Prof Downes' research is focused primarily on the dynamics of flows in star forming regions. This includes addressing questions associated with (molecular cloud) structure formation and magnetic field behaviour in astrophysical turbulence, as well as the dynamics of proto-planetary disks. He is also involved in studying questions surrounding Fermi acceleration of relativistic particles at supernova shocks and gamma-ray bursts. The tool of choice for addressing these questions is numerical simulations and, where necessary, some of the world's largest supercomputers are used in the pursuit of a greater understanding of these systems.
I work on understanding the formation of the first black holes in our universe. Black Holes form from the collapse of massive stars, the gravitational force as the star collapses overwhelms the pressure forces generated by the nuclear reactions and tears a hole in spacetime itself. The result is a singularity and the region around the singularity becomes the black hole. As matter then falls into (accretes onto) the black hole it gets heated up through dynamical friction. These friction forces are driven by the extreme gravity of the black hole and can result in large luminosities. As a result the massive black holes can be seen out to very large distances. The most massive black holes can be observed back to a time when the Universe was less than a billion years old. This presents an interesting challenge for astrophysics. How could such massive objects form so early in the Universe? What are the black hole seeds - were they supermassive stars to begin with or more modestly sized black holes which then grew at prodigious rates? It is this question that my research tries to answer - see my latest paper for a compelling solution!
My research focusses on different aspects of black holes and space-time singularities in the context of General Relativity (GR). This theory – Einstein’s geometric theory of the gravitational field – describes our universe as a 4-dimensional curved space-time. It provides the mathematical tools for the analysis of such phenomena as the gravitational collapse of stars to form black holes, the interaction and collision of black holes, the generation and propagation of gravitational waves (ripples in the curved space-time) and the evolution of the universe as a whole. In particular, I am interested in the Cosmic Censorship Hypothesis (CCH), the propagation of waves in curved space-times and the description of black holes (and other extended bodies) in isotropic universes. Studying the Cosmic Censorship Hypothesis involves trying to resolve the question of whether or not the singularity that inevitably forms as the end state of gravitational collapse is always hidden inside a black hole. The analysis of wave propagation in curved space-times is a non-trivial matter due to the fact that the waves don’t just spread out uniformly from their source (as on the surface of a pond): they can get trapped, distorted and pulled all the way around the black hole under the influence of its extreme gravitational field. Understanding the propagation of such waves is crucial for a full understanding of the generation and propagation of gravitational waves. In mathematical terms, both areas of research are linked by the study of wave equations in curved space-time, a topic that combines various different areas of mathematics (differential geometry, partial differential equations, Fourier analysis,…). In a different vein, I am also interested in using Einstein’s theory to find the appropriate mathematical description of black holes that exist not (essentially) in isolation, far from other material sources, but that are embedded in our expanding, galaxy-filled universe.
My main scientific interests lie in the area of high energy astrophysics, which studies the most energetic events in the Universe. In my work I combine a theoretical approach of modelling of high energy sources with the analysis of multi wavelength experimental data. In particular I am interested in the mechanisms leading to particle acceleration and very high energy (VHE) emission in gamma-ray loud binaries and Galactic Centre (GC). While about half of the Galactic X-ray sources are binary systems, only few (less than 10) binary systems are able to produce TeV emission. The aim of my studies is to understand what makes these systems so special. Understanding of the origin of the high energy emission of the GC (e.g. hadronic or leptonic, diffusive or not) is important for understanding of the origin and properties of the cosmic rays in the central region. In addition to that I am also involved in the simulation of these VHE sources for Cherenkov Telescope Array (CTA).
My research is mainly concerned with classical and quantum aspects of black holes. From a quantum perspective, I am interested in quantum field theory in curved spacetimes and the associated theory of semi-classical gravity. The most famous prediction of this approximation--and one of the most surprising and far-reaching predictions of theoretical physics--is that black holes emit quantum thermal radiation, the so-called Hawking effect. On the classical front, I'm interested in the problem of motion in General Relativity including strong self-interaction effects. This is particularly important for modeling binary black hole systems where one black hole is much larger than the other, a key astrophysical source of gravitational waves being targeted by the European Space Agency's eLISA mission.
My research mainly concerns quantum field theories (QFTs) in curved spacetimes and black hole thermodynamics/Hawking radiation. QFTs form the basic language in which the standard model of elementary particle theory is formulated. The past few decades have seen remarkable theoretical advances and QFTs with perturbative interactions, like the standard model, can now be treated within a perfectly satisfactory generally covariant framework. This allows us to consider particle theory in any fixed external gravitational field. Nevertheless, there are many exciting open questions to work on, both at the mathematical and at the conceptual level. My own interests lie with mathematical problems surrounding the basic structure of axiomatic (non-perturbative) generally covariant QFTs, aspects of Hawking radiation, entanglement, and the fundamental nature of spacetime itself.
My research focuses mainly on general relativity and other classical field theories. One major theme has been the theory of motion: How do the details of an object's internal structure affect its bulk movement or spin? How about a body's "own" electromagnetic, gravitational, or other fields? This is in essence the "self-force problem." There is no self-force at all in Newtonian gravity, although that changes in relativistic settings where Newton's third law fails and fields propagate at finite speeds. I have also been interested in the propagation of gravitational and electromagnetic waves in various contexts: gravitational lensing, wave optics in curved spacetimes, the consequences of caustic formation, and the buildup of nonlinear corrections over large distances. Most recently, I have been investigating the nonlinear structure of Einstein's equation in a broad sense, and finding that in various physically-relevant cases, the "intrinsic nonlinearity" is considerably less than might have been expected; traditional approaches employ variables which are poorly adapted to the equations at hand. I am working to generalize this and to better understand how it can be used to develop a more effective perturbation theory.