Cosmic Topological Defects

What Are Cosmic Topological Defects?	So; what is a "phase transition" or a "topological defect"? A phase transition occurs when a material changes its form. Perhaps the most common example is water. If it freezes, there is a phase transition as the water turns to ice. If you heat it, there is a phase transition as the water turns to steam. The same is true in the opposite direction; condensing steam or melting ice. Consider an ice tray. When you freeze it to form ice cubes, the cubes are rarely perfect. The phase transition causes crack lines, bubbles, and irregular planes to appear embedded in the cube. These are a form of topological defect. Similarly, when the very early Universe was cooling, the phase transitions could have caused cosmic topological defects to occur. They are classified based on their dimensionality; zero, one, two or three dimensions.
Cosmic String	Not to be confused with Superstrings, Cosmic Strings are hypothetical, one dimensional, topological defects in the fabric of the Universe that form when an axial or cylindrical symmetry is broken. They are billions of light years long, but smaller than a proton in diameter. Depending on various assumptions, a piece between 1 and 6 miles long could weigh as much as the Earth! They would have been created as a result of phase changes in the very early Universe, and would contain remnants of the high energy vacuum of the early universe preserved inside. Cambridge University articles on Cosmic Strings; a bit heavy going in places. Cosmic String article with links to several other interesting articles in Science Daily.
Domain Walls	These are two-dimensional objects, effectively 2-dimensional singularities, that form when a discrete symmetry is broken at a phase transition. A network of domain walls effectively partitions the universe into various cells, similar to a foam. Interestingly, the gravitational field of a domain wall is repulsive rather than attractive. The Horava-Witten domain wall acts as a boundary of eleven-dimensional spacetime in M-theory.
Monopoles	These are zero-dimensional and, hypothetically, form when a spherical symmetry is broken. Theory predicts that they would carry magnetic charge, either a north or south pole in isolation,and would be massive; essentially, a magnet with only a single pole. Several Grand Unified Theories and String theory require their existence, but despite several intensive searches, none has been found. If they do exist, they would be far too massive to produce in any particle accelerator, and are likely to be extraordinarily rare in the Universe at large for anyone to detect one.
Textures	These objects form when more complex symmetry groups are broken. They are delocalized and are unstable. If they exist, they are a distortion of the fabric of space-time that would have occured in the very early Universe as a result of phase transitions. Think of it as the three-dimensional analogue of monopoles, cosmic string, and domain walls. They would have had a significant affect on the structure of the Universe on very large scales.
Evidence	Cosmic Topological defects could not be produced with the energies available to particle accelerators, and none has every been observed occurring naturally in the Universe. Indeed, the existence of Domain Walls, or monopoles in vast numbers would contradict what we actually see. This would not prevent these phenomena from existing outside of the observable universe. Cosmic Strings have been proposed as one way in which the large scale clumping of matter into galaxies and clusters of galaxies could have formed, although this theory is somewhat out of fashion just now as this clumping would seem to be the result of random fluctuations. In late 2007, observers found a "cold spot" in the cosmic microwave background. Some observers have suggested that this could indicate a Texture in the direction of the colder area. It could also be the result of a very large supervoid, but this would have to be of the order of 300 Mpc across, which is twice the size of the largest supervoid found to date.

Science - Non-Baryonic Matter

Cosmic Topological Defects