This month the Faculty hosted a conference in honour of its first female professor, Anne-Christine Davis, who retired in September 2018. The conference explored the latest research in cosmology and celebrated Davis' long and pioneering career in theoretical physics.
You would expect a conference which honours an individual to celebrate their work and professional achievements. You may also expect a few personal anecdotes. What you wouldn't necessarily expect is for all of this to happen in the atmosphere of a joyous family reunion. But that's what this one-day event felt like.
Speakers included Davis' current PhD student, former students now well into their own research careers, as well as peers from the very start of Davis' professional life. They had come from as far as Chile, Canada and the US, and one of the speakers, long-time collaborator Ruth Gregory, turned up as a surprise. It was clear that Davis was being celebrated not only for her work, but also for the support she has given to her students and the role she has played in bringing the UK cosmology community together.
The very small and the very large
Davis' research has led her from one end of theoretical physics to the other. "I remember, as an undergraduate, I was interested both in the physics of the very small and the physics of the very large," she says. When she arrived at Cambridge in 1983 she was working on the former; exploring theories that describe the fundamental particles of nature. In particular, she was looking at how symmetries that are inherent in these theories are broken.
The idea that theories contain symmetries may sound puzzling at first, but becomes clear when you think of a pencil balancing on its tip. Once it has toppled over, as we all know it's bound to, it will point in a particular direction. This isn't because the laws of nature favour that direction, but because something — a bump, quantum fluctuations, a gust of wind — has pushed it over in a particular way. In other words, the laws of nature are symmetric under rotations in space, but the outcomes of these laws, which we observe, are not: they break the symmetry.
"[Similarly], theories in particle physics usually have some fundamental symmetry about them, but the nature we observe doesn't necessarily have this symmetry," explains Davis. In her early work she was interested in mechanisms which can break the symmetries of a theory and which are themselves part of the theory — something known as dynamical symmetry breaking.
Once at Cambridge Davis learnt about hypothetical objects called cosmic strings which, as their name suggests, were relevant in the physics of the very large. "At the time it looked as though cosmic strings could explain some of the cosmology we observe, [for example, they could explain how large structures such as galaxies and clusters of galaxies formed in the Universe]." says Davis. The theory that postulates the existence of cosmic strings has them emerging from a symmetry breaking event in the early Universe. "This sounded very interesting to me. It involved symmetry breaking and so combined what I understood in particle physics with cosmology."
The chameleon
Cosmic strings marked Davis' entry to cosmology, but she has since moved on to other topics too. A favourite involves dark energy and a theory called the chameleon. Dark energy is the name given to the mysterious force, the so-called fifth force, that causes our Universe to expand at an accelerating rate today and the chameleon is a mechanism that can explain dark energy. It postulates an extra force of nature which drives the acceleration, but which we cannot detect because it is "screened" around dense bodies like the Earth.
Davis and others developed laboratory experiments with which to detect whether the chameleon, as well as other so-called modified gravity theories, are correct. We now know that the chameleon does not explain dark energy, but Davis' lab tests have given us ways of testing the predictions of the general theory of relativity on unprecedent scales. "Who would have thought that you can test general relativity at scales of 30 micrometres? [You'd think there's] no way, but you can," says Davis. So far, Einstein's theory has passed these micro-tests with flying colours.
The pioneer
The boundaries of theoretical physics weren't the only ones that yielded to Davis during a career that spanned over four decades. She was the first woman with a position in the theory group at CERN, the first woman to get a permanent position in DAMTP (in 1996), the first female professor in the Faculty of Mathematics, and the first female professor in theoretical physics at the University of Cambridge (to name just a few firsts).
"I've never seen myself as being a female pioneer," Davis protests. When she learnt that she was the first female professor in the Faculty she felt overwhelmed at first and then angry for her predecessors who never had that chance. "But I hope that I have helped others to become professors themselves," she says.
Davis' retirement conference clearly demonstrated this: it was a joy to see a number of female speakers being introduced by a female chair, in front of an audience with a substantially female component. All the women who spoke had warm praise for Davis as an encouraging and supportive supervisor or colleague.
But Davis also worked to support women on an institutional level, starting from the early 1990s when, she says, DAMTP was a rather hostile environment for women, who were feeling isolated and unsupported. To remedy the situation Davis helped open the debate about gender equality in DAMTP in the early 1990s. She has also helped raise awareness of gender balance in the undergraduate population and been the University's gender equality champion for STEMM (science, technology, engineering, mathematics and medicine). Women entering the Faculty today would not be able to imagine the conditions they would have faced only 25 years ago, and that is to no small extent due to Davis' efforts.
But what does the future as a professor emeritus hold? "I'm still doing research, I still have collaborators, and I'll still have research students for another academic year," she says. And while it took her a while to get used to not having a time table, she enjoys the newly-won freedom to go away in term time. Since October she has been helping to look after her first grandchild and in June she is planning to walk a section of the southwest coast path. "That's when my dear colleagues will be marking their exams. I'll send them a postcard: 'hope the marking is going ok. I'm on the coast path. Love, Anne.'"
The image above shows the core of the merging galaxy cluster Abell 520. Credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University).
