Astronomers using the NASA Hubble Space Telescope's Advanced Camera for Surveys (ACS) have found two supernovae that exploded so long ago they provide new clues about the accelerating universe and its mysterious "dark energy." The supernovae are approximately 5 and 8 billion light-years from Earth. The farther one exploded so long ago the universe may still have been decelerating under its own gravity.

(Left) A Hubble Space Telescope image of a portion of the Hubble Deep Field North as originally photographed in 1995 with the Wide Field Planetary Camera 2. (Right) A composite image of the same field as imaged by Hubble's Advanced Camera for Surveys (ACS), combined with the original WFPC2 image. The ACS observations were made in May and June 2002. The red spot is the glow of a very distant supernova captured exploding in the field. The supernova is estimated to be 8 billion light-years away. The supernova appears deep red in this composite image because it was photographed by the ACS at far-red wavelengths. Credit: NASA and J. Blakeslee (JHU)

"We're trying to fill in a blank region where the universe's rate of expansion switched from deceleration due to gravity to acceleration due to the repulsive force of dark energy," says John Blakeslee, an associate ACS research scientist at Johns Hopkins University, Baltimore, Md., and lead author of a new paper due out in the June Astrophysical Journal. "That's a real challenge, but the ACS is making it very straightforward to find distant supernovae and get detailed information about them."

"This beautifully demonstrates that the ACS is a 'supernova machine' for probing the early universe," says co-investigator Holland Ford, who headed the team that developed the ACS camera that was installed on Hubble in March 2002. According to the Johns Hopkins astronomers, the supernovae they discovered will be just the first of many to be identified with the ACS.

Coupled with Hubble's powerful vision, the ACS can pick out the faint glow of the distant supernovae. The ACS can then dissect their light (by spectroscopy) to measure their distances, study how they fade, and confirm that they are a special type of exploding star that are reliable distance indicators.

In 2001 Hubble astronomers found a supernova even farther away. It offered the first evidence the expanding universe was once decelerating. Astronomers are using Hubble's new camera to go supernova hunting for supporting evidence. "We have enough data on the new supernovae to constrain both their distance and the amount of dust obscuration," says Blakeslee. The filtering effects of interstellar dust can lead to misinterpretation of the cosmic distances unless carefully taken into account.

Type Ia supernovae are believed to be white dwarf stars that pull in gas from an orbiting companion star. The white dwarf siphons off mass until it hits a critical point where a thermonuclear "burning" wave of oxygen, carbon, and heavier elements immolates the star in a few seconds. The physics of the explosions is so similar from star to star that all Type Ia supernovae glow at a predictable peak brightness. This makes them reliable objects for calibrating vast intergalactic distances.

The supernovae were found when ACS team members Daniel Magee (University of California at Santa Cruz) and Zlatan Tsvetanov (Johns Hopkins University) compared earlier Hubble images of the same patch of sky with new ACS images and identified the two supernovae. Follow-up observations were then conducted with ACS and other Hubble instruments to get a detailed fix on their intensities and distances from Earth.

Information from studies of Type Ia supernovae confronted astronomers about five years ago with the stunning, unexpected revelation that galaxies appeared to be moving away from each other at an ever-increasing speed. They've attributed this accelerating expansion to a mysterious factor known as dark energy that is believed to permeate the universe.

Looking farther away into the universe (and, because of the distances involved, further into the past), they've seen evidence that gravity was at that time slowing the expansion of the universe. Astronomers have very little data, though, on the period of transition between these two phases, when the repulsion produced by dark energy began to surpass the tug of gravity.

"Continued studies of supernovae will allow us to uncover the full history of the universal expansion," Blakeslee says. "The sharper images, wider viewing area, and keener sensitivity of ACS should allow astronomers to discover roughly 10 times as many of these cosmic beacons as was possible with Hubble's previous main imaging camera."