 Solution

The plot uses comoving wavenumber in $\mathrm{Mp}{\mathrm{c}}^{-1}$. Using the inverse comoving particle horizon from part (ii) gives $1/r_{p,\mathrm{eq}}\simeq 9\times 10^{-3}\mathrm{Mp}{\mathrm{c}}^{-1}$. The conventional matter-radiation equality scale is insteadThese are consistent order-one conventions for locating the turnover near $10^{-2}\mathrm{Mp}{\mathrm{c}}^{-1}$. The horizon-entry condition is $k/(aH/c)\sim 1$; equating an entire Fourier wavelength $2\pi /k$ to the particle horizon would attach a different numerical factor.

For adiabatic initial conditions, the asymptotic cold-dark-matter power spectrum has $P(k)\propto k^{n_s}T(k{)}^2$, where the scalar spectral index is $n_s\simeq 1$ and $T$ is the cosmological transfer function. Modes below $k_{\mathrm{eq}}$ enter during matter domination and have little relative suppression, giving approximately $P\propto k$. Modes above $k_{\mathrm{eq}}$ enter during radiation domination, when logarithmic growth of matter perturbations during radiation domination is much slower. Their transfer is approximately $T\propto \ln (k/k_{\mathrm{eq}})/k^2$, giving $P\propto k^{-3}{\ln }^2(k/k_{\mathrm{eq}})$ well above equality. This explains the broad turnover; baryon acoustic oscillations add structure, and nonlinear evolution modifies sufficiently small scales.