The universe's Friedmann Models) determine if the
universe will "barely" expand forever (a Flat Universe,
with space's expansion at exactly its escape velocity),
or is expanding more than that (an Open Universe) or will
eventually contract (a Closed Universe),
and if a Cosmological Constant (additional factor) is in play,
how it plays into this.
The critical density shifts as the universe ages, i.e., when a distant object is observed, the critical density at the time/place of the object depends on its Redshift. However, if gravity is the one force controlling expansion (which is currently believed untrue, the other factor being Dark Energy) then the density of the universe remains either above or below its critical density, or at the value of the critical density current at that time.
Astrophysics determines the critical density by observation of the
universe's expansion and the universe's mass, the latter by direct
observation of matter and by observation of apparent local effects
of Gravity on observed matter. The current estimate
for the current critical density is five Hydrogen atoms per
cubic meter. Actual density of ordinary matter (
The
The a d astrophysics,cosmology,measure)Referenced by:
Astronomical Quantities Cluster Radius |