A new study of distant galaxies is adding a fresh perspective to the debate over whether a fundamental physical constant has actually changed over time. The work suggests the number has not varied in the last 7 billion years, but more observations are still needed to settle the issue.
The controversy centres on the fine-structure constant, also called alpha, which governs how electrons and light interact. Alpha is an amalgam of other constants, including the speed of light. So any change in alpha implies a change in the speed of light – and indeed in the entire standard model of physics – with string theories touting extra spatial dimensions stepping in to fill the breach.
So it caused a sensation in 2001 when a team led by John Webb, an astrophysicist at the University of New South Wales in Australia, announced the constant had changed by about one part in 100,000 over 12 billion years. Webb’s team studied about 140 clouds of gas and dust that absorb light from distant, bright quasars. Subtle changes in the relative position of absorption lines from elements in the clouds’ spectra suggested alpha had changed over time.
But another team using higher-quality quasar absorption data, though fewer observations, failed to find any change in 2004. And different researchers studying a natural nuclear reactor in Oklo, Gabon in Africa have also come to opposing conclusions about alpha’s constancy.Slippery customer
John Bahcall, an astrophysicist at the Institute for Advanced Study in Princeton, New Jersey, US, helped pioneer quasar absorption studies in the 1960s, but he says this method can be slippery to interpret.
“The absorption line data is subject to misidentification of the lines and to the introduction of theoretical assumptions which may not be correct,” he told New Scientist. He adds that the spectral lines may be faint and can overlap, making it difficult to tell the lines’ source, and that astronomers must assume that all of the clouds share the same basic composition. Similarly, he says, interpreting the Oklo data involves making assumptions about other physical constants that might be subject to change.
Now, astronomers have used another method Bahcall developed to probe alpha. Rather than using absorption lines from clouds in space, a team led by Jeffrey Newman of Lawrence Berkeley National Laboratory in California, US, has focused directly on a pair of emission lines from ionised oxygen in the galaxies themselves.
“With emission lines, you can choose systems that have a very strong, characteristic appearance and are isolated to be sure you have the right objects,” says Bahcall. And because the average wavelength of the light emitted by the oxygen ions is a direct measurement of alpha, he says, “the fine-structure constant pops out without any interpretation”.
Clean and simple
However, the new study cannot yet claim the precision of Webb’s result. Newman’s group used observations of 300 galaxies, lying between 4 billion and 7 billion light years away, to find that alpha has changed by no more than one part in 30,000 – the resolution limit of the data – in 7 billion years.
But Newman says the method is superior to the absorption line alternative, telling New Scientist: “It’s simpler, it’s cleaner, and to date people haven’t gotten contradictory results from it.”
Bahcall says the team has done an “absolutely superb job” and adds that more observations – including more distant galaxies – will improve the precision of the measurement still further.
Newman presented the results, from a survey of distant galaxies called DEEP2, at a meeting of the American Physical Society in Tampa, Florida, on Monday.
(Article from New Scientist published on 15:33 19 April 2005 by Maggie McKee at http://www.newscientist.com)