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The universe's **critical density** (i.e.,
**critical density of the universe**
or **ρ _{c}**)
is the density of mass across the universe that would leave it flat.
The principle of general relativity (Einstein's theory
that gravitational force is equivalent to space-time
reacting to mass by curving)
and the models of the universe based on it
(

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
(**baryonic mass density**)
is estimated at 0.2-0.25, but a density including
dark matter, dark energy, and other energy (e.g., electromagnetic radiation)
is very close to the critical density.

The **density parameter** of the universe, denoted by **Ω**,
is the density of the universe scaled so that a value
of 1 indicates the universe is at the critical density, i.e.,
**Ω _{c}** = 1.

The **deceleration parameter** (**q**) indicates the
universe's acceleration/deceleration of expansion: above zero
is a measure of deceleration, zero means expansion is steady,
and below zero means the expansion is actually accelerating.
The deceleration parameter is defined in terms
of the scale factor, "a",
and is a (potentially constant) function of time:

a d^{2}a/dt^{2}q(t) = - ————————— (da/dt)^{2}

If gravity were fully counteracting expansion, the
expansion would be decelerating and q would be greater than zero
and the universe would eventually contract, presumably
resulting the universe returning to a point,
a theoretical future event termed the **Big Crunch**.
Deceleration parameter q is defined such that 0.5 indicates
a flat universe, i.e., **q _{c}** = 0.5,
which is what would be expected if the universe's density is
exactly at its critical density, i.e., Ω = 1.
Measurements have revealed acceleration of the expansion with
q ≈ -0.55,
which has led to the coining of the term

http://en.wikipedia.org/wiki/Critical_density_(cosmology)

http://en.wikipedia.org/wiki/Deceleration_parameter

astronomical quantities

cluster radius

dark energy

Friedmann model

Lambda-CDM model (ΛCDM)

mass density

virial theorem