Hubble keeps on earning its upkeep

The Hubble site puts it like this:

Scientists using NASA's Hubble Space Telescope have discovered that dark energy is not a new constituent of space, but rather has been present for most of the universe's history. Dark energy is a mysterious repulsive force that causes the universe to expand at an increasing rate. Investigators used Hubble to find that dark energy was already boosting the expansion rate of the universe as long as nine billion years ago. This picture of dark energy is consistent with Albert Einstein's prediction of nearly a century ago that a repulsive form of gravity emanates from empty space. Data from Hubble provides supporting evidence to help astrophysicists to understand the nature of dark energy. This will allow them to begin ruling out some competing explanations that predict that the strength of dark energy changes over time.

Researchers also have found that the class of ancient exploding stars, or supernov used to measure the expansion of space today look remarkably similar to those ae,that exploded nine billion years ago and are just now being seen by Hubble. This important finding gives additional credibility to the use of these supernovae for tracking the cosmic expansion over most of the universe's lifetime. Supernovae provide reliable measurements because their intrinsic brightness is well understood. They are therefore reliable distance markers, allowing astronomers to determine how far away they are from Earth. These snapshots, taken by Hubble reveal five supernovae and their host galaxies. The arrows in the top row of images point to the supernovae. The bottom row shows the host galaxies before or after the stars exploded. The supernovae exploded between 3.5 and 10 billion years ago.

The AP's Matt Crenson wrote yesterday:

The Hubble Space Telescope has shown that a mysterious form of energy first conceived by Albert Einstein, then rejected by the famous physicist as his "greatest blunder," appears to have been fueling the expansion of the universe for most of its history.

This so-called "dark energy" has been pushing the universe outward for at least 9 billion years, astronomers said Thursday.

"This is the first time we have significant, discrete data from back then," said Adam Riess, a professor of astronomy at Johns Hopkins University and researcher at NASA's Space Telescope Science Institute.

He and several colleagues used the Hubble to observe 23 supernovae _ exploding white dwarf stars _ so distant that their light took more than half the history of the universe to reach the orbiting telescope. That means the supernovae existed when the universe was less than half its current age of approximately 13.7 billion years.

Because the physics of supernova explosions is extremely well-known, it is possible for the astronomers to gauge not just their distance, but how fast the universe was expanding at the time they went off.

"This finding continues to validate the use of these supernovae as cosmic probes," Riess said.