The Research Of Cosmic Stirng Loop

From the GUT theory, we know that some topological defects, such as domain walls, cosmic strings, monopoles and textures, might have been formed at the phase transition in the very early history of the Universe. And the cosmic strings are of great interest , which are thought to be the seeds that led to the formation of the large-scale structures we observe today. Cosmic strings can be classified by the boundary condition into two categories: infinite long strings and closed loops. The study of the evolution of cosmic strings is of the greatest significance in the cosmic string theory. 1998 astronomers found that the Universe is not only expanding, it is accelerating. Therefore, scientists proposed a novel cosmological model which contains a new composition, dark energy. So, how do cosmic strings evolve in the dark energy Universe is a big problem today and also is the motivation of this thesis.We solved the string loop equation in the standard cosmology by numerical methods, and found no matter what the initial radius is the loop will collapse to a point, finally. And we did the same approach the string loop in the dark energy cosmology. However, we discovered that if the initial radius is large enough, the loop will expand in an accelerating way, eventually. Furthermore, we found whether the loop will expand or collapse finally depends on the initial radius and the less state equation parameter w will lead loops to become the expanding state earlier and easier. And then, we studied the loops'critical state between collapsing state and expanding state at the background of different state equation parameters and found that we are able to observe the cosmic string loops today. Finally, we discussed the time when the we can observe the last cosmic string loop, and when loops will enter into the extremely rapid expanding state.