2.1 General Relativity 13 Another important quantity is the Density Parameter with the Critical Energy Density being defined as ρ$_{crit}$ ( t ) ≡ 3 M 2 $_{P}$H $^{2}$( t ). The critical energy density is the total density that a spatially flat Universe would have for a given value of the Hubble parameter. We also define Precise measurements of the geometry and energy density of the Universe made by the Planck spacecraft, when combined with other data [40], yield the current value which is consistent with the time-independent value of Ω = 1. We therefore have that the energy density of the Universe is very close to the critical density ( ρ = ρ$_{crit}$ ) and that the geometry of the Universe is very close to spatially flat ( K =0). The observed energy density of the Universe is made- up of three components: Baryonic Matter (5%), Dark Matter (26%) and Dark Energy (69%), the percentages indicating the approximate relative amount of each component. Ω − 1 ≡ K ( aH ) 2 ≡ K ˙ a 2 (2.29) Ω$_{0}$ = 1 . 000 ± 0 . 005 (2.30) Ω( t ) ≡ ρ ( t ) ρ$_{crit}$ ( t ) (2.28)