Dimensionless cosmological power spectrum (${\Delta }_X=k^3{P}_X/(2{\pi }^2)$)

The dimensionless spectrum is the contribution to a statistically isotropic field variance per logarithmic wavenumber interval: $⟨X^2⟩=\int d\log k{\Delta }_X(k)$, with any necessary smoothing or regulators. Some conventions denote this same quantity ${\Delta }_X^2$. The definition, not the superscript convention, fixes its meaning.

The dimensionless spectrum is $\mathcal{P}(k)=k^{3}P(k)/(2{\pi }^2)$. If it rises as $k^4$ below equality and as ${\ln }^{2}k$ above equality, then the dimensional spectrum rises as $k$ and falls as $k^{-3}{\ln }^{2}k$. Thus the dimensional spectrum has a turnover while the dimensionless spectrum only bends to a much slower rise. A schematic transition must smoothly join the two asymptotic regimes.