Perturbation theory of large scale structure in the ΛCDM Universe: Exact time evolution and the two-loop power spectrum

We derive exact analytic solutions for density and velocity fields to all orders in Eulerian standard perturbation theory for ACDM 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-)Einstein-de Sitter (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 ACDM cosmology. We show that the difference between the exact ACDM and the (quasi-)EdS approximated result can reach the level of several percent (at redshift zero, for wave numbers k <1 h/Mpc). This deviation can be partially mitigated by exploiting the degeneracy with the effective field theory counterterms. As an additional benefit of our algorithm for the solutions of time-dependent coefficients, the computational complexity of power-spectra loops in ACDM 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.

Automated summary

In this paper, we provide exact analytic solutions for density and velocity fields in ACDM cosmology using Eulerian standard perturbation theory. The solutions are derived to all orders and have a separable form in time and momenta at each perturbative order.