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Critical density
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Density separating open and closed universes.
ρ_c = 3H²/8πGρ_c = 3H²/(8πG) ≈ 9×10⁻²⁷ kg/m³ is the density for a spatially flat universe. Observations give Ω ≈ 1.
About 5 hydrogen atoms per cubic metre.
Total matter + energy matches ρ_c.
In more detail
The critical density is the average density of matter that makes the universe spatially flat. Observations show the actual density is very close to the critical value.
Real-world uses
- Critical density sets the reference for cosmological parameters.
- Measurements of flatness test inflation predictions.
- Cosmologists use the critical density to decide whether the universe is open, flat or closed on large scales.
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Frequently asked questions
What is the value?
About 9 hydrogen atoms per cubic metre.
Why is flatness important?
It distinguishes between universes that expand forever and those that recollapse.
What do current measurements suggest?
They suggest the universe is very close to flat, meaning its density is near the critical value.
Related topics in Particle Physics & Cosmology
Inflation · Dark matter · Dark energy · Fate of the universe · Exoplanets · Habitable zone