Cosmological Perturbations from Cosmic Strings
  Some aspects of the theory of cosmological perturbations from cosmic strings and other topological defects are outlined, with particular reference to a simple example: a spatially flat CDM-dominated universe. The conserved energy-momentum pseudo-tensor is introduced, and the equation for the density perturbation derived from it. It is shown how the scaling hypothesis for defect evolution results in a Harrison-Zel'dovich spectrum for wavelengths well inside the horizon. 
Mark Hindmarsh