Using Cosmic Beasts To Uncover The Nature Of Dark Matter

Astronomers have only observed 5% of the content of the Universe so far: the luminous (or baryonic) matter. The remaining 95% is invisible, consisting of so-called Dark Matter and Dark Energy. The nature of dark matter and dark energy is one of the most pressing fundamental questions in modern physics. This Dark Sector of our Universe has remained impossible to detect directly, because neither component interacts with standard matter particles. Moreover many theories predict dark matter will remain fundamentally undetectable in terrestrial experiments, and can only be probed by astrophysical laboratories. If these theories are correct, dark matter can only be studied where it gathers in sufficient quantities for its gravity to affect things around it we can see. I propose to track the behaviour of dark matter in galaxy clusters (the most massive observable structures in the universe, also called 'cosmic beasts'), to distinguish between the 3 leading models: cold, warm and self-interacting dark matter.

My FLF project exploits a dramatic increase over the past decade in observations of galaxy clusters by the world's biggest telescopes, reflecting the field's recognition as a top priority goal. I got awarded observing time on the Hubble Space Telescope in the largest category of programme (>100 orbits) to obtain the deepest ever imaging of clusters' surroundings, plus follow-up spectroscopy from the largest telescope on Earth (VLT). I designed these observations to map clusters' dark matter, via the effect of 'gravitational lensing', which distorts and magnifies objects behind the cluster. I use these data (i) by themselves, (ii) to calibrate the largest (but shallow) Hubble imaging (iii) to set the agenda for, and optimise facilities like Euclid, Athena and the James Webb Space Telescope through the 2020s.

Cosmological simulations indicate that galaxy clusters are the best laboratories to distinguish between models of dark matter, because they are still growing. Clusters grow by merging with each other; every merger acts like a gigantic particle collider. The properties of dark matter are revealed by its trajectory through a collision, which should be between that of stars and of (hydrogen) gas. The properties of stars and hydrogen are well understood, so they bookend measurements of dark matter.

Traditional research programmes usually separate measurements of dark matter, stars and gas, because they require observations from different (infrared, ultraviolet, X-ray) telescopes. I have developed a multiwavelength analysis, to enable previously impossible measurements such as the time-scale on which dark matter and gas are funneled into clusters, how quickly clusters reach equilibrium, and constraints on possible dark matter particle interactions.

I have also led the establishment of a new research area, which I have expanded since the start of my FLF. When transient events (such as supernova explosions) happen behind a galaxy cluster, light from the explosion can be gravitationally lensed and visible along more than one line of sight. Measuring the time delay between multiply-imaged versions of a supernova increases the resolution with which the cluster's dark matter can be mapped. I intentionally scheduled my HST and VLT observations to enable the discovery and monitoring of such events. They also offer the (high risk/high reward) possibility of discovering electromagnetic counterparts to lensed gravitational waves, a new field in which our team has become a leader by developing the theoretical framework and observational strategy to detect the first gravitationally lensed gravitational wave.

My UKRI FLF research programme exploits the latest multiwavelength data from world-class facilities. It uses my high-precision techniques to analyse big data, and is interpreted within the world-leading theoretical framework of Durham's state-of-the-art cosmological simulations.