Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here: http://www.routledge-ny.com/books.cfm?isbn=0415923549
Topological defects of various kinds are predicted to have occurred during phase transitions in the early Universe. The exact character of the defect that would have been produced depends exactly on the nature of the phase transition and the configuration of the fields involved in spontaneous symmetry-breaking. Their existence can be argued on general grounds because of the existence of horizons in cosmology. If a phase transition happens more or less simultaneously in all regions of the Universe, then there is no possibility that regions separated by more than the scale of the cosmological horizon at the time can exchange light signals. Whatever the configuration of the vacuum state of whatever field is undergoing the transition in one region, the state in a different, causally disconnected part of the Universe would be expected to be independent. This incoherence of the field would have resulted in defects, much like the defects that appear when liquids are rapidly cooled into a solid phase. Solids formed like this tend to have only short-range order within domains separated by defects in the form of walls. Other types of defect are possible, depending on the type of phase transition. Some of the cosmological defects that have been suggested are described overleaf.
Monopoles (sometimes called magnetic monopoles) are hypothetical point-like defects in the fabric of spacetime, produced in the early Universe according to some grand unified theories (GUTs). No monopoles have yet been detected in the laboratory. These objects are historically important (even though their existence is entirely speculative), because the inflationary Universe model was originally suggested as a means of reconciling the present lack of observed monopoles with GUT theories. The rapid expansion of the Universe associated with inflation simply dilutes the number of monopoles produced in the phase transition to an unobservably small value.
Cosmic strings are one-dimensional (line-like) defects, slightly similar to the vortex tubes that can be produced in liquid helium phase transitions. If produced in the framework of a GUT, such a string would be about 10-31 metres thick, and have a mass of about ten million solar masses per light year. Because of their strong gravitational effect on nearby matter, it has been suggested that cosmic strings might play a significant role in cosmological structure formation by generating large enough primordial density fluctuations. It is now generally accepted, however, that this of large-scale structure and the fluctuations in the cosmic microwave background radiation disagree with the predictions of cosmic-string theory.
Domain walls would be two-dimensional (sheet-like) defects. In essence, they are wall-like structures in which energy is trapped, rather like the Bloch wall formed between the Weiss domains in a ferromagnet. Any theory of the fundamental interactions that predicts large numbers of domain walls would predict a highly inhomogeneous universe, contrary to observations, so these particular defects are to be avoided at all costs.
Cosmic textures are by far the hardest kind of defect to visualise; they involve a kind of twisting of the fabric of spacetime. Like cosmic strings, these entities have been suggested as possible sources for the primordial density fluctuations, but they have fallen out of favour because they fail to reproduce the so-called Doppler peak seen in observations of the cosmic microwave background radiation (see Sakharov oscillations).
FURTHER READING:
Kolb, E.W. and Turner, M.S., The Early Universe (Addison-Wesley, Redwood City, CA, 1990).