Perturbation theory of LSS in the $\Lambda$CDM Universe: exact time evolution and the two-loop power spectrum

TL;DR

This paper derives exact analytic solutions for density and velocity fields in $\\Lambda$CDM cosmology, enabling precise two-loop power spectrum calculations and revealing deviations from approximations that can be mitigated by EFT counterterms.

Contribution

It provides a novel exact recursive method for solving perturbation theory in $\\Lambda$CDM, improving accuracy and computational efficiency for power spectrum predictions.

Findings

Exact two-loop dark matter power spectra in $\\Lambda$CDM computed.

Deviations from EdS approximation can reach several percent.

Method efficiently implements IR and conservation cancellations.

Abstract

We derive exact analytic solutions for density and velocity fields to all orders in Eulerian standard perturbation theory for $\Lambda$CDM cosmology. In particular, we show that density and velocity field kernels can be written in a separable form in time and momenta at each perturbative order. The kernel solutions are built from an analytic basis of momentum operators and their time-dependent coefficients, which solve a set of recursive differential equations. We also provide an exact closed perturbative solution for such coefficients, expanding around the (quasi-)EdS approximation. We find that the perturbative solution rapidly converges towards the numerically obtained solutions and its leading order result suffices for any practical requirements. To illustrate our findings, we compute the exact two-loop dark matter density and velocity power spectra in $\Lambda$CDM cosmology. We show that the difference between the exact $\Lambda$CDM and the (quasi-)EdS approximated result can reach the level of several percent (at redshift zero, for wavenumbers $k<1h/$Mpc). This deviation can be partially mitigated by exploiting the degeneracy with the EFT counterterms. As an additional benefit of our algorithm for the solutions of time-dependent coefficients, the computational complexity of power spectra loops in $\Lambda$CDM is comparable to the EdS case. In performing the two-loop computation, we devise an explicit method to implement the so-called IR cancellations, as well as the cancellations arising as a consequence of mass and momentum conservation.