pressing problems, such has how stars and planets form, and how black holes can grow to masses many millions of times greater than the Sun.