What is dark energy?

Like the "dark side of the Moon," dark energy represents the unknown. In the late 1990s, astronomers discovered that the universe is expanding faster today than they had expected. But they don’t know what is causing the acceleration, so for now, they simply call it "dark energy."

"Dark energy is our ignorance of what’s going on in the universe right now," says Karl Gebhardt, a professor of astronomy at The University of Texas at Austin and one of the principal investigators for the Hetdex project

"What I always like to say is that dark energy is only a phrase, and don’t get hung up on the words dark and energy. [Dark energy] may not be dark, and it may not be energy. All it is is our ignorance of how the universe may be expanding, and we don’t know what it is at this point."

Even so, theorists have already developed several explanations for dark energy. These explanations include an energy born from space itself, new kinds of subatomic particles, and even a flaw in the theory of gravity. Hetdex and later experiments will allow physicists to select the correct one.

"Whatever the answer is," says Gebhardt, "it’s going to be a fundamental change in our understanding of the basic properties of the universe."

Vacuum Energy, or Einstein’s Blunder

If Las Vegas were taking bets on dark energy, the odds would favor a concept known as vacuum energy or the cosmological constant. In essence, it suggests that space itself produces energy, which is "pushing" the universe outward.

New Physics, or Particle X

Physicists have observed lots of particles in the universe: the protons, neutrons, and electrons that make up atoms; the quarks that make up protons and neutrons; and particles with such exotic names as muons, leptons, and neutrinos.

Flawed Gravity, or Relaxing the Grip

For almost a century, Albert Einstein’s Theory of General Relativity has reigned as the explanation for how gravity works. Einstein’s equations showed that gravity is a property of matter, and that matter "warps" the space-time around it.