Cosmic string: Difference between revisions

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During the early days of string theory both string theorists and cosmic string theorists believed that there was no direct connection between [[superstrings]] and cosmic strings (the names were chosen independently by analogy with [[twine|ordinary string]]). The possibility of cosmic strings being produced in the early universe was first envisioned by quantum field theorist [[Tom Kibble]] in 1976,<ref name="Kibble 1976" />, and this sprouted the first flurry of interest in the field. In 1985, during the [[first superstring revolution]], [[Edward Witten]] contemplated on the possibility of fundamental superstrings having been produced in the early universe and stretched to macroscopic scales, in which case (following the nomenclature of Tom Kibble) they would then be referred to as cosmic superstrings. He concluded that had they been produced they would have either disintegrated into smaller strings before ever reaching macroscopic scales (in the case of [[Type I superstring]] theory), they would always appear as boundaries of [[Domain wall (string theory)|___domain walls]] whose tension would force the strings to collapse rather than grow to cosmic scales (in the context of [[Heterotic string|heterotic superstring]] theory), or having a characteristic energy scale close to the [[Planck energy]] they would be produced before [[cosmological inflation]] and hence be diluted away with the expansion of the universe and not be observable.

Much has changed since these early days, primarily due to the [[second superstring revolution]]. It is now known that string theory in addition to the fundamental strings which define the theory perturbatively also contains other one-dimensional objects, such as D-strings, and higher-dimensional objects such as D-branes, NS-branes and M-branes partially wrapped on compact internal spacetime dimensions, while being spatially extended in one non-compact dimension. The possibility of [[Large extra dimension|large compact dimensions]] and large [[Randall–Sundrum model|warp factors]] allows strings with tension much lower than the Planck scale. Furthermore, various dualities that have been discovered point to the conclusion that actually all these apparently different types of string are just the same object as it appears in different regions of parameter space. These new developments have largely revived interest in cosmic strings, starting in the early 2000s.