The existence of cosmic strings was first proposed in 1976 by Tom Kibble, who
drew on the theory of line vortices in superconductors to predict the formation
of similar structures in the Universe at large as it expanded and cooled during
the early phases of the Big Bang. The critical assumption is that the strong and
electroweak forces were first isolated by a symmetry-breaking phase transition
which converted the energy of the Higgs field into the masses of fermions and
vector bosons. Under certain conditions, it is possible that some of the Higgs
field energy remained in thin tubes which stretched across the early Universe.
These are cosmic strings.
The masses and dimensions of cosmic strings are largely determined by the
energy scale at which the relevant phase transition occurred. The grand unification
(GUT) energy scale is at present estimated to be about 1015 GeV, which indicates
that the GUT phase transition took place some 10−37–10−35 s after the Big Bang,
when the temperature of the Universe was of the order of 1028 K. The thickness of
a cosmic string is typically comparable to the Compton wavelength of a particle
with GUT mass or about 10−29 cm. This distance is so much smaller than the
length scales important to astrophysics and cosmology that cosmic strings are
usually idealized to have zero thickness.