What is dark energy?

Dark energy may sound like an evil force straight out of the movie Star Wars, but the concept is anything but science fiction. Dark energy is the name given to the force that is driving the universe apart. No one has a clue to what this force (or combination of forces) could be. It represents a whopping 70 percent of the universe's energy-matter budget, and it is the biggest mystery in astronomy.

The concept of dark energy has its roots in the theory of general relativity and our old friend Albert Einstein. When Einstein published this theory in 1916, the universe was thought to be "static" -- that is, not expanding nor collapsing. This is what astronomers observed through the best telescopes of the day. Yet general relativity implied that the "attractive" force of gravity should cause the universe to collapse. To compensate for the fact that the universe appeared static, not collapsing, Einstein added a new force to his equations -- a repulsive force called the cosmological constant created by the vacuum of space. This force was just enough to compensate for gravity and keep the universe static.

By the late 1920s, however, Edwin Hubble discovered that the universe wasn't static after all. Hubble discovered that the universe was expanding and that our Milky Way galaxy was only one among billions of other galaxies. There was no need for a cosmological constant. The "Big Bang" set forth the expansions of the universe and gravity would ultimately slow it down, scientists thought. Ever modest, Einstein called the cosmological constant his "greatest blunder."

For decades to follow, astronomers attempted to learn if the universe would ultimately collapse in the distant future or merely teeter out, expanding at an ever-slowing rate. This depended on the total mass of the universe. If there was enough mass, gravity would cause the universe to collapse in a "big crunch."

Then, in 1998, two independent teams of scientists made a startling discovery. Not only was the universe expanding, the expansion was accelerating. We live in a runaway universe. This discovery has been confirmed with several different techniques now -- with ground-based telescopes, the Hubble Space Telescope, and the NASA Wilkinson Microwave Anisotropy Probe. Something was driving space apart.

Calling Einstein. Come in, Einstein. Suddenly his cosmological constant idea wasn't so wacky. Could the vacuum of space -- that is, its emptiness -- create a sucking or repulsive force that overcomes gravity? Some scientists see dark energy as the gravity of empty space. Others see it as an entirely new type of force -- a fifth force or "quintessence" to accompany the known forces of gravity, electromagnetism (light), weak forces (radioactivity), and strong forces (binding atoms). Some theories predict that dark energy changes over time, that it "turned on" about five billion years ago as space evolved from a dense fog after the Big Bang to the expansive hierarchy of galaxy and galaxy clusters we see today. The idea is that Big Bang accelerated all matter, then gravity slowed the expansion, and then dark energy took over and sent the universe into overdrive.

Understanding dark energy is a top priority in scientific initiatives, such as the proposed NASA Beyond Einstein roadmap and the NASA-NSF-DOE "Quarks to Cosmos" initiative. The first step in understanding the nature of dark energy is to precisely measure the density and pressure of the universe over time. One way to do this is to study light from a special type of star explosion called a Type Ia supernova. These explode with a characteristic energy, or brightness; and because they are staggered throughout the universe -- both near and far -- scientists can use them to determine how and when the universe began to accelerate.
The mystery will take years to solve. Those readers of "Science Question of the Week" in grade school and high school may very well be the ones who discover what dark energy is. Study hard, because NASA needs you.

This week's question is from Christopher Wanjek. Mr. Wanjek is a science writer supporting the Beyond Einstein initiative, a roadmap to understand the forces of nature beyond General Relativity and Quantum Mechanics through the study of the Universe from the Big Bang to black holes.