What is Dark Energy?

A landmark discovery of the 1990s was that the expansion of the Universe is accelerating. The source of this mysterious force opposing gravity we call "dark energy."

Because he originally thought the Universe was static, Einstein conjectured that even the emptiest possible space, devoid of matter and radiation, might still have a dark energy, which he called a "Cosmological Constant." This dark energy would keep a static Universe from collapsing under gravitational attration. When Edwin Hubble discovered the expansion of the Universe, Einstein rejected his own idea, calling it his greatest blunder.

As Richard Feynman and others developed the quantum theory of matter, they realized that "empty space" was full of temporary ("virtual") particles continually forming and destroying themselves. Physicists began to suspect that indeed the vacuum ought to have a dark form of energy, but they could not predict its magnitude.

Through recent measurements of the expansion of the Universe, astronomers have discovered that Einstein's "blunder" was not a blunder: some form of dark energy does indeed appear to dominate the total mass-energy content of the Universe, and its weird repulsive gravity is pulling the Universe apart. We still do not know whether or how the highly accelerated expansion in the early Universe (inflation) and the current accelerated expansion (due to dark energy) are related.


Dark Energy: Astronomers Still 'Clueless' About Mystery Force Pushing Galaxies Apart

Dark energy entered the astronomical scene in 1998, after two groups of astronomers made a survey of exploding stars, or supernovas, in a number of distant galaxies. These researchers found that the supernovas were dimmer than they should have been, and that meant they were farther away than they should have been. The only way for that to happen, the astronomers realized, was if the expansion of the universe had sped up at some time in the past.

Until then, astronomers had generally believed that the cosmic expansion was gradually slowing down, due to the gravitational tugs that individual galaxies exert on one another. But the supernova results implied that some mysterious force was acting against the pull of gravity, causing galaxies to fly away from each other at ever greater speeds.

It was a stunning realization.

At first, other researchers questioned the result; perhaps the supernovas were dimmer because their light was being blocked by clouds of interstellar dust. Or maybe the supernovas themselves were intrinsically dimmer than scientists thought. But with careful checking, and more data, those explanations have largely been put aside, and the dark energy hypothesis has held up.

In one sense, the idea is not completely new. Einstein had included such an "anti-gravity" effect in his theory of general relativity, in his so-called cosmological constant. But Einstein himself, and later many other astronomers, came to regard this as a kind of mathematical contrivance that had little relationship to the real universe. By the 1990s no one expected that the effect would turn out to be real.

Still, anti-gravity isn’t the right way to describe dark energy, says Virginia Trimble of the University of Southern California at Irvine.

"It doesn’t act opposite to gravity," Trimble says. "It does exactly what general relativity says it should do, if it has negative pressure."

Trimble has a fairly simple way of imagining the phenomenon.

"If you think in terms of the universe as a very large balloon," she says, "when the balloon expands, that makes the local density of the [dark energy] smaller, and so the balloon expands some more …. because it exerts negative pressure. While it’s inside the balloon it’s trying to pull the balloon back together again, and the lower the density of it there is, the less it can pull back, and the more it expands. This is what happens in the expanding universe."

The supernova evidence suggests that the acceleration kicked in about 5 billion years ago. At that time, galaxies were far enough apart that their gravity (which weakens with distance) was overwhelmed by the relatively gentle but constant repulsive force of dark energy. Since then, dark energy's continuing push has been causing the cosmic expansion to speed up, and it seems likely now that this expansion will continue indefinitely.

"It means that if you look out at the universe today, and if we wait many billions of years," says Hogan, "everything will be flying away faster and faster, and eventually we’ll be left quite alone."

Aside from such grim forecasts, dark energy is causing quite a bit of upset for astronomers who have to adjust to an unexpected and outlandish new view of the universe. Already, they have had to accept the notion of dark matter, which is now thought to far outnumber ordinary matter in the universe, but which has never been detected in any laboratory. Now, the arrival of an unknown force that rules cosmic expansion has added insult to injury.

"I'm as big a fan of dark matter and dark energy as anybody else," says astronomer Richard Ellis of Caltech. But, he adds, "I find it very worrying that you have a universe where there are three constituents, of which only one [i.e., ordinary matter] is really physically understood."

"When you teach undergraduates, and they say, 'Well, what is dark matter?' Well, nobody's really sure. 'What is dark energy?' We're even less sure. So you have to explain to a student, that … 90 percent of the universe, 95 percent, is in two ingredients that nobody really understands," says Ellis. "This isn't really progress."

No one argues that dark energy is difficult to comprehend. And as Trimble points out, it is hardly the first strange idea scientists have had to accept.

"It took two generations for people to be comfortable with quantum mechanics," she says. "The fact that you do not have good intuition about [dark energy] is true for quantum mechanics, general relativity, and lots of other things, because we can’t easily mock them up in the laboratory."

And for cosmologists, dark energy has solved at least one cosmological conundrum raised by studies of the Cosmic Microwave Background, or CMB.