9 Billion-Year-Old ‘Dark Energy’ Reported
Published: November 17, 2006
A strange thing happened to the universe five 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 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 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 nine billion years with the Hubble and were able to discern the nascent effects of antigravity. The group reported their observations at a news conference yesterday and in a paper to be published in The Astrophysical Journal.
The results, Dr. 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.”
The paper by Dr. 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 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 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 seven 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, Dr. Riess, who was a member of Dr. Schmidt’s team, and his colleagues have used the Hubble telescope 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 eight billion years in the past, before dark energy came to dominate the cosmos. The spectra of those distant supernovas, Dr. 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, Dr. 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.
Dr. Carroll of Caltech said, “Dark energy makes us nervous.”
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