Cosmic string: Difference between revisions

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'''Cosmic strings''' are hypothetical 1-dimensional [[topological defect]]s which may have formed during a [[Symmetry breaking|symmetry-breaking]] [[cosmological phase transition|phase transition]] in the early universe when the [[topology]] of the [[Vacuum state|vacuum]] manifold associated to this symmetry breaking was not [[Simply connected space|simply connected]].

In less formal terms, they are hypothetical long, thin defects in the fabric of space that might have formed according to string theory. They might have formed in the early universe during a process where certain [[symmetry breaking|symmetries were broken]]. Their existence was first contemplated by the theoretical physicist [[Tom Kibble]] in the 1970s.<ref name="Kibble 1976">{{cite journal |doi=10.1088/0305-4470/9/8/029 |title=Topology of cosmic domains and strings |year=1976 |last1=Kibble |first1=Tom W K |journal= Journal of Physics A: Mathematical and General |volume=9 |issue=8 |pages=1387–1398 |bibcode=1976JPhA....9.1387K }}</ref>

The formation of cosmic strings is somewhat analogous to the imperfections that form between crystal grains in solidifying liquids, or the cracks that form when water freezes into ice. The phase transitions leading to the production of cosmic strings are likely to have occurred during the earliest moments of the universe's evolution, just after [[cosmological inflation]], and are a fairly generic prediction in both [[quantum field theory]] and [[string theory]] models of the [[early universe]].

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==Dimensions==

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==