Nonlinear wavenumber
= Nonlinear wavenumber
{title2=$k_{\rm NL}$}
The nonlinear <wavenumber> is the inverse scale at which linear <density contrast> fluctuations become of order unity. For the one-dimensional convention $\langle\delta(k)\delta(k')\rangle=2\pi\delta^{(D)}(k+k')P_L(k)$, a usual estimate is $k_{\rm NL}P_L(k_{\rm NL})/\pi\sim1$. Loop momenta above this scale require the <effective field theory of large-scale structure> even when external modes are perturbative.