A plot from Dr Kimmy Wu’s recent paper which shows areas where there is either more matter than average (over-densities) or less matter (under-densities) along the line of sight.

Measuring the mass of the universe

17 May 2024

Dr Kimmy Wu was awarded a Croucher Fellowship in 2015 at the University of California, Berkeley, soon after she visited Antarctica to help deploy the telescope facility she used for data collection.

She’s now at the US Department of Energy’s SLAC National Accelerator Laboratory which is operated by Stanford University and Croucher News talked to her recently to discuss her recent research.

Croucher-funded scientists tend to ponder big questions in their work, and they don’t come any bigger than “measuring the total mass of the universe,” which is a focus of Wu’s current research. Wu, who is also a Panofsky Fellow at SLAC, is part of a team of scientists using data from the new camera for the South Pole Telescope (SPT). This is a 10-metre-diametre telescope located at the Amundsen-Scott South Pole Station in Antarctica. “This work uses the first-season data from the SPT-3G camera and is essentially a measurement of the total matter density in the universe from these observations of the oldest light. And it’s pretty exciting because this measurement can now tell us how much matter there is and how clumpy the matter is in the universe, as precisely as the data sources people used to rely on, or even in more precise ways,” Wu explained.

This measurement can now tell us how much matter there is and how clumpy the matter is in the universe.

The term “clumpy” refers to the clustering of matter in specific areas rather than being evenly spread out. The “clumpiness” of the universe is a matter of debate among cosmologists. The vastness is not just of space but of time, as the measurements look at light emitted when the universe was in its very early stages.

When Croucher News asked how a cosmologist spends her time, Wu replied that in her case, a lot of it was spent coding or in meetings. The coding she does is important because a lot of her work involves data analysis for large, complex, and highly specialised data sets with their own particular structures. “For many of the tasks, we can’t use off-the-shelf solutions,” she told us.

And the meetings are important because science, including cosmology, is increasingly collaborative. “It’s the best way to keep up with where everyone has got to in their research. Although I have moved institutions a couple of times, I still talk to a lot of the same scientists who are working on similar projects in other places,” Wu said.

“You know, people think that astrophysicists or cosmologists sit in solitude thinking or are working out stuff on the whiteboard, but in reality, we spend a lot of time talking to each other, updating each other, and learning from each other,“ she added.

Wu initially became a physicist to satisfy her curiosity about the universe. “At school, I read The Universe in a Nutshell by Stephen Hawking. I remember there were a lot of pictures, and I recall reading about black holes and the twin paradox and really not understanding how all of that works.”

“So that made me really want to understand it. There was a desire, a drive, for me to understand how it all works. So that started me on this journey,“ Wu said. After that, her interest and motivation have been sustained by a series of good mentors and colleagues.

There was a desire, a drive, for me to understand how it all works. So that started me on this journey.

In addition, there’s the inherent wonder in what she is researching: “It’s incredible that the universe we inhabit gives us a way to probe the earliest 10 to the power of minus 33 seconds of the universe (that’s 32 zeroes after the decimal point!). And I think, in a way, what we do is similar to art in the sense that you can’t really place a price tag on it, that it shows all of us how much bigger the universe is and how small we are,” Wu told us.

Wu says that her Croucher Fellowship in 2015 was pivotal in her career. “It came at a time when I was about to get my PhD, and I was thinking about next steps. It enabled me to start a postdoc at Berkeley and join SPT. And that’s what allows me to work with these two data sources—the larger telescope, SPT, along with BICEP, which I was working with before.”

The ability to make more and more precise measurements and thus do more stringent tests of our standard cosmological models has been one of the biggest developments in her field in recent years. Another tool that is likely to become increasingly important is AI. Wu has been working on the application of machine learning to her work for some time and thinks that AI systems may benefit from using cosmological data sets while also offering a way to help analyse those same huge and complex data sets. “There’s huge interest in my field about all this, but there’s a way to go before we see a big impact,” she told us.

However, when thinking of what advice she might give to young students or researchers entering her field, she feels that being familiar with AI is one thing they should work on. “It’s likely to be an essential tool for us in the years to come,” she said. “But equally, communication skills that support high-quality collaborations with other scientists will be very important for them too,” she added.

Dr Kimmy Wu was awarded a Croucher Fellowship in 2015. To view her Croucher profile click here.