  Two-parameter perturbation theory (2PPT) is a framework designed to include
the relativistic gravitational effects of small-scale nonlinear structures on
the large-scale properties of the Universe. In this paper we use the 2PPT
framework to calculate and study the bispectrum of matter in a spatially-flat
$\Lambda$CDM cosmology. This is achieved by deploying Newtonian perturbation
theory to model the gravitational fields of quasi-nonlinear structures, and
then subsequently using them as source terms for the large-scale cosmological
perturbations. We find that our approach reproduces some of the expected
relativistic effects from second-order cosmological perturbation theory, but
not all. This work therefore provides a first step in deploying a formalism
that can simultaneously model the weak gravitational fields of both linear and
nonlinear structures in a realistic model of the Universe.
