Matter-dominated era

The matter-dominated era was the epoch in the evolution of the Universe that began when the radiation-dominated era ended, when the Universe was about 47,000 years old.^[1] Although it was often said that we still live in the matter-dominated era, it is more correct to say that when the Universe was about 9.8 billion years old,^[2] this era ended and was replaced by the dark-energy-dominated era. This change of the perspective occurred when the cosmological constant or dark energy was first observed in the late 1990s.

In the matter-dominated era, the energy density of matter exceeds the energy density of radiation in the universe (and it should exceed the vacuum energy density i.e. dark energy, too).^[3]

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

$a(t)\propto t^{2/3}$

References

1. Ryden, Barbara, "Introduction to Cosmology", 2006, eqn. 6.41
2. Ryden, Barbara, "Introduction to Cosmology", 2006, eqn. 6.33
3. Zelik, M and Gregory, S: "Introductory Astronomy & Astrophysics", page 497. Thompson Learning, Inc. 1998