Focus: A Constant Worthy of the Name
A few years ago, astronomers announced that the laws of physics may have changed slightly since the big bang. Light arriving from distant corners of the Universe suggested that a fundamental constant related to the speed of light was a bit different in the past. But in the 26 March PRL, a different team applies the same technique to newer data and concludes that if there has been a change, it is much smaller than previously claimed.
Look at the spectrum of light from a distant object and you will see characteristic bright and dark lines that appear similar to those from earthly material, except for one difference: the positions of the lines are shifted toward the red. This Doppler shift–akin to the famous acoustic effect–occurs because the Universe has been expanding during the billions of years that the light has been en route to Earth. For some of the ancient galaxies called quasars, this shift suggests the light may have been in transit for more than 90% of the age of the Universe.
Over the past few years, a group at the University of New South Wales in Australia has analyzed the light from quasars recorded at Hawaii’s Keck observatory. They found that the dark lines in the spectrum, which result from absorption by intervening gas, did not all have the same red shift . The researchers ascribed the deviations to changes in the so-called fine-structure constant, a number calculated from the speed of light, Planck’s constant, and the charge of the electron. They deduced that, billions of years ago, this constant was smaller by about 6 parts per million than it is today. Some theorists trying to reconcile the theory of relativity with quantum mechanics have explored theories in which this “constant” varies with time, so they were encouraged by these observations.
Now another group of astronomers has applied the same technique to absorption lines measured at the European Southern Observatory’s Very Large Telescope in Chile. Instead of doing a complex analysis of all the data, the team selected only simple-looking absorption lines. They avoided lines that appeared to include many components and those with complete absorption at the center of the line, to make it easier to extract shifts that are smaller than one tenth of the width of an absorption line. “Our data are of uniform quality and far superior to those used in earlier studies,” says team member Raghunathan Srianand, of the Inter University Center for Astronomy and Astrophysics in Pune, India. The researchers concluded that the fine-structure constant might well be constant, and that the variation seen in the prior work was highly unlikely.
The Australian team isn’t convinced. Michael Murphy, who is now at the University of Cambridge, says selecting data that looks simple might give an unrealistically small estimate of the error. And if the fine-structure constant really hasn’t changed, he says, “we still have to explain these data from Keck.” In view of the extensive efforts to rule out systematic errors, he says the explanation “may be mundane, but it has to be bizarre.” Murphy and his Australian colleagues hope to publish their own analysis of Very Large Telescope data soon.
Lennox Cowie of the University of Hawaii in Honolulu suspects that both groups are underestimating their possible errors. But Cowie, who recently coauthored an essay on related work  adds that “the fact that the first independent measurement is not in agreement has to make one feel the variation claim is now weak.”
- J. K. Webb et al., “Further Evidence for Cosmological Evolution of the Fine Structure Constant,” Phys. Rev. Lett. 87, 091301 (2001); described in Phys. Rev. Focus 8, story 9 (2001)