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Cosmic strings, if they exist, would be extremely thin [[topological defect]]s with diameters of the same order of magnitude as that of a proton, i.e. {{nobreak|~ 1 fm}}, or smaller. Given that this scale is much smaller than any cosmological scale, these strings are often studied in the zero-width, or Nambu–Goto approximation. Under this assumption, strings behave as one-dimensional objects and obey the [[Nambu–Goto action]], which is classically equivalent to the [[Polyakov action]] that defines the bosonic sector of [[superstring theory]].
In field theory, the string width is set by the scale of the symmetry breaking [[phase transition]]. In string theory, the string width is set (in the simplest cases) by the fundamental string scale, warp factors (associated to the spacetime curvature of an internal six-dimensional spacetime manifold) and/or the size of internal [[compact dimension]]s. (In string theory, the universe is either 10- or 11-dimensional, depending on the strength of interactions and the curvature of spacetime.)
==Gravitation==
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===Negative mass cosmic string===
The [[Standard Model|standard model]] of a cosmic string is a geometrical structure with an angle deficit, which thus is in tension and hence has positive mass. In 1995, [[Matt Visser|Visser]] ''et al.'' proposed that cosmic strings could theoretically also exist with angle excesses, and thus negative tension and hence [[negative mass]]. The stability of such [[exotic matter]] strings is problematic; however, they suggested that if a negative mass string were to be wrapped around a [[wormhole]] in the early universe, such a wormhole could be stabilized sufficiently to exist in the present day.<ref>{{cite journal |arxiv=astro-ph/9409051 |bibcode=1995PhRvD..51.3117C |doi=10.1103/PhysRevD.51.3117 |pmid=10018782 |title=Natural wormholes as gravitational lenses |year=1995 |last1=Cramer |first1=John |last2=Forward |first2=Robert |last3=Morris |first3=Michael |last4=Visser |first4=Matt |last5=Benford |first5=Gregory |last6=Landis |first6=Geoffrey |journal=Physical Review D |volume=51 |issue=6 |pages=3117–3120|s2cid=42837620 }}</ref><ref>{{cite press release |url=http://www.geoffreylandis.com/wormholes.htp |title=Searching for a 'Subway to the Stars' |url-status=dead |archive-url=https://web.archive.org/web/20120415100921/http://www.geoffreylandis.com/wormholes.htp |archive-date=2012-04-15 }}</ref>
===Super-critical cosmic string===
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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.
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.
In 2002, [[Henry Tye]] and collaborators predicted the production of cosmic superstrings during the last stages of [[brane cosmology|brane inflation]],<ref>{{cite journal |arxiv=hep-th/0204074 |bibcode=2002PhLB..536..185S |doi=10.1016/S0370-2693(02)01824-5 |title=Cosmic string production towards the end of brane inflation |year=2002 |last1=Sarangi |first1=Saswat |last2=Tye |first2=S.-H.Henry |journal=Physics Letters B |volume=536 |issue=3–4 |pages=185|s2cid=14274241 }}</ref> a string theory construction of the early universe that gives leads to an expanding universe and cosmological inflation. It was subsequently realized by string theorist [[Joseph Polchinski]] that the expanding Universe could have stretched a "fundamental" string (the sort which superstring theory considers) until it was of intergalactic size. Such a stretched string would exhibit many of the properties of the old "cosmic" string variety, making the older calculations useful again. As theorist [[Tom Kibble]] remarks, "string theory cosmologists have discovered cosmic strings lurking everywhere in the undergrowth". Older proposals for detecting cosmic strings could now be used to investigate superstring theory.
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