More than that, he said, dark energy lies at the crossroads of the two best theories scientists have to describe how the universe operates - quantum mechanics, which describes the realm of the sub-atomically small, and Einstein's general theory of relativity, which describes the physics of the very large, especially gravity.
"And these two theories don't work together," Riess said. "Somehow, we need to understand how Nature manages to do physics on all scales without having two sets of rule books." Theorists have offered plenty of guesses, "but none of them are very good."
The importance of continuing Riess' work was clear to everyone. The committee that allocates observing time on Hubble to the world's astronomers - turning away 9 of 10 applicants - had awarded him large segments of time to search for Type Ia supernovae.
Without ACS, however, he was stymied. And a repair seemed just out of reach.
Three months before the camera broke down, NASA Administrator Mike Griffin had finally authorized one last servicing mission by shuttle astronauts, but their to-do list was already long and arduous.
Astronauts John Grunsfeld, Drew Feustel, Mike Massimino and Mike Good were training for six spacewalks - each seven hours long - to replace Hubble's batteries and gyroscopes, install two new scientific instruments and attempt to repair the Space Telescope Imaging Spectrograph, which had quit working in August 2004.
Now Riess and others had to persuade NASA to squeeze an ACS repair onto the work list. It was no shoo-in. ACS was never built to be opened up in orbit and repaired.
But engineers at the Goddard Space Flight Center began to consider the challenge. It would take specialized new tools, and a repair choreography would have to be developed. But everyone wanted to leave Hubble in full working order. Engineers, astronauts and NASA managers all agreed to attempt the ACS repair, on the mission's third spacewalk.
"I am working with a team that's just total overachievers," Riess said.
But in September, another glitch arose. Hubble's science data downlink computer failed, and NASA delayed the repair mission's planned October launch. Goddard engineers needed to test a spare computer and figure out how to add another repair to a jam-packed mission.
Now, they think they're ready. "We just have to pray we have a nice, clean mission," Riess said. "If we lose one [spacewalk] we probably lose an instrument repair ... and that may be ACS."
Hubble servicing missions are always high drama for astronomers everywhere, and especially at the Space Telescope Science Institute in Baltimore. NASA managers give the ACS repair only a 50/50 chance for success.
But "if things go really well," Riess said, "Hubble could be better than it's ever been."
