Joel Primack delivers annual Buhl Lecture at Carnegie Mellon

Joel R. Primack, professor of physics at University of California at Santa Cruz, delivered this year’s Buhl Lecture at Carnegie Mellon last Tuesday. The event was held in the auditorium of the Mellon Institute building in Oakland.

An annual physics presentation, the Buhl Lecture is aimed at a broad audience of individuals, in both science and non-science fields.

The speaker is invited by the Buhl chair, a position filled by a theoretical scientist who has exceptional research and accomplishments to his name. The Buhl chair is currently held by Fred Gilman, dean of the Mellon College of Science.

“The Buhl Lecture Series is a great tradition at Carnegie Mellon University, and was made possible in 1961 by a gift from the Buhl Foundation,” said President Jared L. Cohon. “It is an [effort] to honor the far-reaching philanthropy that continues to shape this college and the world of physics.”

During the lecture titled “A Brief History of Dark Matter,” Primack walked his audience through the various theories proposed over the years to explain the existence and function of dark matter, an invisible or hypothetical form of matter that can only be detected through its gravitational effects on visible matter in space.

Primack earned his Ph.D. from Stanford University in 1970. He served as a junior fellow of the Society of Fellows at Harvard University for three years and has co-authored several papers on cosmology and the properties of dark matter, including a paper titled “Formation of Galaxies and Large-Scale Structure with Cold Dark Matter.”

Presently, Primack is a fellow of the American Physical Society (APS) and the American Association for the Advancement of Science (AAAS).

Dark matter has long been one of the greatest mysteries in cosmology, Primack said.

The history of dark matter began in the early 1930s, when Fritz Zwicky, a professor of astrophysics at the California Institute of Technology, noted that the gravitationally confined Coma Cluster, encompassing 1000 known galaxies, orbited at a rapid rate of approximately 1000 kilometers per second. By applying a variety of mathematical equations and theorems to the cluster’s properties, Zwicky concluded that the cluster of galaxies had more mass than previously thought. Based on this evidence, Zwicky suggested that an additional form of matter (dark matter) was responsible for holding the galaxies together in a cluster.

By studying dark matter and other aspects of space, cosmologists hope to understand and explain the origins of the universe.

“When we look out into space, we look back in time,” Primack said.

In the early 1980s, scientists were almost certain that dark matter existed around clusters of galaxies, Primack said. However, by 1984, a new theory of “cold” dark matter versus the original — supposedly “hot” — dark matter was proposed. The cold dark matter theory reinvented the Big Bang theory — that the universe expanded from dense and hot matter that is continually expanding today — and enforced the idea that the universe contains dark matter made up of slowly moving cold particles.

The theory of dark matter was defended, disputed, and diversified by many scientists after Zwicky. But it was not until the turn of the 21st century that cosmologists successfully established the “double dark” cosmological model. The double dark model states that cold dark matter and dark energy constitute 95 percent of the universe.

“We now know the cosmic recipe. [Cold] dark matter makes up 25 percent of the matter in the universe,” Primack said. “Another 70 percent of the matter occurs as a substance called dark energy,” he added.

Dark energy is the force that speeds up the expansion of the universe. With the help of high-tech supercomputers, delving deeper into the subject of dark matter poses fewer difficulties today than several decades ago. Computer simulations help re-create and display the distribution of dark matter in outer space.

“We need the power and efficiency of supercomputers, considering the daunting amount of data we deal with on a regular basis,” Primack said.

Upcoming experiments that explore dark matter will be conducted using telescopes that are larger than existing ones and provide sharper resolution. The experiments will focus on direct and indirect detection of dark matter around the galaxies.

The evening came to an end with a series of interactive astronomical videos that helped the audience understand the underlying mechanisms of the galactic universe and how dark matter plays a vital role in shedding light on the origins of the universe.