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==Gravitation==
{{refimprove section|date=September 2016}}
A string is a geometrical deviation from [[Euclidean geometry]] in spacetime characterized by an angular deficit: a circle around the outside of a string would comprise a total angle less than 360°. <ref>{{cite journal| last=Gott| first=J. Richard| title=Closed timelike curves produced by pairs of moving cosmic strings: Exact solutions| journal=Phys. Rev. Lett.| date=1991| volume=66| issue=9| page=1126| doi=10.1103/PhysRevLett.66.1126| pmid=10044002| bibcode=1991PhRvL..66.1126G}}</ref> From the [[general theory of relativity]] such a geometrical defect must be in tension, and would be manifested by mass. Even though cosmic strings are thought to be extremely thin, they would have immense density, and so would represent significant gravitational wave sources. A cosmic string about a kilometer in length may be more massive than the Earth.
 
However [[general relativity]] predicts that the gravitational potential of a straight string vanishes: there is no gravitational force on static surrounding matter. The only gravitational effect of a straight cosmic string is a relative deflection of matter (or light) passing the string on opposite sides (a purely topological effect). A closed cosmic string gravitates in a more conventional way.{{clarify|date=September 2019}}
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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.<ref name="witten-cosmic-superstrings">{{cite journal |last1=Witten |first1=Edward |title=Cosmic Superstrings |journal=Phys. Lett. B |date=1985 |volume=153 |issue=4–5 |pages=243–246 |doi=10.1016/0370-2693(85)90540-4|bibcode=1985PhLB..153..243W }}</ref> 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 contains, in addition to the fundamental strings which define the theory perturbatively, 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.