UW astronomers have hand in ‘Science’ top breakthrough

Astronomy professors Christopher Stubbs and Craig Hogan.

Two UW astronomy professors and two UW graduate students were among dozens of scientists on two teams who this year showed that the expansion of the universe is actually accelerating, a discovery lauded by the journal “Science” as the most important science advance of 1998.

A second international project involving UW physics researchers also made the Top 10 list. That project provided evidence indicating that subatomic particles known as neutrinos have mass.

Astronomy professors Craig Hogan and Christopher Stubbs and students David Reiss and Al Diercks are part of the High-Z Supernova Research Team, a collaboration of 21 scientists and 11 institutions around the world. The loosely organized team was started in 1995 by Brian Schmidt of the Mount Stromlo and Siding Spring observatories in Australia and has studied one type of supernova, which essentially is the blast that happens when a dead star transforms into a natural thermonuclear bomb.

In the physics project, the finding that neutrinos have mass counters assumptions in the Standard Model of particle physics. It had been believed that the electrically neutral, weakly interacting particles have no mass, but the Super Kamiokande Observatory in Japan detected the neutrinos “oscillating” from one form to another, something that could happen only if they do possess mass. UW researchers involved include physics professor Jeffrey Wilkes and the late professor Kenneth Young, along with post-doctoral researcher Larry Wai, graduate students Jeff George and Andy Stachyra, and research engineer Hans Berns.

Above from left are students Eric Zager and Qi Ling Huang, research engineer Harry Berns, Physics professor Jeffrey Wilkes, and student Andy Stochyra.

The finding could have major implications for astrophysicists trying to find the “missing” matter in the universe. Some estimates are that perhaps 90 percent of the universe’s mass is “missing,” much of it assumed to be dark matter that emits no light and therefore is not visible with current observing equipment. Wilkes said the finding of neutrino mass indicates “a broken symmetry” in existing models, and added that “such deviations from simplicity in nature are usually a good sign that there is much more to be learned.”

In the “Science” breakthrough of the year, astronomers observed light from supernovas that has been traveling 5 billion to 7 billion years, or between one-third and one-half the life of the universe. In each supernova observed, the team measures how much the light is stretched—the so called “red shift,” or elongation of the wavelengths toward the red end of the spectrum caused by expansion of the universe. (Z is used by astronomers to denote red-shift, thus the name of the team.) The group also looks at how much the light has spread out and been diluted over time.

“We know how bright they’re supposed to be, intrinsically, pretty accurately,” Hogan said, “so their observed brightness tells us what the universe is doing.”

What they’ve found is that the supernovas are fainter than expected. For the light to have taken as long as it has to reach telescopes on Earth, the universe in the past must have expanded more slowly than scientists believed. In fact, it must have actually picked up speed more recently. This can result from an exotic, little-understood force that some have mistakenly referred to as antigravity, Hogan said.

“It’s a component of gravity that pushes things apart instead of pulling them together,” he said. “The repulsive gravity can be caused by a new form of energy that’s never been observed before. But you can’t use it for antigravity boots.”

Accelerating expansion of the universe fits with the Cosmological Constant, a theory Albert Einstein formulated but later discounted as the biggest blunder of his career. However, the problem wasn’t in the theory, Hogan said, but in Einstein’s expected conclusion. He formulated the Cosmological Constant to explain a universe that neither expanded nor contracted, so the repelling gravitational force he envisioned would exactly counter the attracting gravitational force in the universe. It now appears the repelling force is actually greater.

Even the emptiest space, Hogan said, contains gravitational energy that helps make the universe fly apart.

“If even empty space has energy in it, that’s totally new physics—if we are right. And we might not be right.”

However, another team called the Supernova Cosmology Project, headed by Saul Perlmutter of the Lawrence Berkeley National Laboratory, has carried out similar studies since 1988. Using different methods, that team has reached the same conclusion as the High-Z team. Both groups have published their work in scientific journals and it is their combined work that is being hailed as “The Science Breakthrough of the Year.” It is No. 1 on the “Science” list of the top 10 scientific advances for the year.

The High-Z team used observatories in Chile and Hawaii to make their initial observations of supernovas. In each case, members then used smaller telescopes around the world to monitor the stars for several weeks, until the supernova effect disappeared. Among the smaller telescopes is one at the Apache Point Observatory in New Mexico, operated by a consortium that includes the UW. A key to the project, Hogan said, is being able to make remote observations from that telescope using the Internet.

“It’s the perfect tool for this kind of project,” he said. ¶

Vince Stricherz, News & Information