Strings that blow bubbles in the cosmos.

Strings that blow bubbles in the cosmos

How the galaxies and clusters of galaxiesformed in a universe where matter was smoothly and homogeneously distributed in the beginning is one of the great questions of cosmology. Cosmic strings, which are topological defects in the structure of space, are the latest things to be suggested as triggers of galaxy formation.

One recent theory proposes that thestrong gravitational forces exerted by the strings gather matter around them and so start galaxy formation (SN: 5/12/84, p.294). A newer theory, by Jeremiah P. Ostriker, Christopher Thompson and Edward Witten of Princeton (N.J.) University, proposes just the opposite. Electromagnetic radiation from the strings, they suggest, blows bubbles in the primordial matter, compressing it in sheets between the bubbles so that galaxies form in the bubble walls.

In the view of modern cosmologists,space has properties similar to those of matter. It can be stretched, compressed, curved and twisted. It can also undergo phase changes analogous to freezing or boiling, and those phase changes can leave behind topological defects like the defects and dislocations that sometimes occur when crystals form. In the case of space, these defects are strings that form closed loops. These strings generate strong gravitational effects.

The new theory, as Witten described itrecently at the 13th Texas Symposium on Relativistic Astrophysics in Chicago, proposes that the strings can also carry electric currents and generate strong electric and magnetic effects. The currents are formed by extremely heavy subatomic particles, of varieties yet unknown to experiment, with which the strings are endowed at their formation. If primordial magnetic fields exist, they can start these particles moving as electric currents inside the strings, and the currents are supercurrents --once started, they persist forever.

The strings also vibrate as if they hadbeen plucked. As vibrating electric currents, they emit electromagnetic radiation at an extreme rate--for one typical string about 10,000 times as much as a bright quasar. The electromagnetic radiation couples to the matter surrounding the string and pushes it away, blowing a bubble in the cosmos. A lot of such strings blowing bubbles produces a froth in which the matter is compressed into narrow walls between the bubbles, and there galaxies and clusters form. This picture corresponds to, and may explain, an observation of a slice of the universe reported about a year ago by Valerie de Lapparent of the University of Paris and the Ecole Normale Superieure in Paris, and Margaret Geller and John P. Huchra of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. (SN: 1/18/86, p.38).

The current in a given string has an upperlimit, and when the limit is reached, the string begins to expel charged particles. If they belong to the class of particles called fermions, as Christopher Hill of Fermi National Accelerator Laboratory in Batavia, Ill., describes it, they will decay into a burst of very energetic particles. This burst, according to Hill, will consist of a mixture of charged particles, photons, neutrinos and particles yet unknown, with energies on the order of 10(15) billion electron-volts.

Such bursts would be rare and comefrom long distances, more than 100 megaparsecs away. A detector might expect to record one per square kilometer of detecting surface per century, but there is a hope of detecting them with terrestrial equipment such as the Fly's Eye telescope in Utah.

If such an observation ever succeeded,astrophysicists would have evidence for the existence of superconducting cosmic strings. Furthermore, if such expulsion and creation of particles by cosmic strings really goes on, it could contribute a sizable amount to the total of dark matter that cosmologists think the universe needs in order to make it closed.