New Data on 2 Doomsday Ideas, Big Rip vs. Big Crunch
Published: February 21, 2004
The energy, whose source remains unknown, was named the cosmological constant by Einstein. In a prediction in 1917 that he later called "my greatest blunder," Einstein posited a kind of antigravity force that was pushing galaxies apart with a strength that did not change over billions of years of cosmic history.
Theorists seeking to explain the mysterious force have suggested that it could, in fact, become stronger or weaker over time — either finally tearing the universe apart in a violent event called "the big rip" or shutting down in the distant future — tens of billions of years from now.
If the force somehow shut down, gravity would again predominate in the cosmos, and the universe would collapse on itself. That version of oblivion is sometimes called "the big crunch."
On Friday, Dr. Adam Riess of the Space Telescope Science Institute in Baltimore presented the first broad set of observational figures that gauge the strength of the antigravity force over time. The information, he said, suggests that the cosmos will gradually expand, cool and darken, more akin to a slide into senescence rather than a violent apocalypse.
Dr. Riess and his team, which included Drs. Louis Strolger of the science institute and Alexei V. Filippenko of the University of California at Berkeley, used the Hubble to search for exploding stars, or supernovae, that are swept up in the cosmic push of the dark energy. They discovered 42 new supernovas in their survey area, including six of the seven most distant known.
Rather than seeing the changes in the push that many theorists had predicted, Einstein's unchanging cosmological constant fits the data better than any of the alternatives.
"What we've found is that it looks like a semipermanent kind of dark energy," Dr. Riess said. "It appears like it's been with us for a long time. If it is changing, it's doing so slowly. Einstein's theory is looking a lot better than before this data."
A cosmologist not involved in the work, Dr. Michael S. Turner of the University of Chicago, said: "This is the biggest mystery in all of science, whether or not dark energy varies with time. It's a big, big clue, and this is the first information we have."
Although the new results favor Einstein's nearly century-old prediction, Dr. Turner said, they still do not rule out some alternative theories. The information specifically leaves open the chance that the antigravity force will eventually strengthen and tear apart planets, stars and even atoms in a big rip, among other exotic possibilities.
Dr. Turner said that future measurements were quite likely to turn up smaller changes in the force over time and that those subtleties could help unravel the mystery.
The results do suggest that any ultimate cataclysm could not occur for, perhaps, 30 billion years, Dr. Riess said. Several physicists said that those estimates were highly uncertain, but that the findings could lead to a wide-ranging reassessment of models that predict strange variable energy densities in space.
"Models which predict wild dark energy densities which change a lot with time don't look so good," Dr. Yun Wang, a cosmologist at the University of Oklahoma, said. "Everybody will go back to the drawing board."
Dr. Filippenko presented some of the results at a conference here on Sources and Detection of Dark Matter and Dark Energy in the Universe.
The measurements raise new questions about the NASA decision, which is under review, to let the Hubble die a slow death in space rather than try another service mission with a space shuttle. The administrator of National Aeronautics and Space Administration, Sean O'Keefe, has said a service mission would be too risky after the disaster of the Columbia space shuttle.
Dr. Riess said he disagreed with the decision to stop the Hubble, which would halt the research for years to come. He pointed out that the remarkable clarity of the work depended on the Advanced Camera for Surveys, which spacewalking astronauts installed in Hubble two years ago.
The organizer of the conference here, Dr. David B. Cline of the University of California, Los Angeles, said that in view of the team's results, he was inclined to agree with Dr. Riess's assessment.
"They really shocked everybody by showing they could do this," Dr. Cline said. "You have to say obviously it's shame that the Hubble can't continue its life."
All the scientists agreed that because the problem had become so knotty, a complete solution might have to wait for a proposed satellite, tentatively called the Supernova/Acceleration Probe, or SNAP, which is planned to observe thousands of exploding stars from space.
"It's so tantalizing and so beautiful to see this kind of data set," said Dr. Saul Perlmutter, a physicist at Lawrence Berkeley National Laboratory who is leading the SNAP team. "This kind of result is so exciting for those of us who are eager to get to that next step."
Einstein invented the cosmological constant 1917 to explain why the universe, filled with countless stars and galaxies that are attracted to one another by gravity, did not collapse on itself. He added a constant term, called lambda, to his equations of general relativity, which describe the workings of gravity and the curvature of space on large scales. Lambda provided a repulsive force to counteract gravity.
In 1929, the astronomer Edwin Hubble, for whom the space telescope is named, discovered that the universe was expanding — from the primeval explosion called the Big Bang, scientists would learn. In that picture, gravity would slow the expansion and, possibly, reverse it at a far-off time. Einstein abandoned lambda as unnecessary and called it a blunder.
Six years ago, however, two supernova groups — one led by Dr. Perlmutter and the other by several scientists, including Drs. Filippenko and Riess — found that cosmic expansion was speeding up rather than slowing. The evidence was that distant supernovas, swept up in some sort of cosmic repulsion, were farther away than they would otherwise be and therefore appeared dimmer.
Lambda was reborn, but astronomers still had little information on whether the antigravity force was truly constant. The latest results suggest, within the accuracy of the measurements, at least, that something like Einstein's cosmological constant is indeed at work.
"His greatest blunders are our greatest ideas," Dr. Sean M. Carroll, a cosmologist at the University of Chicago, said. "It is a triumph of general relativity."
Einstein's theory says nothing about why the energy of the cosmological constant, or anything else, should be filling space in the first place, Dr. Carroll said. Theorists have turned to explanations as bizarre as parallel universes that exert an influence on our own or extra unseen dimensions in the fabric of space.
Dr. Carroll said the supernova findings might raise the stock of certain ideas that have emerged from string theory, which assumes that fundamental physics is a manifestation of minuscule strings that vibrate in an 11-dimensional space. String theorists have calculated what they call cosmic "landscapes," in which empty space contains energy densities that resemble a cosmological constant.
Because dark energy has never been directly detected and all these explanations are so bizarre, a few scientists say the ultimate explanation might be something else entirely.
Others conceded that there was a bit of a letdown that nearly a century after Einstein invented the cosmological constant nary a crack in his theory had turned up.
"This gives me a sinking feeling," Dr. Max Tegmark of the University of Pennsylvania said. "My nightmare is that we're going to be forever stuck with this puzzle."
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