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 | Astronomical |
| microgram per liter | 8.5300000e-21 μg/L |
| milligram per cubic meter | 8.5300000e-21 mg/m³ |
| milligram per liter | 8.5300000e-24 mg/L |
| pound per cubic yard | 1.4377784e-26 lb/yd³ |
| kilogram per cubic meter | 8.5300000e-27 kg/m³ |
| gram per liter | 8.5300000e-27 g/L |
| pound per cubic foot | 5.3251050e-28 lb/ft³ |
| pound per imperial gallon | 8.5491182e-29 lb/imp gal |
| pound per US gallon | 7.1186300e-29 lb/gal |
| slug per cubic foot | 1.6550932e-29 slug/ft³ |
| 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 |
| tonne per cubic meter | 8.5300000e-30 t/m³ |
| US short ton per cubic yard | 7.1888918e-30 ton/yd³ |
| pound per cubic inch | 3.0816580e-31 lb/in³ |
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.