Critical Density of the Universe
| Value | 8.53e-27 kg/m³ |
| Status | Measured: ± 1.30e-28 kg/m³ (0.015 relative) |
| Source | Derived from the Planck 2018 H₀ and CODATA 2022 G |
| Categories | Astronomicalcosmologyexpansion |
| kilogram per cubic meter | 8.5300000e-27 kg/m³ |
| gram per cubic centimeter | 8.5300000e-30 g/cm³ |
| gram per milliliter | 8.5300000e-30 g/mL |
| kilogram per liter | 8.5300000e-30 kg/L |
| pound per cubic foot | 5.3251050e-28 lb/ft³ |
| pound per US gallon | 7.1186300e-29 lb/gal |
Learning zone
The critical density is the dividing line in Friedmann's equations between a universe that expands forever and one that recollapses, and it is absurdly small: five hydrogen atoms in a cubic metre, a vacuum far better than any laboratory can produce. Measurements of the microwave background show the universe sits at that value to within a fraction of a per cent, so space is flat on the largest scales — which is what the theory of inflation predicts.
What supplies the density is the surprise. Ordinary atoms account for only about 5 per cent of it, dark matter for 27 per cent, and dark energy for the remaining 68 per cent. The value scales as H₀², so it inherits the Hubble tension: pick the distance-ladder H₀ instead of the microwave-background one and the critical density rises by about 17 per cent.