= Solution
Without radiation, the <Friedmann acceleration equation> changes sign when $\rho_m=2\rho_\Lambda$. The <cosmological redshift> satisfies $1+z=a_0/a$, and conservation of <pressureless matter> gives $\rho_m(z)=\rho_{m,0}(1+z)^3$. Hence
$$
\boxed{z_{\rm acc}=\left(\frac{2\rho_\Lambda}{\rho_{m,0}}\right)^{1/3}-1
=\left(\frac{2\Omega_{\Lambda,0}}{\Omega_{m,0}}\right)^{1/3}-1.}
$$
The expanding spatially flat model is decelerating above this redshift and accelerating below it. Positive $\rho_\Lambda$ is required for such a transition; a negative value gives no accelerated expanding epoch. If the displayed redshift is negative, the transition lies in our future.
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