cosmic string warps the space-time around it.

Is it a cosmic string we're seeing?

THE case for the existence of cosmic strings has just been boosted. If confirmed, these one-dimensional threads of energy that can span millions of light years could be the first sign of extra dimensions in the universe. Cosmic strings are predicted by string theory. They are gigantic counterparts of the strings that are thought to give rise to the fundamental particles of matter. String theory suggests that our universe may be a three-dimensional island, or "brane", and that the big bang was the result of a collision between our universe and another 3D brane. The collision would have given rise to one-dimensional cosmic strings, and finding such a string would strengthen the theory and support the idea that extra dimensions exist.

The immense energy of a cosmic string would warp the space-time around it. If one existed somewhere between us and a distant galaxy, say, the warped space-time would create two possible paths for the light from the galaxy to reach Earth. This would result in two identical images of the galaxy in our sky, just a whisker apart. Last year, that's exactly what Mikhail Sazhin of Capodimonte Astronomical Observatory in Naples, Italy, and the Sternberg Astronomical Institute in Moscow, Russia, and his colleagues found. They named the pair CSL-1 (New Scientist, 18 December 2004, p 30).

Many astronomers were sceptical of Sazhin's claim that a string was creating the images. Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, said that CSL-1 is merely two very similar galaxies that happen to be close together. Now, Sazhin's team has presented more evidence that the two images are of the same galaxy. In March, the team used the European Southern Observatory's Very Large Telescope at Paranal, Chile, to record detailed spectra of the two galaxies and found that they are identical (www.arxiv.org/astro-ph/0506400 ). This adds further weight to the possibility that CSL-1 is an artefact of a string, he says. "We are 99.9 per cent sure of this."

Loeb remains unconvinced. "It is not clear whether the quality of the spectra is sufficient to separate, for example, the Milky Way galaxy from the Andromeda galaxy in the local group of galaxies," he says. "Both the Milky Way and Andromeda might have similar spectra." He adds that if the astronomers could use their technique to tell these neighbours apart, then it would make their case for CSL-1 much stronger. Sazhin believes his team's technique would be precise enough to distinguish the Milky Way from Andromeda, even if they were as far away as CSL-1, but admits more work needs to be done to demonstrate this.

If a string is producing the twin galaxy images, the edges of the images should be extremely sharp, but our turbulent atmosphere prevents telescopes on Earth from detecting this. Now Sazhin has been granted turbulence-free observation time on the Hubble Space Telescope. "The resolution of the HST will allow us to detect the specific signature produced by the cosmic string," he says. "We hope it will reduce the scepticism of other astronomers." ###

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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 30 JULY 2005

Author: Marcus Chown

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