Astronomers have learned that the mean density of matter in the universe is within a factor of a few of the critical density.
We can calculate ρcrit by inserting the gravitational constant, G = 6.67 \xd7 10-11 Nm2 / kg2, and adopting H0 = 65 km/s/Mpc..
If the density of the universe is less than the "critical density", which is proportional to the square of the Hubble constant, then the universe will expand forever..
The 'critical density' is the average density of matter required for the Universe to just halt its expansion, but only after an infinite time..
For relativistic particles, known in cosmology as “radiation” (although any relativistic species counts, not only photons or even strictly massless particles), the energy density is the number density times the particle energy, and the latter is proportional to a-1 (redshifting as the universe expands); the radiation .
Density parameter
To date, the critical density is estimated to be approximately five atoms (of monatomic hydrogen) per cubic metre, whereas the average density of ordinary matter in the Universe is believed to be 0.2–0.25 atoms per cubic metre..
In the big bang model, the expansion of the universe is slowed down by gravity
If there is enough matter in the universe, then the expansion can be overcome and the universe will collapse in the future
The density of matter that is just sufficient to eventually halt the expansion is called the critical density
The mean density of matter that is required for gravity to halt the expansion of the Universe, equivalent to about 10 –29 g/cm 3
If the cosmological constant is zero, a universe with a density below the critical density will expand for ever, whereas a universe with a density greater than the critical density will eventually collapse
In a Universe where the density of matter is the ‘critical density’ (flat space), locally parallel lines remain parallel
The critical density for the Universe is approximately 10 -26 kg/m 3 (or 10 hydrogen atoms per cubic metre) and is given by: where H is the Hubble constant and G is Newton’s gravitational constant
Topics referred to by the same term
