Understanding the observed late-time acceleration of the cosmological expansion of the Universe is perhaps one of the most compelling problems facing physics today. The University of Waterloo's newly created Distinguished Chair in Astrophysics, Prof. Will Percival, coming from the Institute of Cosmology & Gravitation at the University of Portsmouth, is leading an effort to understand why the Universal expansion is accelerating.
Prof. Percival holds an Associate Faculty position at the Perimeter Institute (PI) for Theoretical Physics in Waterloo. Dr. Neil Turok of the PI institute says: “All the direct observational evidence we have on physics beyond the Standard Model and Einstein’s theory of gravity comes from cosmology – for example, on dark matter, dark energy, and the nature of the big bang. Will Percival is a world-leader in learning about fundamental physics from galaxy surveys, an area poised for major progress."
Upon joining the faculty at Waterloo, Prof. Percival’s immediately launched into his first initiative – to create the University of Waterloo Centre for Cosmology and Astrophysics.
We asked Prof. Percival to tell us about his path that led him to become a world-renowned astrophysicist. Here is what he told us:
“I’ve met astronomers who came into the field from a general desire to do physics, or who started as amateur astronomers wanting to know more. For me however, I became a professional astrophysicist due to a mix of loving mathematics and loving practical problems. My practical nature matches that of my father, a retired automotive production engineer, while my academic interest comes from my mother. My mother’s education was cut short because of a need to work at a young age to look after her siblings, and the world lost out as I think she would have wanted to be, and would have made a great scientist. Wiping a tear from eye, this is one reason why I’m so passionate that everyone has equal chance today at a career in science. Unfortunately, there remains an imbalance in physics, with the lack of female physicists particularly apparent, and so this is a problem that still requires addressing.
Anyway, back to my personal journey. As a child finishing school, I knew I wanted to do something practical and to do with math. So (eventually) I choose to do cosmology. There is no greater challenge than trying to understand the Universe on the largest scales observable, and I’m always ready to take on a challenge! The language we use to explain the properties of the Universe is mathematics, and we need to apply practical algorithms to make measurements to compare with theoretical predictions, so it meets both of my criteria. And the Universe is an awesomely interesting place, with huge questions still unanswered. For example, we know that we don’t know what most of the Universe is made of. If this sentence doesn't make sense, read it again. It’s like being driven in a car and not knowing if it’s made of plastic or metal. As the son of an automotive engineer I’d be embarrassed in such a situation! Thankfully, we can do clever tests and find out what stuff fills the Universe.
Many of these tests are statistical in nature, and require great care in the process applied to avoid biases. Part of what I love day-to-day is developing techniques that allow rigorous and robust cosmological measurements. Finding a good way of removing a particular bias or optimally making a measurement is very satisfying. There are many sources of potential bias - physical processes that can mimic cosmological signals, and developing tests that can robustly say something about the nature of the Universe is tricky. So, in answering the big questions, it’s often the case that I spend time answering the small questions - how to avoid contaminating a measurement, or how to optimally combine different results. But I’m fine with this. If your car doesn’t start, you don’t just stare at the engine as a whole, you start checking each of the pieces in turn – is there fuel? Is there a spark? It’s the same in cosmology – to understand the big questions you have to solve the pieces.
The next decade will be a huge one for cosmology. There are some great experiments coming online - particularly huge surveys of galaxies such as the Dark Energy Spectroscopic Survey (DESI) and the Euclid satellite mission, which will be more than 10 times better at measuring interesting cosmological parameters than the data we have in hand today. The big question that we’re trying to answer is what is the physical reason why the expansion of the Universe is accelerating. Gravity is an attractive force, so in most models dominated by gravity acting on matter we would expect deceleration as gravity pulls material back together. Instead we see acceleration in our Universe. There are many theories to explain this, but none is compelling. To be a compelling theory, there has to be the beauty of simplicity and the power of prediction without fine-tuning: tweaking a theory to match data is not as nice as have a reason why the Universe is as it is. To explain acceleration, we need a theory that either weakens gravity on large-scales, or that postulates a dominant material in the Universe with really weird properties - if you squeeze it, it gets less dense. Neither has so far been ruled out, and we would like to tell which is correct.
The field is being led by the observations, and we’re on the verge of a transformation in the amount of data available. This will be able to tell apart these options for the accelerated expansion, and I can’t wait to see the results from the new surveys. There’s a whole new branch of physics waiting to be discovered.”
The Department of Physics & Astronomy is proud and excited to have Prof. Percival joined the rank its faculty members and that his research activities over the next few years will, indeed – result in “a whole new branch of physics to be discovered”.
