Astronomy & Cosmology -

Non-Baryonic Matter

WILLIAM & DEBORAH HILLYARD

Cosmic Topological Defects

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Overview

A topological defect occurs as a result of a phase transition; so what is a phase transition?  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. 

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.  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.


Links

A series of articles from Cambridge University on Cosmic Strings and other cosmological defects.  It is a bit heavy going in places, while strangely lacking in others.  Useful overall. 

Cosmic string article with links to several other interesting articles in Science Daily. 

An article in Cosmos about the CMB cold spot, and whether it could be due to a topological defect.  Another article on the same topic in Space magazine.

Details

Monopoles
Magnetic Monopoles are hypothetical, zero-dimensional particles, that 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.  Paul Dirac first suggested a quantum theory of magnetism in 1931.  Today, several Grand Unified Theories as well as String theory require their existence, but despite several intensive searches, none has been found.  If they do exist, they would be far too massive at up to 1026 eV to produce in any particle accelerator.  Experimental evidence points to their extreme rarity in the Universe at large; probably only one monopole to every 1029 nucleons (protons and neutrons) at most, so it is unlikely that we would detect one. 

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 would have formed at the end of the inflation period in the very early stages of the Universe.  While billions of light years long, they are smaller than a proton in diameter; possibly as small as the Planck length.  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.  At one time, it was suggested that strings could have contributed to the "clumping" evident in the Universe, but analysis of the CMB has ruled this out.  Recent Superstring research has shown that certain macroscopic superstrings, known as one-dimensional Dirchlet branes, behave in a manner similar to cosmic srings. 

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.

Textures
These objects form when more complex symmetry groups are broken, and are unstable.  If they exist, they are a distortion of the fabric of space-time that would have occurred 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.