Editing Scale factor (cosmology) (section)

===Dark-energy-dominated era===
In [[physical cosmology]], the '''dark-energy-dominated era''' is proposed as the last of the three phases of the known universe, beginning when the Universe was about 9.8 billion years old.<ref>Ryden, Barbara, "Introduction to Cosmology", 2006, eqn. 6.33</ref> In the era of [[Inflation_(cosmology)|cosmic inflation]], the Hubble parameter is also thought to be constant, so the expansion law of the dark-energy-dominated era also holds for the inflationary prequel of the big bang.

The [[cosmological constant]] is given the symbol Λ, and, considered as a source term in the [[Einstein field equations|Einstein field equation]], can be viewed as equivalent to a "mass" of empty space, or [[dark energy]]. Since this increases with the volume of the universe, the expansion pressure is effectively constant, independent of the scale of the universe, while the other terms decrease with time. Thus, as the density of other forms of matter – dust and radiation – drops to very low concentrations, the cosmological constant (or "dark energy") term will eventually dominate the energy density of the Universe. Recent measurements of the change in Hubble constant with time, based on observations of distant [[supernova]]e, show this acceleration in expansion rate,<ref>[https://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html The Nobel Prize in Physics 2011]. Retrieved 18 May 2017.</ref> indicating the presence of such dark energy.

For a dark-energy-dominated universe, the evolution of the scale factor in the [[Friedmann–Lemaître–Robertson–Walker metric]] is easily obtained solving the [[Friedmann equations]]:
<math display="block">a(t) \propto \exp(H_0t)</math>
Here, the coefficient <math>H_0</math>in the exponential, the [[Hubble constant]], is
<math display="block">H_0 = \sqrt{8\pi G \rho_\mathrm{full} / 3} = \sqrt{\Lambda / 3}.</math>
This exponential dependence on time makes the spacetime geometry identical to the [[de Sitter universe]], and only holds for a positive sign of the cosmological constant, which is the case according to the currently accepted value of the [[Cosmological_constant#Positive_value|cosmological constant]], Λ, that is approximately {{val|2|e=−35|u=s-2}}.
The current density of the [[observable universe]] is of the order of {{val|9.44|e=−27|u=kg/m3}} and the age of the universe is of the order of 13.8&nbsp;billion years, or {{val|4.358|e=17|u=s}}. The Hubble constant, <math>H_0</math>, is ≈{{val|70.88|u=km/s/Mpc}} (The Hubble time is 13.79&nbsp;billion years).