UF distinguished professor of physics, Dr. Pierre Sikivie, is an expert in cold, dark matter. The kind of cold, dark matter that likely makes up 85% of the mass of the universe, but no one has yet observed. Its theoretical presence helps account for the expansion rate of the universe and explains why many spiral galaxies, as they spin around their cores, don’t seem to be following Newtonian laws of gravitation. According to some physicists, this mysterious dark matter is made up of “axions” – huge quantities of axions – that individually weigh next to nothing, don’t have any charge, and thus would be almost impossible to detect. Each might weigh ~10-42 kg. We could all be swimming in a sea of axions and not know it. But on galactic scales, their numbers really add up and would impact the motion of the stars.
But first, some backstory. Dr. Sikivie was born in 1949 in the Flemish part of Belgium. His father was a medical doctor by training but a dentist in practice. At age twelve, the young Pierre received a radio kit, full of wires and transistors, as a gift from his father. That was really a turning point in stimulating his interested in science and technology. Later, when it was time for college, instead of going into medicine, he decided to become a physics major at the University of Liège. He really liked the prospect of doing scientific research, especially as a theoretician. He received his License in Physical Sciences from Liège in 1970.
Resources in academia were scarce in post-war Europe, so the logical place for graduate school was the United States. Pierre was accepted at several American universities and decided to go to Yale because they offered him a fellowship and because theoretical physicist (and future Nobel Laureate) Dr. Lars Onsager was there. However, Onsager turned out to be in the wrong department – chemistry, not physics! – so Pierre could not study under him. Oops, oh well. Luckily, he was unperturbed. Despite not knowing much English, which made for a tough first year as a grad student, Pierre sailed through Yale’s physics qualifying exam and established a productive collaboration with his thesis advisor, Professor Feza Gürsey, and a junior faculty member, Dr. Pierre Ramond. Shortly after Dr. Sikivie received his doctorate in 1975, the three of them published a paper on a grand unified theory (GUT) of particle physics, which helped explain the interrelationship between the weak nuclear, strong nuclear, and electromagnetic forces in quantum mechanics.
Dr Sikivie worked on a variety of mainstream topics in theoretical particle physics while in postdoc positions at the University of Maryland, Stanford Linear Accelerator, and CERN, including grand unified theories, CP violation by the weak interactions, and the “technicolor” mechanism by which W and Z bosons acquire mass. In 1981, he came to the University of Florida as an assistant professor, lured by the fact that one of his GUT collaborators, Dr. Pierre Ramond, had also recently accepted a faculty position there. “It seemed like a good opportunity at UF, with a critical mass of theorists there.” His wife, Dr. Cynthia Chennault, an authority in medieval Chinese literature, was also offered a position at UF.
At UF, Dr. Sikivie began to work on the cosmological properties of axions. The axion had been postulated in the 1970’s to explain why the strong interactions obey the discrete symmetries P and CP found in many GUT formulations. He realized that, unless certain conditions were met, axions would have long ago caused a cosmological disaster, such as a runaway expansion rate for the observable Universe. Following up on this, he and others theorized that very low mass axions could be present in the Universe as dark matter – out there, but seemingly undetectable.
In an interesting example of serendipity, during his second year at UF Dr. Sikivie was assigned to teach a class on electricity and magnetism. As many educators can attest, the best way to become familiar with a subject is to teach it. For Dr. Sikivie, as he made sure he understood the subtleties of Maxwell’s equations, it led to an “aha” moment. He realized that in the presence of a strong magnetic field the axion would, with some probability, convert to a photon. What quickly followed at UF was the development of the axion haloscope, which uses a resonant microwave cavity and a superconducting magnet to search for axions in the local galactic dark matter halo. This was soon followed by an axion helioscope, where axions presumably produced in the sun can be converted into x-rays in a laboratory setting.
The ADMX (Axion Dark Matter eXperiment) at the University of Washington and CAST (CERN Axion Solar Telescope) in Geneva are just two examples of ongoing experiments based on Dr. Sikivie’s theories. The search for axions has now stretched on for thirty years without success. Although there may be literally a quadrillion axions per cubic meter, even where you sit, the predicted rate of their conversion to photons is fairly miniscule. The signal to noise ratio of the current equipment may still be too low. Asked if he is discouraged by the lack of detection so far, Dr. Sikivie says “not really.” As he notes, “Further discoveries [in physics] are going to be very hard. Gravitational waves took a hundred years [to detect], and the Higgs boson took almost 50 years.”
When not theorizing, Dr. Sikivie likes to play the violin, which he has done since his teens. “I’m bad at it,” he says, yet he has organized a small ensemble to play with him for seniors at local retirement homes. “Life in an American college town is just about the best for me. Big cities have too many distractions.” Well, Gainesville seems to have provided him with the right environment. His advice to prospective physicists is “Follow your own path, and fight very hard for your ideals. You may be doing great work, even if no one recognizes it. That’s OK.” In 2019, Dr. Sikivie was awarded one of physics highest honors, the APS Sakurai Prize “for seminal work recognizing the potential visibility of the invisible axion, devising novel methods to detect it, and for theoretical investigations of its cosmological implications.” Now that’s an accomplishment we can all recognize and “detect.”