Rate of Expansion of The Universe
Plugging this information to the Friedmann-Lemaître-Robertson-Walker equations of motion and neglecting both the cosmological constant and the curvatue parameter, which is justified for the early universe, one gets the following equation:
is the energy density, and one finds the following behavior:
- In a radiation-dominated universe:
- In a matter-dominated universe:
One can further show that the universe was radiation-dominated as long as the energy density was of the order of 10 eV to the fourth, or higher. Since the energy density keeps going down, this was no longer true when the universe was 70,000 years old, when it became matter dominant.
In the universe today, matter is mainly in forms of galaxies and dark matter, while the radiation is the cosmic microwave background radiation, the cosmic neutrino background (if the neutrino rest mass is high enough then the latter is formally matter), and finally, mostly in the form of dark energy.
Read more about this topic: Thermodynamics Of The Universe
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—Max Lerner (b. 1902)
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thy pow’r thro’out the universe displayed.
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—Stuart K. Hine (b. 1899)