Gallagher, CSClifton, TClarkson, C2022-08-012022-08-012021Gallagher, Christopher & Clifton, Timothy & Clarkson, Chris. (2021). Multi-scale perturbation theory II: Solutions and leading-order bispectrum in the ΛCDM universe. Journal of Cosmology and Astroparticle Physics. 2021. 048. 10.1088/1475-7516/2021/08/048.1475-7516http://iopscience.iop.org/1475-7516http://hdl.handle.net/10566/7650Two-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 ΛCDM 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 secondorder 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 Universeencosmological perturbation theorygravitynon-gaussianitypower spectrumArticle