# Modern Physics: Friedmann equation

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Homework Statement:
Consider a flat expanding universe with no cosmological constant and no curvature (k=0 in the Friedmann equation). Show that if the Universe is made of "dust", so the energy density scales like 1/R^3 , then the scale factor, R(t), grows as t^(2/3). Show if it is made of radiation (so the energy density scales as 1/R^4 -- the extra factor of R comes from the redshift), then it grows as t^(1/2). In both cases, show that for early times, the scale factor grows faster than light. Is this a problem?
Relevant Equations: