...$\rho_{ \rm Q}(z)$

One has also to make the distinction between the equation of state parameter $w_{ \rm Q}= P_{ \rm Q}/ \rho_{ \rm Q}$ and the ``sound speed'' squared $c_{{\rm s}~{ \rm Q}}^2 \equiv \dot P_{ \rm Q}/ \dot \rho_{ \rm Q}$, which are identical when w is constant.

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