  In a previous paper [1] we considered the possibility that (within the
early-radiation epoch) there has been (also) a short period of a significant
presence of cosmic strings. During this radiation-plus-strings stage the
Universe matter-energy content can be modelled by a two-component fluid,
consisting of radiation (dominant) and a cosmic-string fluid (subdominant). It
was found that, during this stage, the cosmological gravitational waves (CGWs)
- that had been produced in an earlier (inflationary) epoch - with comoving
wave-numbers below a critical value (which depends on the physics of the
cosmic-string network) were filtered, leading to a distorsion in the expected
(scale-invariant) CGW power spectrum. In any case, the cosmological evolution
gradually results in the scaling of any long-cosmic-string network and, hence,
after a short time-interval, the Universe enters into the late-radiation era.
However, along the transition from an early-radiation epoch to the
late-radiation era through the radiation-plus-strings stage, the
time-dependence of the cosmological scale factor is modified, something that
leads to a discontinuous change of the corresponding scalar curvature, which,
in turn, triggers the quantum-mechanical creation of gravitons. In this paper
we discuss several aspects of such a process, and, in particular, the
observational consequences on the expected gravitational-wave (GW) power
spectrum.
