Two competing teams of astronomers announced the accelerating universe in 1998. The groups, known as the Supernova Cosmology Project and the High-Z Supernova Search team, used the same method. They observed distant star explosions known as Type Ia supernovae. These cataclysms occur when dense stars known as white dwarfs blow themselves to smithereens. Type Ia supernovae are special because they all explode with about the same luminosity and differences in luminosity can be corrected by measurements of their light intensity versus time. In this way, astronomers can use them as approximate “standard candles.” Measuring the apparent brightness of a Type Ia supernova thereby gives astronomers a measure of its distance.

By painstakingly observing Type Ia supernovae at different distances from Earth, and by seeing how their light has been "stretched out" by cosmic expansion, the teams calculated how the universe's expansion has changed over time. The two teams made most of their observations with ground-based telescopes. NASA's Hubble Space Telescope (HST) was also used to extend the supernova observations to higher redshift (larger distances) and improve the accuracy of the expansion measurements. Since 1998, completely independent studies with NASA's Chandra X-ray Observatory and Wilkinson Microwave Anisotropy Probe (WMAP) have confirmed the initial findings: we live in a universe whose expansion is accelerating.

Whatever it is, modern astronomical measurements show that about 72% of the total energy in the universe is dark energy. The rest consists of dark matter (23%) and the familiar atomic matter that makes up our bodies (4.6%). Not knowing the nature of dark energy would be akin to an alien scientist trying to understand Earth's surface without knowing the nature of water. Scientists cannot claim to understand the universe at its deepest level if they can't explain its dominant form of energy. Unlocking the mysteries of dark energy will have profound implications for both physics and astronomy.

Perhaps more important, dark energy controls the fate of the universe. In some theories, dark energy will cause the universe to accelerate forever, eventually spreading out galaxies so thinly that our cosmos will be a dark, cold, and lonely realm. In other theories, dark energy could reverse its properties in the distant future, causing the universe to collapse back upon itself in a "big crunch."

Unlike some mysteries, where nobody has any idea how to solve it, the problem with dark energy is that scientists have developed too many possible solutions. It's possible that one of their ideas is correct, but so far the scientific community has not settled on a single explanation. Here are three of the leading contenders:

1 The Energy of Empty Space
Scientists have known for decades that even supposedly "empty" space could have energy. The energy contained in this vacuum, known as the cosmological constant, could be providing the oomph that is pushing galaxies apart from one another at an ever-faster rate.

2 Modified Gravity
Our theory of gravity, based on the laws of Newton and Einstein, is very well tested on the scale of the solar system. But perhaps over very large distances, such as those between galaxies, our understanding breaks down. A new law of gravity may naturally explain why cosmic expansion is speeding up.

3 A Dynamical Energy Field
Some theorists think that an invisible energy field known as quintessence pervades our universe, and that the nature of this field varies with time. At the moment, quintessence is causing cosmic acceleration. But in the future, it may reverse itself, leading to a big crunch.

Supernova 1994D in
Galaxy NGC 4526

By observing many Type Ia events in their host galaxies, both near and far, scientists can determine how fast galaxies are moving away from us, and this in turn yields crucial information about how fast our universe was expanding at different epochs. In fact, this is how two independent teams of astronomers discovered dark energy in 1998.