Another slinky candidate for galaxy seeds.

The uneven sprinkling of galaxy clusters in the universe today raises a question still unanswered after decades of study: How did the uniform soup of the newborn universe develop the initial, tiny density ripples that ultimately led to present-day lumpiness?

The theoretical field is thick with candidates such as magnetic monopoles--particles with a single north or south pole--and endless wires of tightly bunched energy called cosmic strings (SN: 12/7/96, p. 364). Such objects, if indeed they existed, might have attracted enough matter to start the clustering of larger clumps.

Now, another type of energy strand, called the semilocal string, has wriggled into the limelight. Researchers carrying out the first three-dimensional supercomputer simulations of these filaments of magnetic energy have found that they may have been abundant and durable in the first blaze of the Big Bang.

"What we've demonstrated so far is that semilocal strings are interesting and viable," says Julian Borrill of Lawrence Berkeley (Calif.) National Laboratory (LBNL), a member of the research team.

According to the simulations, which will be described in a future issue of Physica B, semilocal strings first appeared at [10.sup.-35] of a second after the Big Bang. Both cosmic and semilocal strings could have formed as the newborn universe cooled, theorists say. As water changes from steam to liquid to ice, so cosmic phase changes cause the fundamental forces of nature--gravity and the strong, weak, and electromagnetic forces--to condense progressively out of the unified force with which the universe began (SN: 10/15/94, p. 248). This fracturing of forces would have led to energy concentrations such as strings.

Theory forbids cosmic strings from having ends, so they must appear as loops or infinite strands. Semilocal strings, however, can have ends, each of which is studded with a magnetic monopole. Semilocal strings can also curl back on themselves into loops, but then they quickly wink out of existence.

Only recently have supercomputers become capable of simulating semilocal strings in three dimensions. Borrill, Ana Achucarro of the University of the Basque Country in Bilbao, Spain, and Andrew R. Liddle of the University of Sussex in Brighton, England, modeled them on a computer at LBNL. It performed simultaneous calculations on 3 billion points in fields of quantum force and matter.

The researchers are planning additional simulations to test whether semilocal strings could have been seeds for the universe's transition from smooth to lumpy, Borrill says. Cosmic strings have recently run into trouble in that arena because their expected pattern matches neither the distribution of galactic clusters nor the patchiness of background radiation left over from the Big Bang (SN: 1/27/96, p. 63).

It's too soon to say whether semilocal strings can overcome these difficulties, but the new findings "extend the range of possibilities substantially," says Tom Kibble of the Imperial College in London, who pioneered cosmic-string theory in the late 1970s. Semilocal strings might also help explain the universe's overwhelming preference for matter over antimatter, Borrill adds.

Aside from potentially seeding galaxies, semilocal strings promise to link cosmology and particle physics in tantalizing ways, says Tanmay Vachaspati of Case Western Reserve University in Cleveland, who invented semilocal strings with Achucarro 7 years ago. Particle physicists have postulated a nearly identical string, called an electroweak string, composed of known and proposed elementary particles. Accelerators only about 10 times more powerful than the strongest available today could produce low-energy forms of these strings, he predicts.