Final answer:
The critical density refers to the average density of matter in space that is needed for the universe to stop expanding. It is estimated to be about 1 to 2 × 10-27 kg/m³, which is only 10 to 20% of the critical density.
Step-by-step explanation:
The critical density refers to the average density of matter in space that is required for the universe to stop expanding. It is calculated by counting all the galaxies out to a given distance and estimating their masses, including dark matter. Current estimates suggest that the density is about 1 to 2 × 10-27 kg/m³, which is only about 10 to 20% of the critical density.
Measuring the local density of mass relative to the critical density involves considering the amount of dark matter and other types of indirectly observed normal matter. The total amount of matter detected accounts for only 10% to 40% of what is needed for closure, with dark matter contributing about 27%. This suggests that there is a significant amount of matter that is yet to be accounted for. Further measurements and refinements in expansion rates and other factors are needed to determine the exact curvature of space and to justify the observations made regarding the critical density and dark matter.