Minimal Basis for Exact Time Dependent Kernels in Cosmological Perturbation Theory and Application to $\Lambda$CDM and $w_0w_a$CDM

TL;DR

This paper develops a minimal basis of kernels for exact time-dependent cosmological perturbation theory applicable to arbitrary expansion histories, improving accuracy over the EdS approximation and enabling better EFT matching.

Contribution

It introduces a minimal set of kernels for perturbative expansion in cosmology that relaxes the EdS approximation and explicitly derives their properties up to fifth order.

Findings

Explicit minimal basis functions up to order 5

Demonstrates improved EFT matching for arbitrary cosmologies

Analyzes differences from EdS approximation in $\\Lambda$CDM and $w_0w_a$CDM

Abstract

We derive a minimal basis of kernels furnishing the perturbative expansion of the density contrast and velocity divergence in powers of the initial density field that is applicable to cosmological models with arbitrary expansion history, thereby relaxing the commonly adopted Einstein-de-Sitter (EdS) approximation. For this class of cosmological models, the non-linear kernels are at every order given by a sum of terms, each of which factorizes into a time-dependent growth factor and a wavenumber-dependent basis function. We show how to reduce the set of basis functions to a minimal amount, and give explicit expressions up to order $n=5$. We find that for this minimal basis choice, each basis function individually displays the expected scaling behaviour due to momentum conservation, being non-trivial at $n\geq 4$. This is a highly desirable property for numerical evaluation of loop corrections. In addition, it allows us to match the density field to an effective field theory (EFT) description for cosmologies with an arbitrary expansion history, which we explicitly derive at order four. We evaluate the differences to the EdS approximation for $\Lambda$CDM and $w_0w_a$CDM, paying special attention to the irreducible cosmology dependence that cannot be absorbed into EFT terms for the one-loop bispectrum. Finally, we provide algebraic recursion relations for a special generalization of the EdS approximation that retains its simplicity and is relevant for mixed hot and cold dark matter models.