The stars and their secrets: Will Percival and his journey to understanding the universe

Humanity’s quest to uncover the secrets of our vast universe is one that has been long, fraught with difficulty, and largely unsuccessful. As is often characteristic of us, however, it also seems to be a quest that has not stopped or even shown any signs of slowing down. From its embryonic beginnings of Copernicus and Galileo theorizing about how the planets move around the sun, to the present day of scientists and physicists devising ever ingenious ways to calculate the distance and chemical composition of things very, very far away, astronomy was never a field for the faint of heart. That fact in and of itself seems to keep most people away, but not Will Percival the director of the Waterloo Centre for Astrophysics.

Percival, originally hailing from England, had always been interested in mathematics — pure mathematics to be exact, with its elegant theorems and beautiful proofs. In fact, it’s what would ultimately become his major for his undergraduate career at the University of Nottingham. After finishing his degree, he knew he wanted to do something in research but remained unsure of what area to go into. To get his feet wet and see more of what the market offered, he started working for a company. While he didn’t stay long in the position, the experience ultimately proved valuable as it helped him realize his lack of interest in industry, seeing it as too “applied” for his taste.

Following this, Percival started looking around for PhDs in what he considered to be interesting topics. He would eventually find Lance Miller at the University of Oxford who agreed to take him on for a cosmology-related project, and the rest is history.

“It was an interesting project… nothing related to my research now,” Percival reminisced. “It was in the era where you still went out [with] telescopes… so we did a number of observing runs and analyzed the data looking at a particular class of object called a quasar.” Quasars are “extremely luminous galactic cores [resulting from] gas and dust falling into a supermassive black hole… and becoming luminous as a result of the extreme gravitational and frictional forces exerted on them.” Quasars can emit thousands of times more light than the entirety of the Milky Way, and they are located at extreme distances from us (millions to billions of light years away) as well.

“We took very detailed views of the quasars, and then we could have the detail to tell us what type of galaxy they lived in… [and we looked] particularly [at] when and how structures formed in the universe and we tried to tie this to quasars and the galaxies hosting them… how [they] fitted into cosmology.”

After this brief stint, Percival began his post-doctorate and moved on to bigger things that would ultimately define the rest of his career: he started working on the order of the entire Universe with redshift surveys. These essentially have scientists take the spectra of many millions of galaxies and measure what is called a redshift. From this, they can deduce how fast an object is moving away from us and its approximate distance. This phenomenon is a consequence of the Doppler effect.

Imagine you’re standing still on the sidewalk and an ambulance comes down towards you on a busy street. The sound of its siren as it approaches will appear to be much higher pitched than when it passes you and starts moving away. This is because the sound waves it emits as it approaches bunch up together, and your ears perceive this to be a high frequency (high pitched) sound. As it moves away, the sound waves spread out, leading your ears to perceive a low frequency (low pitched) sound.

This is essentially what’s happening to the light that is being emitted from distant galaxies. As they move away from Earth, their perceived frequency is lower due to the reflected light being spaced out further and further, and this causes it to adopt a red hue when we observe it because red is the lowest wavelength of visible light. From this data, Percival and his team can then construct a galactic map of sorts and use the different patterns and distributions of galaxies to infer cosmological information.

A perfect example of one of these projects was the extended Baryon Oscillation Spectroscopic Survey (eBOSS), of which Percival was a survey scientist at the senior management level. It was the largest galaxy redshift survey at the time, and it used the Sloan Foundation’s telescope to observe the position of galaxies to produce a map of the universe. The results were ultimately consistent with the standard picture in cosmology. Even though there were no anomalies to report, Percival was still proud that the public recognized the work they were doing and that they were able to secure enough money to both continue the experiment to its end as well as start funding the creation of the Dark Energy Spectroscopic Instrument (DESI) and the Euclid telescope.

After eBOSS, Percival shifted his focus towards DESI and began laying the groundwork for the project, submitting proposals to the government and taking up various management positions. The purpose of DESI, like all things in cosmology, is deceivingly simple: find out what dark energy is.

But what is dark energy anyways? To explain that we have to consider the expansion of the universe. Previous observations with supernovae have shown that the rate at which the universe is expanding is accelerating. However, this finding contradicts what theoretically should happen — as more parts of the universe come into being, the force of gravity from an ever-increasing number of celestial objects should actually slow down its expansion. Physicists currently cannot offer a convincing explanation as to why this occurs, but they believe it has to do with the as-of-yet undiscovered physics surrounding “dark energy” and “dark matter.”

“We have three years of data… we are currently working on the first year of data,” Percival said. “We have a standard theory of what dark energy could be, and it’s called the cosmological constant… which can be interpreted as a sort of zero point energy density of the vacuum. We call this lambda and it has various predictive properties of the universe.” Using optical spectra of galaxies and quasars obtained from DESI, Percival and his team hope to test if expansion happens as predicted within this lambda model so they can glean more about the characteristics of dark energy.

Along with DESI, Percival is also currently working with the European Space Agency as a science coordinator for the Euclid telescope, which is conducting a galaxy redshift survey as part of a larger project. The data from Euclid will differ slightly because instead of being ground-based, it’s in orbit around the Earth. The structure contains a grism, a type of grating-based prism that disperses incident light and creates a spectrum from which they can analyze.

At UW, Percival is also serving as the director of the Waterloo Centre for Astrophysics and is involved in promoting astrophysics research at the university. He hopes to establish a vibrant postdoctoral program to enhance the research being conducted now as well as provide more opportunities for future studies to be done. “I enjoy what I do… I mean, it’s incredibly lucky to have a career that’s also a hobby,” he muses. “There’s no such thing as a typical day… different things happening. I could be lecturing, I could have early meetings.” When asked if he had any advice for prospective cosmologists, he said, “It’s a tough career path to pick, but one that’s very rewarding… you have to focus on it, and you need good grades, bottom line. At each stage, you have to keep pushing yourself to get to that next stage.”