# The dark matter bispectrum from effective viscosity and one-particle irreducible vertices

## TL;DR

This paper models dark matter structure formation using an effective field theory approach, calculating the bispectrum with a focus on effective viscosity and vertices, and compares results with standard theory and simulations.

## Contribution

It introduces a novel effective action framework for dark matter evolution, emphasizing the role of viscosity and vertices in bispectrum calculations.

## Key findings

- Effective vertices are subdominant compared to viscosity and sound velocity.
- Reproduces standard perturbation theory results at applicable scales.
- Shows improved agreement with N-body simulations at larger wavenumbers.

## Abstract

Dark matter evolution during the process of cosmological structure formation can be described in terms of a one-particle irreducible effective action at a characteristic scale $k_m$ and a loop expansion below this scale, based on the effective propagators and vertices. We calculate the form of the effective vertices and compute the bispectrum of density perturbations within a one-loop approximation. We find that the effective vertices play a subdominant role as compared to the effective viscosity and sound velocity that modify the (inverse) propagators. For the bispectrum we reproduce the results of standard perturbation theory in the range where it is applicable, and find a slightly improved agreement with N-body simulations at larger wavenumbers.