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In cosmology, a topological defect is a (often) stable configuration of matter predicted by some theories to form at phase transitions in the very early universe. As the universe expanded and cooled, symmetries in the laws of physics began breaking down in regions that spread at the speed of light; topological defects occur where different regions came into contact with each other. The matter in these defects is in the original symmetric phase, which persists after a phase transition to the new asymmetric new phase is completed.
Various different types of topological defects are possible with the type of defect formed being determined by the symmetry properties of the matter and the nature of the phase transition. They include:
- Domain walls, two-dimensional membranes that form when a discrete symmetry is broken at a phase transition. These walls resemble the walls of a closed-cell foam, dividing the universe into discrete cells.
- Cosmic strings, one-dimensional lines that form when an axial or cylindrical symmetry is broken.
- Monopoles, point-like defects that form when a spherical symmetry is broken. They are predicted to have magnetic charge, either north or south, and so are commonly called magnetic monopoles.
- Textures form when larger, more complicated symmetry groups are completely broken. They are not as localized as the other defects, and are unstable.
Other more complex hybrids of these defect types are also possible. Topological defects are extremely high-energy phenomena and are likely impossible to produce in artificial Earth-bound physics experiments, but topological defects that formed during the universe's formation could theoretically be observed.
No topological defects of any type have yet been observed by astronomers, however, and certain types are not compatible with current observations; in particular, if domain walls and monopoles were present in the observable universe they would result in significant deviations from what astronomers can see. Theories that predict the formation of these structures can therefore be largely ruled out. On the other hand, cosmic strings have been suggested as providing the initial 'seed' gravity around which the large-scale structure of the cosmos matter has condensed. Textures are similarly benign.
See also topological soliton
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