Cosmic dark age found in shadows

May 19, 2004

THE earliest structures in the universe may be visible by the shadows they cast in the afterglow of the big bang. The objects have been hidden until now because they formed in the dark age of the universe before the first stars switched on. "The shadows promise to provide the richest ever gold mine of information about the early universe," says Abraham Loeb of Harvard University. The so-called cosmic dark age stretched from the fading of the big bang fireball 13.7 billion years ago to the time when the first stars ignited, several hundred million years later. During this period, hydrogen clouds formed into structures that eventually became the stars and galaxies of today.

Astronomers would dearly like to understand this formation process but, because of the lack of light, this period of the universe's history has seemed impenetrable- until now. Loeb and his Harvard colleague Matias Zaldarriaga think that the hydrogen gas must have absorbed radiation left over from the big bang and that this absorption would have created shadows that astronomers should be able to see today ( The effect is similar to that of a terrestrial cloud blotting out the sun and casting shadows on the ground. Neutral hydrogen gas absorbs radio waves at a characteristic wavelength of 21 centimetres. However, the enormous expansion of the universe since the dark age will have stretched this wavelength tens or hundreds of times. Loeb and Zaldarriaga are therefore advocating searching for shadows at wavelengths of tens of metres, a region of the radio-frequency spectrum astronomers have largely ignored. But the detection of these shadows is a formidable challenge. Loeb and Zaldarriaga predict that the absorption will typically produce a dip in temperature of only a thousandth of a kelvin. Nevertheless, they say that the LOFAR radio telescope, which will become operational in the Netherlands in 2006, may see an effect after a year of observation.

"The expectation is that it will be able to pick up signals of the order of 10 millikelvin, probably in hundreds of hours," says radio astronomer David Hough of Trinity University in San Antonio, Texas. "So getting to 1 mK is plausible- although I don't know whether the LOFAR team would want to give a year's time to one project." Other experiments being planned, including space-based ones to avoid terrestrial interference, may also pick up the signal within the next decade. The pay-off could be dramatic, say Loeb and Zaldarriaga. The shadows will contain a wealth of information not only about how the first objects in the universe formed, but also about the earliest split seconds of the universe after the big bang.
Author: Marcus Chown

New Scientist issue: 22 May 2004


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