Critical Density


The more matter and energy the Universe has in it the more curved space is.
Curvature can be positive or negative.

Critical density is the dividing line:
For larger density curvature is positive - "closed"
For critical density curvature is zero - "flat"
For smaller density curvature is negative - "open"

Also the dividing line between finite and infinite Universe:
For density greater than critical Universe finite.
For density equal or less than critical is infinite.

[Critical density at the current epoch is small - around 6 hydrogen atoms per cubic meter!]