Published online 10 March 2009 | Nature | doi:10.1038/news.2009.143

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Cosmic strings could solve positron mystery

Collapsing defects in the Universe's structure may generate antimatter excess.

A network of 'cosmic strings' criss-crossing the Universe could be responsible for a mysterious flux of antimatter particles which has been puzzling astronomers.

galaxyCosmic strings may have played a role in galaxy formation.NASA

Theoretical astrophysicists have long proposed the existence of cosmic strings, thinner than an atom yet stretching vast distances across the Universe. They are thought to have formed in events known as 'phase transitions' – dramatic shifts in the structure of matter that took place as the Universe cooled down shortly after the Big Bang. These strings would have strong gravitational fields, and could have helped to gather the matter that formed the first galaxies.

The idea fell from favour when detailed observations seemed to prove that strings alone could not account for galactic formation. Instead, a theory called 'inflation' has been invoked to explain how the Universe went through a period of exponential expansion early in its life, magnifying any tiny wrinkles in its structure into the seeds of the first galaxies.

But now, theoretical astrophysicist Tanmay Vachaspati at Case Western Reserve University in Ohio, suggests that space may be threaded instead with a network of much lighter strings – too lightweight to be directly responsible for galaxy formation - that could have formed during phase transitions in the Universe's unseen dark matter1.

Dark matter emits no light and has never been directly observed. But by tracking its gravitational influence on stars and galaxies, astronomers calculate that it makes up around 85% of the matter in the Universe.

Vachaspati believes that the strings themselves would be made of dark matter, and could solve a cosmic mystery. In 2008, physicists using an Earth-orbiting satellite called PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) reported that they had detected a surplus of positrons – the positive, antimatter twin of the electron (see 'Physicists await dark-matter confirmation'). Some scientists proposed that the extra positrons could be produced as debris from collisions between dark matter particles, but subsequent observations have so far failed to confirm the idea.

That's where Vachaspati's cosmic strings come in. If dark matter interacts very weakly with more familiar particles, as predicted by leading models, "then it is quite likely that strings will form in it," says theoretical astrophysicist Alex Vilenkin at Tufts University in Medford, Massachusetts.

Over time, loops of cosmic string should shrink and disappear, radiating away their energy in a puff of particles as they go. According to Vachaspati's calculations, collapsing loops of the lightweight strings can produce the excess of positrons seen by PAMELA.

Mark Pearce, a particle physicist at the Royal Institute of Technology in Stockholm, and part of the PAMELA team, says that it's difficult to assess Vachaspati's model without having more detailed predictions about the positrons - such as their range of energies – to compare with PAMELA's observations. "One would also need to check that predictions of fluxes of other cosmic particles, such as antiprotons, are compatible with existing data," he adds.

Broader revival

Further evidence for dark strings could come from experiments on Earth. If Vachaspati's theory is correct, string loops should be present in our Galaxy and periodically collide with Earth. "The oceans and ice caps are dense enough for the loop to quickly lose its energy [when it collides], which would show up as gamma rays or other radiation," says Vachaspati. He suggests that this radiation could be detected by experiments designed to catch the flashes of light from passing neutrinos, such as IceCube, under construction beneath the South Pole, and Antares, deployed deep in the Mediterranean Sea.

It might even be possible to manufacture the dark strings in particle accelerators, Vachaspati adds.

Another group of theoretical physicists recently claimed to have seen evidence for cosmic strings in data from NASA's Wilkinson Microwave Anisotropy Probe, which has been mapping the temperature variations in the cosmic microwave background radiation for almost eight years2.

That group includes Mark Hindmarsh, a particle physicist at the University of Sussex, UK, who says that Vachaspati's and their own results are part of a broader revival of the cosmic strings concept. "People are more willing to entertain the idea of cosmic strings than before," he says. He attributes this trend to a growing realization that cosmic strings are not a rival theory to inflation, but instead a complement to it. He also notes a wider availability of the computing power needed to "nail down the calculations" in cosmic string theories.

Vilenkin adds that astrophysicists are now paying greater attention to strings because they come with clear predictions of effects including gravity waves and the creation of energetic particles. "Observational techniques are now becoming accurate enough to either detect the strings, or to place stringent constraints on particle theories that predict them," he says. 

  • References

    1. Vachaspati, T. Available at: http://arxiv.org/abs/0902.1764 (2009).
    2. Bevis, N., Hindmarsh, M., Kunz, M. & Urrestilla, J. Phys. Rev. Lett. 100, 021301 (2008). | Article | PubMed | ChemPort |
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