If the idea that the universe is flying apart at ever-increasing speeds makes you seasick, better stock up on Dramamine. The most distant exploding star ever discovered has just given that theory a fresh boost, researchers announced yesterday at a press conference in Washington, D.C.
Most astronomers once thought that gravity would keep the universe from expanding at ever faster rates. Then, in the early 1990s, two international teams made a surprising discovery: At greater and greater distances, light from a type of supernovae called Ia becomes progressively fainter than predicted by the simplest model of a steadily expanding universe. Both teams concluded that the expansion of the nearby universe has to be accelerating, pushed by some unseen "dark energy."
The hitch was that, on a cosmic scale, the Ia supernovae weren't very far away--merely a few billion light-years or so. As a result, the universe hadn't picked up enough speed since the stars exploded to make much difference in their brightness. The slight dimming that was observed might be due not to dark energy but to intervening dust clouds that sop up some of the light. Another alternative was that some quirk of cosmic evolution might have made ancient dying stars less bright than today's supernovae.
Now, a supernova called SN1997ff has provided more decisive evidence for universal acceleration. When astronomers spotted it in a follow-up observation of the Hubble Deep Field in 1997, they only knew it was far away. Digging in the Hubble Space Telescope archives to learn more, a team of astronomers led by Adam Riess of the Space Telescope Science Institute in Baltimore spotted SN1997ff in the corner of a series of infrared images, taken over 35 days during an unrelated research project. The images showed that SN1997ff was in a galaxy over 10 billion light-years away. At that distance, the supernova was dim enough to support an accelerating universe, but too bright to be explained by dust or stellar evolution.
"This is tantalizing evidence" that the universe is accelerating at an ever faster pace, and filled with dark energy, says Robert Kennicutt, an astronomer at the University of Arizona in Tucson. "They have done a very careful job with both the measurement and the error analysis, and that is very important in this game."