Astronomers used the supernovae to measure the expansion rate of the universe and determine how the expansion rate is affected by the repulsive push of dark energy, a mysterious energy force that pervades space. 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.

Pinpointing supernovae in the faraway universe is similar to watching fireflies in your back yard. All fireflies glow with about the same brightness. So, you can judge how the fireflies are distributed in your back yard by noting their comparative faintness or brightness, depending on their distance from you.

Only Hubble can measure these supernovae because they are too distant, and therefore too faint, to be studied by the largest ground-based telescopes.

These Hubble observations show for the first time that dark energy has been a present force for most of the universe's history. A spectral analysis also shows that the supernovae used to measure the universe's expansion rate today look remarkably similar to those that exploded nine billion years ago and are just now seen by Hubble.

These latest results are based on an analysis of the 24 most distant known supernovae, most of them discovered within the last three years by the Higher-z SN Search Team. The images were taken between 2003 and 2005 with Hubble's Advanced Camera for Surveys.

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