Nov 17, 2006
By Dennis Overbye
A strange thing happened to the universe 5 billion years ago. As if God had turned on an antigravity machine, the expansion of the cosmos speeded up, and galaxies began moving away from one another at an ever faster pace.
Now a group of astronomers using the Hubble Space Telescope (http://news.com.com/NASA+weighs+attempt+to+save+Hubble/2100-11397_3-6130449.html) have discovered that billions of years before this mysterious antigravity overcame cosmic gravity and sent the galaxies scooting apart like muscle cars departing a tollbooth, it was already present in space, affecting the evolution of the cosmos.
"We see it doing its thing, starting to fight against ordinary gravity," Adam Riess of the Space Telescope Science Institute (http://www.stsci.edu/resources/) said about the antigravity force, known as dark energy. He is the leader of a team of "dark energy prospectors," as he calls them, who peered back 9 billion years with the Hubble and were able to discern the nascent effects of antigravity. The group reported their observations at a news conference Thursday and in a paper to be published in The Astrophysical Journal.
The results, Riess and others said, provide clues and place new limits on the nature of dark energy, a mystery that has thrown physics and cosmology into turmoil over the last decade.
"It gives us the ability to look at changes in dark energy," he said in an interview. "Previously, we knew nothing about that. That's really exciting."
The data suggest that, in fact, dark energy has changed little, if at all, over the course of cosmic history. Though hardly conclusive, that finding lends more support to what has become the conventional theory, that the source of cosmic antigravity is the cosmological constant, a sort of fudge factor that Einstein inserted into his cosmological equations in 1917 to represent a cosmic repulsion embedded in space.
Although Einstein later abandoned the cosmological constant, calling it a blunder, it would not go away. It is the one theorized form of dark energy that does not change with time.
Sean Carroll, a cosmologist at the California Institute of Technology who was not on the team, said: "Had they found the evolution was not constant, that would have been an incredibly earthshaking discovery. They looked where no one had been able to look before."
Progress report from the dark side
The paper by Riess and his colleagues represents a sort of progress report from the dark side, where astrophysicists have found themselves more and more as they try to understand what is happening to the universe.
This encounter with the invisible began eight years ago, when two competing teams of astronomers were using exploding stars known as Type 1a supernovas as cosmic distance markers to determine the fate of the universe.
Ever since the Big Bang 14 billion years ago, the galaxies and the rest of the universe (http://news.com.com/Images+Far-flung+galaxies+and+newborn+stars/2009-1008_3-5683122.html) have been flying apart like a handful of pebbles tossed in the air. Astronomers reasoned that gravity would be slowing the expansion, and the teams were trying to find out by how much and, thus, determine whether all would collapse one day into a "big crunch" or expand forever.
Instead, to their surprise, the two teams, one led by Saul Perlmutter of the University of California, Berkeley, and the other by Brian Schmidt of the Mount Stromlo and Siding Spring Observatories (http://www.mso.anu.edu.au/home.php) in Australia, found that the universe was speeding up instead of slowing down.
But the ground-based telescopes that the two teams used could track supernovas to distances of just 7 billion light-years, corresponding to half the age of the universe, and the effect could have been mimicked by dust or a slight change in the nature of the supernova explosions.
Since then, Riess, who was a member of Schmidt's team, and his colleagues have used the Hubble telescope (http://news.com.com/NASA+weighs+attempt+to+save+Hubble/2100-11397_3-6130449.html) to prospect for supernovas and dark energy farther out in space or back in time.
The new results are based on observations of 23 supernovas that are more than 8 billion years in the past, before dark energy came to dominate the cosmos. The spectra of those distant supernovas, Riess reported, appear to be identical to those closer and more recent examples. By combining the supernova results with data from other experiments like the NASA Wilkinson Microwave Anisotropy Probe (http://map.gsfc.nasa.gov/), Riess and his colleagues could begin to address the evolution of dark energy.
"That's one of the $64,000 questions," he said. "Is dark energy changing?"
So far, he said, the results are consistent with the cosmological constant, but other answers are also possible. The possibility that it is the cosmological constant is a mixed blessing. Physicists concede that they do not understand it.
Carroll of Caltech said, "Dark energy makes us nervous."
Einstein invented his constant to explain why the universe does not collapse. After he abandoned it, the theory was resuscitated by quantum mechanics, which showed that empty space should be bubbling with staggering amounts of repulsive energy. The possibility that it really exists in the tiny amounts measured by the astronomers has flummoxed physicists and string theorists.
Because it is a property of empty space, the overall force of Einstein's constant grows in proportion as the universe expands, until it overwhelms everything. Other theories of dark energy like strange force fields called quintessence or modifications to Einstein's theory of gravity can change in more complicated ways, rising, falling or reversing effects.
The equation of state
Astronomers characterize the versions of dark energy by their so-called equation of state, the ratio of pressure to density, denoted by the letter w. For the cosmological constant, w is minus one.
Riess and his group used their data to make the first crude measurement of this quantity as it stood 9 billion years ago. The answer, he said, was minus one--the magic number--plus or minus about 50 percent. By comparison for more recent times, with many more supernovas observable and thus more data, the value is minus one with an uncertainty of about 10 percent.
"If at one point in history it's not minus one," Riess said, "then we have killed the very best explanation."
Getting to the precision needed to kill or confirm Einstein's constant, however, will be very difficult, he conceded. One of the biggest sources of uncertainty is the fact that the Type 1a explosions are not completely uniform, introducing scatter into the observations.
The Hubble is the sole telescope that can pursue supernova explosions deeply enough to chart the early days of dark energy. The recent announcement that the National Aeronautics and Space Administration will send astronauts to maintain and refurbish the Hubble once again, enabling it to keep performing well into the next decade, is a lift for Riess's project. A new camera could extend observations to 11 billion or 12 billion years back.