TITLE:"Newtonian cosmology and Friedmann's equation." AUTHOR: A R Thatcher CITE: European Journal of Physics 3.4 (1982): 202. Cosmology is the study of the universe, from it's birth, very early times, evolution up until now and perhaps even its future and/or demise. The fundamental theory that allow us to study the universe is the Einstein's Theory of General Relativity (GR). GR not only give us a new perspective of gravity, but also allows one to study the dynamics of the universe. A. Friedmann in 1922, proposed a solution to Einstein's Field Equation (now called Friedamnn equation), that predicted an expanding universe and this was verified later by Vesto Slipher and E. Hubble. Although he derived the equation from GR, in 1930's Milne and McCrea and McCrea derived this equation from pure Newtonian Mechanics. In this paper, the author describes the Newtonian derivation of the Friedmann equation using the method of hydrodynamics. The galaxies in this model is considered to be discrete particles of fluids, that do not interact with each other. The evolution of the universe depends on the density of matter in the universe. If the universe has finite amount of matter, then the universe will contract, or if the universe has an infinite amount of matter, then the universe will expand indefinitely. In this paper, the author considers only finite amount of matter in both cases. The final result of this work was that the universe should either expand or contract, an exact equation to GR but different in interpretation.
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