Oct 1, 2007
University of Chicago cosmologist Michael Turner calls the acceleration "the most profound mystery in all of science." It was Turner who coined the term "dark energy" for the unknown substance that provides this cosmic push. Studies have shown that it comprises 74% of all the mass and energy of the universe.
Scientists say that if they can understand dark energy, they may learn the fate of the universe — whether it will keep on expanding, tear itself apart or implode cataclysmically billions of years from now.
Science writers took part in a workshop last month at the Space Telescope Science Institute in Baltimore to focus on the question of why gravity, on the largest scales, has switched roles — pushing out instead of holding in.
NASA plans to explore the question in a big way as well. The National Research Council recommended in early September that a dark-energy probe be the first spacecraft NASA launches in its delayed "Beyond Einstein" series of missions designed to explore the formation of the universe and some of its most unique features.
Jointly sponsored by the Department of Energy, the series has three proposed missions, one of which would be selected and launched around 2015.
"It's not very often that theorists face a situation in which they need to explain something that is 74% of everything there is and they don't have a clue," says Mario Livio, theorist for the space telescope institute.
Coming to the opposite conclusion
Astronomer Adam Riess remembers the moment he realized that instead of pulling galaxies together, gravity was pushing them apart.
A decade ago, Riess was a 26-year-old postdoctoral researcher at the University of California-Berkeley, part of a team using the light from distant supernovae to study how rapidly the universe has been expanding over the past several billion years.
Researchers had known since the 1920s that the universe has been expanding in all directions but had assumed that the expansion was slowing, weighed down by the combined tug of all the galaxies.
But when Riess examined the supernova data in the fall of 1997, he found just the opposite: Cosmic expansion was speeding up. Something was undermining gravity's pull, turning it into a push and inflating the universe like a balloon. "I still recall feeling very excited — excited that it was true and also very anxious … because most things you discover in science are wrong, and they have a half-life of about five minutes."
But this time, the finding didn't fizzle. Instead, a rival team studying supernovae, led by Saul Perlmutter of the University of California's Lawrence Berkeley National Laboratory, had arrived at the same conclusion.
The most compelling evidence of dark energy relies on some of the most brilliant objects in the cosmos. Type 1a supernovae are stars that have blown themselves to smithereens and are bright enough to be seen in galaxies several billion light-years from Earth.
Like light bulbs of similar wattage, these supernovae all have about the same intrinsic brightness. This enables astronomers to calculate the distance to each eruption by observing how dim it appears on the sky. In addition, astronomers also determine how fast the host galaxy of each supernova is speeding away from Earth.
By combining this information, scientists find that the 13.7-billion-year-old universe began revving up its rate of expansion about 5 billion years ago.
Over the past 10 years, other studies have reinforced the dark-energy picture, Turner notes. These include images of the microwave glow left over from the universe-forming Big Bang and studies of X-rays that bathe distant clusters of galaxies.
"Now the evidence is rock-solid that the universe is speeding up," Turner says. "We live in this kooky universe."
'Big Rip' or 'Big Crunch'?
Dark energy presents a tremendous opportunity for physicists, in much the same way that the mysteries of the subatomic world did in the past century, Turner and others say.
In fact, dark energy may be tied to the subatomic world. Studies so far hint that dark energy might be sprinkled evenly throughout space. According to quantum mechanics, the laws that govern the motion and behavior of atomic particles such as electrons and protons, empty space isn't really empty. It seethes with pairs of particles and negatively charged antiparticles that constantly pop in and out of existence.
That activity imbues the nothingness of space with energy. And that energy could be just the hidden culprit behind dark energy.
So what are the possible fates awaiting the universe?
If dark energy has a constant density, as hinted by the supernova studies, the universe would continue to expand at an accelerated rate. Galaxies would be separated by such great distances and flee from one another at such high speeds that in 30 billion years, Turner says, residents of our own Milky Way would look out on a lonely sky. Only six other galaxies would be visible, compared with the billions seen today.
If dark energy becomes more powerful with time, its cosmic push would eventually become a cosmic killer. In 30 billion years, the runaway expansion would tear asunder every galaxy, star, planet, molecule and atom in what theorists call the "Big Rip." If, however, dark energy were to fade away, gravity's tug eventually would take back the reins. The universe would collapse in a "Big Crunch."
Now, scientists are designing new experiments to elucidate the properties of dark energy. The proposed space missions would variously hunt for several thousand supernovae, record the positions of 100 million galaxies and scour the heavens to study the image-distorting properties of gravity. Scientists acknowledge that there's a chance the missions could discover there's another reason altogether for the universe's accelerated expansion.
•The Supernova/Acceleration Probe would study the expansion history of the universe by recording 2,000 type 1a supernovae a year, using a mirror slightly bigger than that of the Hubble Space Telescope and the biggest camera ever launched into space.
•The Advanced Dark Energy Physics Telescope would use the echoes of primordial sound waves to examine cosmic expansion.
•The Dark Energy Space Telescope would be used to detect more than 3,000 type 1a supernovae over two years. It would then survey the sky to determine how the distribution of galaxies has evolved since the Big Bang.
The estimated $1 billion price tag for these projects comes with no guarantee, but Turner says he's optimistic that in 10 years, space and ground-based telescopes will crack the mystery. "This puzzle seems to be (related) to a number of other puzzles. It's the nexus," he says. "We can't understand the universe until we discover what dark energy is."ç