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The experiment that confirmed—and almost beat—COBE

30 August 2018

Led by innovative Canadian astrophysicist Herbert Gush, a 1990 sounding rocket experiment acquired definitive evidence that the cosmic microwave background has a thermal spectrum.

The history of characterizing the cosmic microwave background (CMB) seems straightforward enough. Arno Penzias and Robert Wilson serendipitously detected the CMB in 1964. The Cosmic Background Explorer (COBE) spacecraft delivered evidence in the early 1990s that the CMB has a blackbody spectrum and subtle anisotropies, offering overwhelming evidence for the Big Bang and strong support for the theory of cosmic inflation. The Wilkinson Microwave Anisotropy Probe (WMAP) and Planck, complemented by ground-based and aerial studies, subsequently delved into those anisotropies with ever-greater sensitivity.

Herbert Gush and COBRA
Herbert Gush looks over the outer skin of COBRA after the successful 1990 launch. Credit: Martin Dee, courtesy of Mark Halpern

But as is often the case with history, the real story contains far more complexity. One such wrinkle in the CMB time line got renewed attention a few months ago when University of British Columbia astronomer Mark Halpern received the Carlyle S. Beals Award from the Canadian Astronomical Society. Halpern’s award citation described his important work building instruments for the COBE and WMAP missions. But he was also part of a team that came tantalizingly close to scooping COBE and attaining the first precision energy spectrum of the CMB.

On 20 January 1990, Halpern, along with Ed Wishnow and principal investigator Herbert Gush, launched a sounding rocket from the White Sands Missile Range in New Mexico. Over the course of five minutes, the payload telescope, interferometer, and bolometer obtained enough data to produce a smooth blackbody curve at a temperature of 2.736 K. The team’s successful launch came a week after COBE principal investigator John Mather presented the satellite’s first spectrum results at a meeting of the American Astronomical Society (AAS). If not for a case of bad luck, Gush and his team probably would have been first to attain that measurement, which was ultimately deemed worthy of the Nobel Prize in Physics.

No waiting for COBE

A dozen years after Penzias and Wilson’s discovery, NASA approved the COBE mission to measure the CMB. Scientists were particularly interested in a search for anisotropies and a measurement of the energy spectrum. By the 1970s the Big Bang model was well accepted, but a blackbody curve for the CMB would definitively demonstrate that an initially hot and dense universe has expanded and cooled over billions of years.

The motivation was compelling enough that scientists weren’t willing to wait for the launch of a satellite. Throughout the 1970s and 1980s, several balloon and sounding rocket experiments measured the CMB spectrum, with limited success. Some groups amassed evidence of a thermal spectrum; another team measured excesses in energy intensity at certain wavelengths that were inconsistent with theorists’ simple blackbody interpretation.

Gush, an astrophysicist at the University of British Columbia, immediately took an interest in the challenge. He was a pioneer in Fourier transform spectroscopy, the method of choice for attaining the CMB spectrum. “I had a lot of experience in spectroscopy,” Gush says. “It was an interesting problem, and I knew what to do technically.”

Unlike the researchers on other CMB projects at the time, Gush had no collaborators dedicated to the experiment. That didn’t stop him from thinking big. He decided to use a rocket rather than a balloon, to rise above more of the atmosphere. And he would cool the bolometer for measuring the radiation with liquid helium, the first time a 3He refrigerator would be launched on a rocket.

Gush sent his first rocket up in 1971 and the next in 1972; both failed to make a clean measurement. He made improvements and tried again in 1978. This time the rocket, after detaching from the payload, overtook the telescope and spectrometer. Gush ended up obtaining a spectrum of the rocket exhaust.

Rocket launch at Churchill
Herbert Gush launched several sounding rockets in Churchill, Manitoba that failed to attain clean measurements of the cosmic microwave background energy spectrum. Credit: NRC Canada, courtesy of Mark Halpern

Gush’s complications went beyond the experiments themselves, according to James Peebles, Lyman Page, and Bruce Partridge in their 2009 book, Finding the Big Bang. One time Gush’s dewar of 3He got clogged on a flight to the launch site in Churchill, Manitoba; he acted quickly to clear the blockage and prevent an explosion. Gush then got stranded in Churchill because hockey-obsessed locals, determined to watch a game that wouldn’t be broadcast in the small town, had booked all the outgoing flights to Winnipeg.

After about a decade going at it alone, Gush finally brought in some help. Halpern joined the team as a postdoctoral researcher, having just helped build a prototype COBE spectrometer at MIT, where he worked for COBE cofounder—and eventual LIGO cofounder and Nobel laureate—Rainer Weiss. Gush and Halpern spent six years at work, focusing on maximizing their instrument’s ability to absorb radiation during the short flight and finding a suitable material to serve as a reference blackbody. Ultimately they settled on glass.

A close second

By the summer of 1989, Gush, Halpern, and graduate student Wishnow were ready for final tests before launching their new and improved instrument, which they called COBRA. They took the rocket and payload to Bristol Aerospace in Winnipeg for a shake test, to ensure that none of the equipment would vibrate uncontrollably during the flight. But the engineers performing the test shook the rocket far more violently than they should have based on its weight. “It shook the apparatus to pieces,” Gush says.

Halpern estimates that the error set the team back six months. “If not for that, we would have flown first,” he says.

As it happened, COBE launched in November 1989 and quickly began collecting data. On 13 January 1990 Mather received a standing ovation at the AAS conference when he showed a curve based on nine minutes of the satellite’s data: a beautiful blackbody spectrum at 2.735 K.

Exactly a week later, Gush, Halpern, and Wishnow launched their rocket. Data collection lasted only several minutes, yet that was more than enough time to lock in on the CMB signal. “We already knew on its way up what we were seeing,” Halpern says. By May the British Columbia researchers had submitted a paper with a measurement that was more precise than the initial results presented by Mather. (COBE, of course, would end up providing far more precise results over the next few years.)

Data from COBE and COBRA
The published energy spectra of the cosmic microwave background, as measured by COBE (left) and COBRA. Credit: J. C. Mather et al., 1990 (left); H. P. Gush et al., 1990

For CMB researchers, the COBRA results came at a perfect time—it’s rare that such a groundbreaking measurement can be confirmed so quickly. Still, Gush and Halpern immediately knew that COBRA would forever be remembered as the confirming decisive measurement of the CMB spectrum, rather than the original one. Mather would go on to share the 2006 Nobel Prize in Physics with George Smoot, another leader of COBE. “It would have been fun and exciting,” Halpern says of coming in first. “But the right science came out in the end.”

Halpern, who in addition to his CMB work is involved in projects such as BICEP2 and CHIME, an effort to map the universe’s expansion with a radio telescope, says he is most disappointed that his former boss hasn’t received the recognition he deserves. “If Herb had won more prizes and become more famous, I’d be happy,” says Halpern, who notes that Gush was also a pioneer in long-baseline interferometry. “He’s done all these amazing things and he’s not a household name. It’s too bad.”

Mather adds his praise: “I have great admiration for what he did. He made a great instrument design, and he made it work.”

Gush, like his former University of British Columbia colleague, has no interest in playing the what-if game. Some years ago he and his wife inherited and moved to a large cattle farm dotted with olive trees in Sicily, where he says he rarely contemplates his physics career. “No, I don’t think about that,” Gush says of coming close to a Nobel Prize. “It’s not useful to make that type of speculation.”

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