We begin, in §2, by describing the simplest examples of cosmic strings, which arise in the breaking of an abelian U(1) symmetry, and discuss the reasons for thinking that cosmic strings might appear during phase transitions in the very early history of the universe, in a process closely analogous to what happens when a superfluid or superconductor is cooled rapidly through its transition temperature. (For reviews see Polchinski 2005 Davis & Kibble 2005 Myers & Wyman 2009 Sakellariadou 2009.) Indeed, it is now very possible that observations of a cosmic superstring might provide the first real direct evidence for M-theory. But in the last few years it has emerged that under certain circumstances they too can grow to macroscopic size and play the same role as cosmic strings. On the other hand superstrings were seen as minute objects, even on the scale of particle physics, far too small to have any directly observable effects. Cosmic strings, if they exist, stretch across cosmological distances and though exceedingly thin are sufficiently massive to have noticeable gravitational effects. Initially, cosmic strings and superstrings were regarded as two completely separate classes. Nevertheless, there are good theoretical reasons for believing that these exotic objects do exist, and reasonable prospects of detecting their existence within the next few years. There is no direct empirical evidence for their existence, though there have been some intriguing observations that were initially thought to provide such evidence, but are now generally believed to have been false alarms. Superstrings are the supposed basic constituents of matter in fundamental string theory or M-theory, which is at present the leading contender as a unified theory of all particle interactions including gravity.īoth cosmic strings and superstrings are still purely hypothetical objects. Similar point-like or planar defects-monopoles or domain walls-may also be formed. They could be produced during one of the early symmetry-breaking phase transitions predicted by many particle-physics models, for example one associated with the breaking of a ‘grand unification’ symmetry. They arise because in certain circumstances the ordering may be frustrated.Ĭosmic strings are analogous objects that may have been formed in the early universe. These structures typically appear when a system goes through a phase transition into a low-temperature ordered phase in which the underlying symmetry is spontaneously broken. There are many examples of linear topological defects in low-temperature condensed-matter systems, including vortices in superfluid helium and in atomic Bose–Einstein condensates, and magnetic flux tubes in superconductors.
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