Galaxies cluster around cosmic strings.

Galaxies cluster around cosmic strings

How the galaxies formed is one of themost serious questions in cosmology and astrophysics. Most theories assume that the universe started out smooth and undifferentiated, but for galaxies to have formed, there had to be some kind of flaw in the smoothness, around which matter could gather. One very new suggestion is that these were topological flaws in space-time itself, the so-called cosmic strings. Now a calculation shows that cosmic strings will cluster hierarchically, and this clustering closely resembles the distribution of galaxies seen in what astronomers call Abell clusters. Neil Turok of Imperial College in London described this development in Berkeley, Calif., at the recent Twenty-Third International Conference on High-Energy Physics.

Physicists today treat space-time asif it were a material substance. It is stretchable, compressible and twistable. It can also undergo phase transitions, analogous to freezing or boiling, in which its basic structure changes radically. Cosmic strings are a relic of such a phase change. Phase changes usually take some time to complete themselves; in the same pot there will be both liquid water and bubbles of steam for several minutes. In the case of the universe, scientists are dealing with eons of time. The supposed phase change mostly occurred eons ago, but a few small relics of the previous structure of space-time may still persist.

These topological flaws appear as"strings," which may be either open-ended and infinitely long or closed loops. It is the closed loops that are of interest in the business of nucleating galaxies. As time goes on, they shrink and disappear, giving up the energy they possess as gravitational radiation, but while they last, they exert strong gravitational forces. Responding to these forces, matter will gather around the string loops, and so the evolution of galaxies could begin. Ultimately the string disappears, but the galaxy persists.

In the beginning, string loops comein various sizes. Now, Turok says, interested physicists have calculated that large loops will attract smaller ones to form clusters around them. These loop clusters look geometrically like the clusters of galaxies, particularly the Abell clusters, which are small and dense as galaxy clusters go. Usually they contain more than 50 galaxies in a space no larger than 1.5 megaparsecs (slightly less than 5 million lightyears).

Rival theories of galaxy formationhave trouble with Abell clusters, Turok says. One of the most common of such theories proposes that the universe wasn't entirely smooth at the beginning, that there were small primordial clumps, density fluctuations, scattered randomly about. There is no explanation why these clumps were there; they are simply primordial. Galaxies could gather around such clumps, and such theories may even have an explanation for the larger galaxy clusters.

But according to Turok, while thistheory dos not predict the formation of Abell clusters, cosmic string theory doe. The clustering of small loops around larger ones mimics the structure of Abell clusters, and it even reproduces the correlation function that describes the sizes of galaxies in the Abell clusters. The correlation function says that the radius of galaxy in an Abell cluster is inversely related to the mean distance between galaxies in the area where it is. The theoretical calculation of the development of clusters of cosmic strings predicts a similar correlation function. As Turok puts it, "The correlation of Abell clusters fits the prediction from the numerial simulation of the formation of strings."