Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 312 / 2 / i / Solution

Use the one-loop matter power spectrum convention in which $P_{13}$ is one ordering of $⟨{\delta }^{(1)}{\delta }^{(3)}⟩$. Thenwith the P22 contribution to the one-loop matter power spectrumand the P13 contribution to the one-loop matter power spectrumThe factors two and three count Wick contractions. The mixed $P_{12}$ contains an odd number of zero-mean Gaussian fields, so $P_{12}=0$.

For $q\gg k$, the ultraviolet softness of the second-order density kernel gives $F_{2,s}=O(k^2/q^2)$, soThe third-order kernel has $F_{3,s}(\mathbf{k},\mathbf{q},-\mathbf{q})=O(k^2/q^2)$, givingThese integrals expose the sensitivity to unresolved short-scale dynamics. The effective sound-speed counterterm in large-scale structure has exactly the required deterministic shape:Here $c_{\mathrm{eff}}^2$ absorbs the time and normalization conventions and has dimensions of length squared. A stochastic correction begins at $k^4$ and can absorb the corresponding short-scale sensitivity of $P_{22}$.