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I want to show that in the radiation-dominated era, $\delta_H^2 \propto a^{1-n}$. I know that the wavenumber corresponding to the Hubble scale goes like $a^{-1}$ and that $\delta_H^2 \propto k^3 P(k)$ and $P(k) \propto k^n$, but I am unsure of how to put these together to get the desired result.

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  • $\begingroup$ Isn't better suited for Physics SE? $\endgroup$ – Alchimista Nov 12 '20 at 10:15

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