LIKE a shout in the dark, a mysterious burst of radio waves from the depths of space has puzzled astronomers since it was discovered last year. Now one physicist is suggesting it came from the cosmic equivalent of a cracking whip.
In 2001, a radio burst lasting about 1 millisecond was picked up by the 64-metre Parkes radio dish in Australia. The signal went unnoticed at first, but last year a team led by Duncan Lorimer of West Virginia University, Morgantown, spotted it during a recap of archived observations.
By analysing the time delays of different frequencies within the burst, they found it had travelled a huge distance 鈥� about 3 billion light years. The presence of a supernova, gamma-ray burster or galaxy would have explained its origin, but curiously none lies in that part of the sky. Though it is possible a stellar object such as a pulsar was the source, evidence for this has yet to be found.
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Tanmay Vachaspati of the Institute for Advanced Study in Princeton, New Jersey, and Case Western Reserve University in Cleveland, Ohio, says a cosmic string may have been the culprit.
Cosmic strings are 鈥渢opological defects鈥� in the fabric of space-time, a bit like geological faults. Though we have never seen one, many theories of fundamental particles predict they formed after the big bang, possibly due to the 鈥渇reezing鈥� of adjacent fields with different orientations as the universe cooled down. Where these fields meet and alignments suddenly change, a line of discontinuity may have formed.
In 1985, Ed Witten of the Institute for Advanced Study pointed out that the thread-thin channels of such strings may have trapped charged particles as they formed. This would create gigantic electrical charges, which circulate endlessly in detached loops formed when two strings cross.
鈥淭he thread-thin channels of cosmic strings would contain gigantic electrical charges which circulate endlessly鈥�
Vachaspati thinks one of these 鈥渟uperconducting鈥� loops could be behind the Parkes radio burst. The idea that cosmic strings emit signals that we would notice is not new 鈥� but previous theories predicted the emission of high-energy particles, such as gamma rays, or even gravitational waves. 鈥淣obody thought to look at low-energy phenomena such as the emission of radio waves,鈥� he says.
In his calculations, Vachaspati considered a string loop about 3 light years across, weighing 100 trillion tonnes per metre and carrying a current of 10 billion amperes. The acceleration of current in this loop would naturally produce electromagnetic waves, he says, but the loop would oscillate far too slowly to create a detectable radio burst. However, Vachaspati points out that strings lash about because they are under enormous tension due to the opposing fields either side. His calculations show that a piece of a string moving like the kink on a cracked whip, and travelling close to the speed of light, could have produced the Parkes burst ().
鈥淚t鈥檚 certainly an intriguing idea and certainly possible,鈥� says Tom Kibble of Imperial College London. 鈥淚t must be regarded as a rather long shot. However, there is no other plausible explanation either, so it could be right. Watch this space, I would say.鈥�
If Vachaspati is right, then such bursts should be relatively common and detecting them would not require a big or very sensitive radio telescope. 鈥淎ll that鈥檚 needed is to observe the whole sky for, say, a year to catch them,鈥� he says. Such bursts could tell us about the era when nature鈥檚 strong and electroweak forces were one, because the string source would have formed at this time. 鈥淚t鈥檚 hard to imagine any other phenomenon that would give us a window into such an early epoch of the universe,鈥� says Vachaspati.
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