The capital Greek letter (Omega) is the symbol for this fraction.
So if (Omega) = 1, then the universe is "flat" and has critical density.
If (Omega) \x26lt; 1, then the universe is "open" and will expand forever.
If (Omega) \x26gt; 1, then the universe is "closed" and will eventually recollapse..
Aside on flatness
Current indications from the cosmic microwave background are that the universe is spatially flat.
That implies that the sum of all of the energy (density) in the universe equals the critical density, i.e. the total Omega is 1..
The vacuum energy density of the Universe based on 2015 measurements by the Planck collaboration is ρvac = 5.96\xd710−27 kg/m3 ≘ 5.3566\xd710−10 J/m3 = 3.35 GeV/m3 or about 2.5\xd710−47 GeV4 in geometrized units..
The ΛCDM model has been shown to satisfy the cosmological principle, which states that, on a large-enough scale, the universe looks the same in all directions (isotropy) and from every location (homogeneity); "the universe looks the same whoever and wherever you are." .
2.1 Cosmological parameters From the Friedmann equation(5) (where henceforth we take the effects of a cosmological constant into account by including:
The fraction of the effective mass of the universe attributed to "dark energy" or the cosmological constant is Ω Λ,0 = 0.73 +/- 0.04.
With 73% of the influence on the expansion of the universe in this era, the dark energy is viewed as the dominant influence on that expansion.
Thus, the Lambda-CDM model, the current standard model of cosmology which uses the FLRW metric, includes ,the cosmological constant, which is measured to be on the order of 10−52 m−2.
It may be expressed as 10−35 s−2 (by multiplication with c2, i.e. ≈ 1017 m⋅s−2) or as 10 −122 ℓP−2 (where ℓP is the Planck length).
Book by Charles Seife
