What is dark energy?

Dark energy is believed to be a component of the Universe whose repulsive gravity is presently accelerating the expansion of the Universe.€а Although it comprises the bulk of the mass-energy of the Universe, its nature is not well understood.€а €аAlbert Einstein originally suggested its existence about 80 years ago.

What is a supernova, and why is it an important tool in gauging the universeтАЩs behavior?

A supernova is the explosive death of a star, which unleashes a burst of light through the cosmos. These violent deaths occur once every 100 years in a typical spiral galaxy like our Milky Way. Some astronomers call some types of supernovas natureтАЩs тАЬ60-watt light bulbsтАЭ because they burn at nearly the same brightness. By measuring their predictable light output, astronomers can estimate how far they are from Earth. Many of them are billions of years away. But supernovas blaze so brightly that they can be seen far across space. ThatтАЩs why some astronomers also call them тАЬcosmic mile markersтАЭ: their light provides important information about the universeтАЩs behavior. Supernovas illuminate the dark corners of space, allowing astronomers to map the history of the universeтАЩs expansion.

How was the discovery of dark energy made?

Two teams of scientists measured about 100 supernovae to gauge changes in the expansion rate.€а They expected to find the expansion rate to be slowing down, but were stunned to find it speeding up!€а A detailed description of how this was done for the High-z Team is HERE. Watch the first video broadcast of this discovery, 02/27/98, PBS NewsHour with Jim Lehrer.

Why do we need to see even more distant supernovae?

Faraway supernovae provide additional, convincing evidence that the expansion of the cosmos is actually accelerating. In 1998 information from a passel of other distant supernovas hinted at an increasing expansion rate as inferred by their excessive faintness. Some astronomers, however, had proposed alternative theories to explain this evidence.€а More distant supernovae are used to test, and ultimately refute these alternatives (specifically that grey dust or evolution was dimming SNe in lieu of dark energy).€а