Dark matter makes up the majority of the matter within our universe, however we are still perplexed by its mysterious nature. To try and make sense of this field of study, we met with Professor Justin Read, Head of the University of Surrey Physics Department, to ask him some questions about his field of research.
Justin completed his PhD in Theoretical Physics at the University of Cambridge in 2004, came to Surrey in 2013 as a lecturer and as an expert in his field of the behaviour and study of dark matter within galaxies. He’s been an author on nearly 100 scientific papers since 2003 and one of his most recent papers made headlines in early 2019 regarding dark matter within a specific kind of galaxy.
What we knew before meeting with Justin is that dark matter is concentrated within galaxies – we know this through looking at the rotation of stars within these galaxies. How we know this is that the speeds of those rotations do not make sense when we consider only the matter that we can see – so we assume there must be additional mass somewhere within the galaxy that does not emit or interact with light.
When this phenomenon was observed, some scientists believed that our understanding of gravity was incomplete and that dark matter did not exist. However, Justin informed us that by measuring the radiation in our universe and observing interactions with different types of matter, that this is probably not the case.
Justin is extremely passionate about his field of research, and informed us that there are two main ways of studying dark matter. Scientists either focus on dark matter as a particle to try and determine its properties or see how its effects on the gravitational properties of systems within space. Justin’s research spans across almost all areas of astrophysics however he specifically focuses on the latter, and the way in which he does this is through studying smaller types of galaxies called dwarf galaxies. Through this research Justin aims to answer the question, ‘Where should the dark matter be, and how does it behave?’ by working on developing new software and techniques to compare his data with the observations and data seen through telescopes around the world.
So why does Justin focus on dwarf galaxies for his study of dark matter? It’s because, surprisingly, they contain less dark matter than all of the predictions up until the early 2000’s had estimated. Through his research he has largely improved upon existing models by taking into account all of the missed-off stars, dust and gas within his own simulations. Earlier predictions wildly over-estimated the amount of dark matter as they simulated only dark matter within these dwarf galaxies. They did this for good reason though, as some dwarf galaxies may just be made from dark matter and all other calculations ignored the gravity from nearby stars and larger galaxies. This gave a general view; however to fully fine-tune and get more accurate data, bodies not comprised of dark matter needed to be taken into account.
From Justin’s improved simulations, he predicted a dark matter ‘heating’ process within these galaxies, which was later verified through observing dwarf galaxies with various star forming lifetimes. This ‘heating’ process isn’t related to temperature like you might assume, but instead refers to the change in speed and energy of the dark matter ‘particles’ because of the explosions that happen at the end of a star’s life. The timescale over which this ‘heating’ process occurs is billions of years. He also found a link between the amount of dark matter within these galaxies and the point at which the galaxies stopped forming stars through this – all of these findings significantly increase our understanding of dark matter and its role in galaxy and star formation.
This research was by no means easy for Justin and his team to carry out – it’s not as if you can create a universe within a lab or just go and visit the other side of a galaxy to see what’s going on. This meant that he had to be creative and innovative with his computational methods and try to ensure that his work didn’t break any laws of physics. The problems that occur when measuring dwarf galaxies is that the most convenient ones orbit our own Milky Way and end up being shredded apart by the large gravitational pull of our galaxy. The ones that aren’t being shredded are sometimes too far away, inactive, or there is something larger obstructing our view so the measurements are a lot harder to get. The way that Justin overcame this was to observe 16 dwarf galaxies, such as Fornax, that were in an ‘intermediate’ zone, so measurements could be taken easier, and the star formation histories were all varied. This gave him a wide range of data that was used to verify his work.
Dark matter is most definitely in our universe, and plays a large part in the evolution of galaxies and the universe as a whole. Justin, alongside the majority of the scientific community, have taken these results to show that dark matter seems to be a cold, non-relativistic particle – which means that it’s slow and heavy and doesn’t really interact with anything beside gravity. We asked Justin where he thinks his research is going towards, and to him it seems that there is still more to be learnt through studying how dark matter interacts with gravity, and he is going to be continuing to follow this path to aid the work being done to create a particle model within a laboratory. He said that by studying these large scale structures within the universe we can learn about the small scale models of dark matter.
We also spoke to Justin about potential dark matter candidates, and he feels that the odds are in favour of it being a new type of neutrino. His justification for this is that neutrinos are strange anyway – they don’t interact with light or other forces – so if we could slow one down to a non-relativistic speed (less than 20% of the speed of light), then this would be a perfect candidate. However, this has never been performed in a laboratory, and the Nobel Prize awaits for the person who finds exactly what dark matter is. Justin feels that if we don’t fabricate dark matter within a lab over the next few years by performing higher energy experiments, then we need to question whether we are on the right path. It is possible that new, unthought-of and untested physics will reveal more answers that Justin or his students will be taking part in.