Jim Peebles and the cosmic microwave background

Jim Peebles is a giant of science. He was studying physical cosmology long before it was considered a serious, quantitative branch of physics, and has done much to establish its respectability. Peebles also has contributed a great deal to the thinking about dark matter and dark energy.

Jim Peebles

Legendary physical cosmologist Jim Peebles makes a point during a lecture at the University of Washington May 19, 2015. Photo: Greg Scheiderer.

Peebles, the Albert Einstein Professor of Science emeritus at Princeton University, gave a lecture titled “Fifty Years of the Cosmic Microwave Background” recently at the University of Washington.

“The last 50 years have seen a truly transformative advance in our understanding of the world around us,” Peebles noted in opening the talk. He explained that the idea of the Big Bang had been bouncing around for a while, and in the early 1960s folks were setting out to prove it as fact. Peebles was a research associate with Bob Dickie at Princeton, and the two of them advanced the idea of the cosmic microwave background. Along with research associates Peter Roll and Dave Wilkinson, they built a microwave radiometer to detect the signature of a hot Big Bang.

Little did they know that the evidence had already been spotted and measured.

Several years earlier, Bell Telephone Laboratories in New Jersey had done an experiment in communication using microwave radiation.

“This was an important forerunner to the sight of our students wandering around campus staring at their cell phones,” Peebles quipped. The experiment also found a lot of background radiation despite the best engineering efforts to eliminate it. By 1963 Bob Wilson and Arno Penzias at Bell wanted to use the technology to do radio astronomy, but they needed to solve the problem of the system noise.

“The Bell people had this constant irritation,” Peebles said. “They were getting more radiation than they expected from their communications experiments.”

It must be the CMB

Peebles had already been doing lectures about the possibility of the cosmic microwave background. By 1964 the Bell folks and the Princeton people got together. Peebles and Dickie figured that the system noise plaguing Wilson and Penzias was actually the cosmic microwave background.

“We had the possibility of a great discovery,” Peebles recalled. “We already knew right away that this was something new. That was exciting because you have a new phenomenon, something new to measure, and something new to make theories about.”

Measuring to prove it

The measurement piece took a quarter century, and was accomplished with spectacular precision by two experiments just months apart in 1990: NASA’s Cosmic Background Explorer (COBE) satellite, headed by John Mather of the Goddard Space Flight Center and George Smoot of Berkeley, and a rocket-borne experiment launched by Herb Gush of the University of British Columbia, along with Mark Halpern and Ed Wishnow. Both projects, in development for about 15 years, made measurements that meshed perfectly with the theoretical predictions for the cosmic microwave background.

COBE all-sky map

COBE all-sky map. Image: NASA.

“It’s a glorious piece of evidence, I would say an iconic piece, that shows tangibly that the universe had to have evolved from a different state, because this is a thermal spectrum,” Peebles marveled. “Our universe as it is now is transparent for this radiation. There is no way it could force the radiation to relax to this thermal equilibrium. The universe had to have evolved from a state in which it was dense and hot enough to have relaxed to equilibrium and then expanded away from it.”

Interestingly, this is a tale of “missed it by that much” when it comes to Nobel Prizes. Dickie, Peebles, and the Princeton team were well on their way to making the measurement when they learned that Wilson and Penzias had already stumbled across it. The latter two won the Nobel in 1978 for their work. Mather and Smoot won the Nobel in 2006 for their COBE measurements, but Gush may have beaten them to it had it not been for equipment troubles that delayed the launch of his experiment.

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