Massdominated Expansion of the UniverseIn the current era, mass is dominant over radiation as a determiner of the expansion of the universe, whereas early times in the big bang were radiationdominated. Some insight into the expansion can be obtained from a Newtonian expansion model which leads to a simplified version of the Friedman equation. Only recently have we been dealing with the fact that "dark energy" may be the dominant factor in the present expansion of the universe, but the examination of the massdominated expansion is important in providing insight about the current universe.
The simplified expansion equation is expressed in terms of the dimensionless scale factor R for the universe. In the case where ρ = ρ_{c}, the critical density, then the curvature parameter k = 0. Since the expanding shell always contains the same amount of mass, we can write and putting the equation in a form where it can be integrated and expressed in terms of the Hubble time t_{H} Then for the present time R=R_{0}=1 the expansion time is and the scale factor R is given by

Index References Carroll & Ostlie Ch. 29  

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