Two University of Washington 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 today by the journal "Science" as the most important science advance of the year.
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.
Light from the supernovas being studied 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.
For more information, contact Hogan at 206-685-2112 or by e-mail at hogan@centaurus.astro.washington.edu.
Journal
Science