When we calculate escape velocity, we set the total energy equal to zero. That is equivalent to setting the curvature term in the Friedmann equation to zero: The Friedmann equation then becomes: \[H^2...When we calculate escape velocity, we set the total energy equal to zero. That is equivalent to setting the curvature term in the Friedmann equation to zero: The Friedmann equation then becomes: H2−8πGρ3=0 The only two adjustable quantities in the equation now are ρ, the average density of the Universe, and the expansion rate, H. Solving for ρ in terms of H we get: \rho_{crit} = \frac{3H}{8 \pi G} \nonumber
