Significance
The tipping-point density: above it space curves closed, below it open, exactly at it flat. Today it is about 8.5 × 10⁻²⁷ kg/m³ taking H₀ = 67 km/s/Mpc — five hydrogen atoms per cubic metre, a better vacuum than any laboratory can make, and yet the density that decides the shape of the cosmos. Take H₀ = 73 instead and it rises to 10 × 10⁻²⁷, which is one more way of saying the Hubble tension is not a small argument. Measurements put the total at Ω = 1.000 ± 0.002, flat to a fifth of a percent, which is the observation inflation was invented to explain.