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Deep2 Data Suggests Fine Structure Constant Doesn't Change

The first part of a new catalog of galaxies offering a snapshop of the universe about 6 to 9 billion years ago has been publicly released, according to Jeffrey Newman of Lawrence Berkeley National Laboratory.

At the APS April Meeting, Newman presented preliminary findings of the DEEP2 Galaxy Redshift Survey, specifically the conclusion that the fine structure constant, which sets the absolute scale of the electromagnetic force, does not appear to change in any statistically significant way even over cosmic timescales. Furthermore, the evolution of galaxy clustering in this distant epoch will soon be used to explore the nature of dark energy.

DEEP2 is a joint project of the University of California, Berkeley, and the University of California, Santa Cruz. It is a five-year survey of galaxies more than 7 to 8 billion light years away, whose light has been redshifted to nearly double its original wavelength by the expansion of the universe. The survey is now more than 80% complete and should finish observations this summer, with full data released by 2007.

Like the Sloan Digital Sky Survey (SDSS) and the 2dF sky survey, the DEEP2 project systematically maps galaxies over part of the sky. However, while SDSS and 2dF study objects with redshifts less than 0.2, DEEP2 has used one of the largest telescopes in the world–the DEIMOS spectrograph on the Keck II telescope in Hawaii–to measure the positions of 40,000 galaxies at a typical redshift of 1 in order to study the evolution of both galaxies and the universe itself.

Image: U. of California at Berkeley
Image: U. of California at Berkeley

The fine structure constant pops up in nearly all equations involving electricity and magnetism. It is equal to the square of the charge of the electron, divided by the speed of light times Planck’s constant. However, despite its fundamental nature, some theorists have suggested that it changes subtly as the universe ages, reflecting a change in the attraction between the atomic nucleus and the electrons orbiting it.

Experimental results have been contradictory. For instance, Australian astronomers a few years ago measured the absorption of light from distant quasars as the light passes through galaxies closer to us. The team reported that the constant has increased over the lifetime of the universe by about one part in 100,000. Other astronomers using the same technique have found no such change.

Newman designed a new experimental approach, drawing on earlier work by the Institute of Advanced Study’s John Bahcall, who pointed out that measuring emission lines from distant galaxies would be more direct and less error-prone than measuring absorption lines. The DEEP2 data allowed Newman and his colleagues to measure the wavelength of the emission lines of ionized oxygen to a precision of better than 0.01 Angstroms out of 5,000 Angstroms. They compared emission lines for 300 galaxies at various redshifts, and found the fine structure constant was no different from its current value: approximately 1/137. They found no change over a 4-billion-year time period, within one part in 30,000.

"Our null result is not the most precise measurement," Newman admitted. "But the alternative method (looking at absorption lines) that gives more precise results also involves systematic errors that cause different people using the method to come up with different results."

The DEEP2 survey has also completed measurements that may shed light on the nature of dark energy, now estimated to account for 70% of all the energy in the universe. Newman and his collaborators are counting the number of small groups and massive clusters of galaxies in a distant volume of space as a function of their redshift and mass. They believe this will make it possible to measure the amount by which the universe has expanded to the present day.

"What they are really trying to get at is how the dark energy density is changing as the universe is expanding," said UC-Berkeley’s Martin White. "If the dark energy density is Einstein’s cosmological constant, then the theoretical prediction is that it doesn’t change. The holy grail now is to get some evidence that it’s not the cosmological constant, that it is in fact changing."

Marc Davis, DEEP2’s principal investigator and a professor of astronomy and physics at UC-Berkeley, is now comparing the DEEP2 measurements with simple predictions of dark energy theory, but hopes to also collaborate with other theoreticians to test more exotic dark energy theory. Some, such as those that involve many extra dimensions, predict a gradual evolution of the fine structure constant.



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