Here is why things must have slowed down:
"Early on the universe had lots of mass in a small volume," explains John Blakeslee of Johns Hopkins University. "The pull from gravity must have been enormous." As the universe expanded, gravity would have become less effective over the larger distances, and dark energy would have taken over.
One previously detected supernova, at about 10 billion light years away, supported this idea when reported in 2001, but the object proved difficult to study. The Hubble observations presented today also support the switch, said Blakeslee, lead author of a paper on the findings that will be published in the June Astrophysical Journal.
Had the universe always been accelerating, the supernova that's 8 billion light-years distant would have been dimmer, Blakeslee said in a telephone interview.
"It's not conclusive at all," Blakeslee said of his work. Another 20 or so very distant supernova are needed to make a strong case, he added.
Those 20, and then some, will not take long.
Hubble's new eyesight "should allow astronomers to discover roughly 10 times as many of these cosmic beacons as was possible with Hubble's previous main imaging camera," Blakeslee said.
Already in the bag
A separate Hubble project, led by Adam Riess of the Space Telescope Science Institute, has already bagged several distant supernovae. Riess, who worked on one team that made the 1998 acceleration breakthrough, reported preliminary results of his latest work at the APS meeting and is expected to publish a paper soon, possibly later this year.
Eleven other distant supernovae have been examined by Hubble in another study headed up by Saul Perlmutter of the Lawrence Berkeley National Laboratory. Perlmutter led the other team involved in the 1998 discovery of acceleration.
Perlmutter was not involved in the two new Hubble discoveries, but he told SPACE.com they are among many important steps that could lead to a firm determination of when acceleration began. Many of the discoveries are coming from ground-based telescopes, he noted, but Hubble "is really becoming a key for everybody in terms of follow-up" to glean the necessary detail.
By building a strong historical timeline of the universe, astronomers and cosmologists hope to answer a pressing question: Do the properties of dark energy change over time?
The answer would help them determine what dark energy is and would allow refined predictions about the origin and fate of the universe. No one expects a quick resolution, however.
Michael Turner, one of the world's foremost cosmologists from the University of Chicago, said solving the dark energy problem "is going to require a crazy idea." While most leading theories project the universe will accelerate forever, perhaps even to the wild point that it rips all matter apart, the notion that it might eventually collapse has not been ruled out, Turner said at the APS meeting.
"The destiny question is wide open," Turner said. |
