D. The Monopole Problem

At early times in the expansion (z > 1000), the physics of the universe is described by particle theory. Many of these theories predict the creation of topological defects. These defects arise when phase transitions occur in particle models. Since the temperature of the universe cools as the expansion proceeds, these phase transitions are natural consequences of symmetry breakings that occur in particle models. Several types of defects are described briefly below [22, Chapter 10].

• Domain Walls - Space divides into connected regions; one region with one phase and the other region exhibiting the other phase. The regions are separated by walls of discontinuity described by a certain energy per unit area.

• Strings - These are linear defects, characterized by some mass per unit length. They can be visualized at the present time as large strings stretched in space that possibly cause galaxies to form into groups. They serve as an alternative to inflation, for explaining the large-scale structure of the universe. However, at the moment they are not favored due to lack of observations of the gravitational-lensing effect they should exhibit (17)

• Monopoles - These are point defects, where the field points radially away from the defect, which has a characteristic mass. These defects have a magnetic field configuration at infinity that makes them analogous to that of the magnetic monopole, hypothesized by Maxwell and others.

• Textures These objects are hard to visualize and are not expected to form in most theories. One can consider them as a kind of combination of all the other defects.

Out of all these defects, monopoles are the most prevalent in particle theories. It becomes a problem in the hot Big Bang model, when one calculates the number of monopoles produced in events, such as the electroweak symmetry breaking. One finds they would be the dominate matter in the universe. This is contrary to the fact that no monopole has ever been observed, directly or indirectly, by humans. These monopoles would effect the curvature of the universe and in turn the Hubble parameter, galaxy formation, etc. Therefore, unwanted relics, such as monopoles, remain an anomalous component of the hot Big Bang theory.

17 Also note that these are not visible objects, they are distortions in the space-time fabric. Back.