Dark-energy-dominated era

The dark-energy-dominated era refers to one of the three phases of the known universe, the other two being the matter-dominated era and the radiation-dominated era. The dark-energy-dominated era began after the matter-dominated era, i.e. when the Universe was about 5 billion years old. As other forms of the matter – dust and radiation – dropped to very low concentrations, the cosmological constant term started to dominate the energy density of the Universe.

For a dark-energy-dominated universe, the evolution of the scale factor in the Friedmann-Lemaitre-Robertson-Walker metric is easily obtained solving the Friedmann equations:

$a(t)\propto \exp(Ht)$

Here, the coefficient H in the exponential, the Hubble constant, is

$H = \sqrt{8\pi G \rho_{full} / 3} = \sqrt{\Lambda / 3}.$

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, the sign that was observed to be realized in Nature anyway. As the current density of the observable universe is of the order of 9.44 x 10⁻²⁷kg m⁻³, and the age of the universe is of the order of 13.7 billion years, or 4.32339 x 10¹⁷s, the present scale factor a(t) is ~2.699738. The Hubble parameter, H, is ~70.88 km s⁻¹Mpc⁻¹. (The Hubble time is 13.79 billion years.) The value of the cosmological constant, Λ, is ~2 x 10⁻³⁵s⁻².

See also

• Dark energy

http://map.gsfc.nasa.gov/news/