Andrew Fruchter/STScI, NASA
Gamma-ray bursts are some of the most powerful explosions in the universe. This burst had the radiance of 100 million billion stars. The fireball was rapidly fading by the time the Hubble Space Telescope captured this image.
From 'Cosmological Constant' to 'Dark Energy'
A Static Universe: When Albert Einstein was writing equations to describe the nature and the shape of the universe, most astronomers believed at that time that the universe was neither growing nor shrinking: it was simply static.
His theory of general relativity, however, predicted that the universe must be changing. So to make his equations fit the physics of the times — to make the universe a static place — Einstein introduced a "cosmological constant" into the equation. He predicted that there must be a force that counteracts gravity and thus keeps the universe from collapsing into itself.
Problem: In 1929, Edwin Hubble published a study based on observations that said the universe actually was expanding. Hubble's study caused Einstein to call his notion of a "cosmological constant" his greatest mistake.
Cosmological Constant Reborn: In 1999, when astronomers discovered that the universe is not just expanding, but the pace of expansion is accelerating, the idea of a cosmological constant came back into consideration. This time, with a sexier name: dark energy.
Astronomers suggested that dark energy — something thought to make up 70 percent of space — may be Einstein's anti-gravity force, i.e. a force that pushes the universe apart, while gravity tries to pull it together. One of the many questions about dark energy is, does it change or is it a steady force?
Problem: In order to be Einstein's cosmological constant, dark energy must be steady. But a new observational study by LSU professor Bradley Shaefer finds that dark energy is not steady, but appears to have changed gradually over time. If Schaefer's observations hold, Einstein's idea of a cosmological constant may be put back on the shelf.
Dark Energy or Just Gravity? One new, radical idea in physics even suggests there's no such thing as dark energy. What astronomers may be seeing is new behavior from a long-known force, gravity. For instance, in the distant, relatively empty hinterlands of the universe, gravity may behave differently than it does inside the crowded Milky Way.
— Vikki Valentine
An astronomer says he may have found an important clue to one of the most profound mysteries in the universe: Why the stars in the sky are moving away from us at an ever increasing pace. Some scientists have suggested that the vacuum of space is filled with a weird force, called "dark energy."
But new results, presented Wednesday at the American Astronomical Society meeting in Washington, D.C., call that idea into question.
When astronomers look out into the sky, they see that everything around is moving away from us. Part of their explanation is that the universe started with a cosmic explosion, known as the Big Bang. In 1999, astronomers set out to measure the speed of expansion by looking at distant supernovae — exploding stars — to see how fast they were moving away.
To everyone's surprise, they found that the material isn't just coasting away from us in space, as expected in the aftermath of an explosion. It's actually picking up speed as it goes.
"Something out there is pushing the universe, making it expand, making it speed up and we don't know what this is really," says Bradley Schaefer, the Louisiana State University astronomer who presented the results.
"It's been given a name, dark energy, and you can say this dark energy is pushing the universe, pushing the acceleration," he says.
A leading notion is that dark energy is something that pervades what was thought of as empty space. It's sometimes called "quantum vacuum energy" or the "cosmological constant."
The idea is that it's a constant through all space and through all time. Schaefer decided to test this idea by probing deeper back in time, to see if the constant was the same way back then. He did this by studying objects called gamma-ray bursts.
"They're an extreme version of supernovae," says Schaefer, "and as such, they are 100 times more luminous than supernovae, and so you can see them much further out in the universe."
Early results from Schaefer's study of the movements of gamma-ray bursts suggest that dark energy is different far out in space, and therefore way back in time.
"The first result of this new method happens to be pointing toward the direction of this cosmological constant not being constant," Schaefer explains, "which would be to say it appears that the dark energy is changing with time."
Schaefer, however, stops short of making a definitive claim about the nature of the universe from these results. And his evidence is not strong enough at the moment to be absolutely convincing.
Even so, it is attracting a lot of attention at the astronomy meeting, including from heavyweights such as Michael Turner from the University of Chicago and the National Science Foundation.
"It's very intriguing," says Turner, "but I don't think it rises to the level of me being able to issue Schaefer a ticket to Sweden."
The Nobel Prize will have to wait for more convincing results, Turner says. But he adds that a breakthrough in this field would clearly be gold-medal material.
"Cosmic acceleration, I believe, is the biggest mystery in all of science."
Turner says it's hard enough for scientists to understand how the supposedly empty vacuum of space can be suffused with some form of cosmic energy. If Schaefer's data hold up and the idea of a constant and pervading energy is thrown out, then what?
"The alternatives really are mind-stretching," says Turner.
Maybe the universe contains hidden dimensions. Turner says the uncertainty about Shaefer's results won't last long; other astronomers have also been trying to measure the expansion of the universe way back in time. In a few years, Turner says we should know whether this is just a statistical fluke, or a real clue about the fabric of our universe.