"It appears to make up about 70 percent of the mass/energy budget of the universe," he said. "But that's about all we know. It's very mysterious, and we're quite desperate to understand what it is, how it operates, and what its physics is."
Riess' paper has become one of the most cited in the cosmology literature. It has earned him and his colleagues a share of some of the richest and most prestigious awards in science, including the Peter Gruber Prize in Cosmology in 2007, and the Shaw Prize in 2006.
In September, he was named a MacArthur Fellow, accepting a $500,000 no-strings grant in recognition of his groundbreaking work. Last month he was elected to the National Academies of Science.
With enough data from as many Type Ia supernovae as he can find, Riess still hopes to unravel the mystery of dark energy. By 2007 he'd found 25 at least 9 billion light years away.
Riess has continued with related work that last week provided more precise estimates of the expansion rate of the universe, an advance that is expected to help scientists test theories about dark energy.
But the ACS was the only instrument anywhere today with the sensitivity and the wide field of view capable of quickly and efficiently cataloging Type Ia supernovae at vast distances before they fade away.
Atlantis astronauts will be installing a new Hubble instrument - the Wide Field Camera 3 - but it's only half as efficient as ACS at this work.
More and better tools may be coming. "It's a very hot topic right now. NASA and the Department of Energy are in the process of planning what could become a billion-dollar space mission to study dark energy," Riess said. But for now, "the Hubble Space Telescope and ACS are the only game in town, and may be so for the next 15 years."
When that fateful e-mail flashed on his screen in 2007 saying that ACS was gone, Riess' work skidded to a halt.
"Space science is not for the faint of heart, or those who demand continual gratification," he said.
Still, it is a seductive quest, not easily abandoned. It's as if humans had always lived far inland, and suddenly ventured out and discovered the oceans, which cover two-thirds of the planet, Riess said. "We'd go, 'Wow! What is this?' And we'd spend a lot of time studying the oceans and what their significance is."
