Measuring the Mysterious ‘Dark’ Force
By DENNIS OVERBYE
Published: December 16, 2008
Measurements of the lack of growth of galaxy clusters over the last five billion years put astronomers one step closer, they say, toward narrowing the possible explanations of dark energy, the mysterious force that is speeding up the universe.
Physicists denote the virulence of dark energy by a number called w, the equation of state. From the stifling of cluster growth, Dr. Alexey Vikhlinin’s multinational team derived a value of w of about minus 1.14, plus or minus 0.21. That is within shooting range of the magic value for the cosmological constant of minus 1.
The constraints on dark energy tighten considerably when the cluster measurements are combined with the supernova measurements and others, including observations of the cosmic microwaves left over from the Big Bang, in which the seeds of cosmic structures like galaxy clusters can be seen, yielding a w of minus 0.991, plus or minus .045.
That looks like an improvement of a factor of 2 on the uncertainty charts that dark energy specialists show to one another in meetings and papers, but critics caution that combining disparate types of measurements can result in an artificially small error that masks underlying uncertainties. Astronomers, for example, still do not have a good theory to explain how their standard candles, supernovas, explode, and theories of cluster growth depend on assumptions about the nature of the dark matter in the universe and the nature of the original fluctuations that give birth to them.
Even the improved error bars still leave a lot of wiggle room for other theories of dark energy, and some astronomers worry that the measurements even from the upcoming dark energy satellite will never get accurate enough to prove or disprove the cosmological constant.
Paul Steinhardt of Princeton University pointed out that the uncertainties in the cluster results are larger if you consider the possibility that w could vary with cosmic time, “which is what you should do if you are trying to test the idea that the dark energy is not a cosmological constant,” he said in an e-mail message.
But the fact that the answer from clusters came out so close to the cosmological constant is a vote of confidence for Einstein’s general relativity, said William Forman of Harvard, a member of Dr. Vikhlinin’s team. “This is a test that general relativity could have failed,” he said.
Indeed, several theorists said the future now looked dim for alternative theories of gravity, in particular a variant from string theory, which incorporates extra dimensions and which predicts enhanced growth of structures like galaxy clusters. David Spergel, a theorist at Princeton University, said that although quantitative tests had not yet been done with the new data, “I suspect that it rules out those models.”
Dr. Vikhlinin lamented that there were not yet very many such theories to knock down yet, but there were sure to be more on the table soon. “The field has just started, I think,” he said.