Remembering Paul L. Richards

Left: Paul L. Richards as a newly appointed assistant professor of physics at UC Berkeley, July 1966. Photo: Marilee B. Baily, courtessy of Lawrence Berkeley National Laboratory; Right: Professor Richards in 1998. Photo: UC Berkeley

“Paul was my thesis adviser on the balloon project, which actually didn’t work right the first time, but became the idea for the Cosmic Background Explorer and my career at NASA,” said Mather, who after COBE took a position as senior project scientist for another highly successful project, the James Webb Space Telescope. “He was a brilliant mentor, a model human being and a great scientist. Paul’s a hero, and his ideas have grown immensely. The COBE mission came directly from his inspiration.”

While COBE was under development, Richards began developing his own set of balloon experiments to probe the CMB’s spatial variation, or anistropy. The Millimeter Anistropy eXperiment (MAX) was the first of those, followed by MAXIMA (Millimeter Anisotropy eXperiment IMaging Array). They detected patterns in the cosmic microwave background (CMB), supporting the idea of inflation in the early universe and proving that the universe is flat, not curved. The results, announced in 2000, confirmed the findings of a competing experiment, BOOMERanG (Balloon Observations Of Millimetric Extragalactic Radiation and Geophysics), co-led by one of Richards’ former students, the late Andrew Lange, which were announced a month earlier. Together, MAXIMA and BOOMERanG provided convincing evidence that the reigning theories of cosmic birth and evolution were correct.

“A subset of cosmological theories, those involving inflation, dark matter and a cosmological constant, fit our data extremely well,” Richards said at the time. “This is a good confirmation of the standard cosmology, and a large triumph for science, because we are talking about predictions made well before the experiment, about something as hard to know as the very early universe.”

MAXIMA used highly sensitive helium-cooled detectors, called bolometers, to measure minuscule fluctuations in the background radiation left over from the Big Bang.

Richards also pioneered detectors based on superconducting sensors and instrumented with superconducting quantum interference devices (SQUIDs), which were used on an experiment called POLARBEAR in the early 2010s to measure the polarization of the CMB. Variations on these detectors are still used today for even more detailed measurements of the CMB, such as those by the South Pole Telescope.

“He needed very sensitive detectors, and of increasingly high frequency, and I eagerly took that on and helped him with these devices,” said John Clarke, one of the pioneers of SQUIDS as detectors and a UC Berkeley professor emeritus of physics. “People in my group built the devices for him. I was always very interested in making the SQUIDS go to ever higher frequencies, and the reason I did that was largely driven by what Paul needed. Our long-running collaboration ultimately resulted in 22 joint publications.”

“That was really a theme in Paul’s career, that he would develop the really difficult, innovative technologies and then go out and measure something very important with them,” said Adrian Lee, a UC Berkeley professor of physics and Richards’ former postdoctoral fellow who later became his collaborator and now leads a group exploring the polarization of the CMB radiation. “Bringing that blockbuster technology forward was a big part of his career.”

A makerspace in the UC Berkeley physics department is being dedicated to Richards in recognition of his belief that building equipment is an important part of learning. Funded in part by a donation from Richards’ family, the Paul L. Richards Innovation Lab will provide valuable hands-on experience with equipment and projects that students are likely to encounter in their scientific career.

From solid-state physics to astrophysics

Richards was first introduced to infrared spectroscopy as a UC Berkeley graduate student in the late 1950s and eventually began building state-of-the-art detectors for far infrared wavelengths — a relatively unexplored region of the spectrum — to study new materials. With detectors he built as a member of the UC Berkeley physics faculty a decade later, he was able to detect a central feature of superconductors — the energy gap — that determines the temperature at which a material becomes superconducting.

In the 1970s, one of his physics colleagues, Charles Townes — the Nobel Prize-winning inventor of the laser who had moved into the field of astrophysics — suggested that these detectors might be used to measure the frequency spectrum of radiation left over from the Big Bang. Though measured in the microwave range in 1965, the radiation had not been measured in the infrared, primarily because infrared detectors at the time were primitive and measuring such radiation was difficult because most of it is blocked by Earth’s atmosphere.

Richards took Townes up on his proposal and built and flew the first helium-cooled infrared spectrometer on balloons, to get above the atmosphere, in the mid-’70s. That and later balloon experiments helped prove the blackbody nature of the CMB, that is, its emission spectrum matched that of an object in thermal equilibrium.

MAXIMA and Boomerang subsequently explored the spatial fluctuations at different scales and by 2000 had data from which other features of the universe could be deduced, including its curvature, its age and the proportions of normal matter (5%), dark matter (27%) and dark energy (68%).

“People like to say that the cosmic microwave background is the Rosetta Stone of cosmology,” Lee said. “That’s a bit hyperbolic, but it’s actually true. We really are learning all these things about the universe from the cosmic microwave background.”

Lee eventually took over the POLARBEAR experiment and leadership of the research group with Richards as a collaborator. Lee currently leads a successor experiment, the Simons Array, based at the James Ax Observatory at 17,000 feet altitude in Chile.