The bispectrum of matter perturbations from cosmic strings
  We present the first calculation of the bispectrum of the matter
perturbations induced by cosmic strings. The calculation is performed in two
different ways: the first uses the unequal time correlators (UETCs) of the
string network - computed using a Gaussian model previously employed for cosmic
string power spectra. The second approach uses the wake model, where string
density perturbations are concentrated in sheet-like structures whose surface
density grows with time. The qualitative and quantitative agreement of the two
gives confidence to the results. An essential ingredient in the UETC approach
is the inclusion of compensation factors in the integration with the Green's
function of the matter and radiation fluids, and we show that these
compensation factors must be included in the wake model also. We also present a
comparison of the UETCs computed in the Gaussian model, and those computed in
the unconnected segment model (USM) used by the standard cosmic string
perturbation package CMBACT. We compare numerical estimates for the bispectrum
of cosmic strings to those produced by perturbations from an inflationary era,
and discover that, despite the intrinsically non-Gaussian nature of
string-induced perturbations, the matter bispectrum is unlikely to produce
competitive constraints on a population of cosmic strings.
