ABSTRACT
The dynamics of the expanding Universe only appeared implicitly in the time dependence of the scale factor R(t). This chapter explains the Friedmann equation which may be integrated to give the age of the Universe in terms of present cosmological parameters. One of the routine duties of an early-Universe cosmologist is to compute the present mass density contributed by some massive stable particle species hypothesized by a particle physicist down the hall, and to determine whether such a particle is at odds with the standard cosmology. Today the radiation, or relativistic particles, in the Universe is comprised of the 2.75 K microwave photons, and the 3 cosmic seas of 1.96 K relic neutrinos. Because the early Universe was to a good approximation in thermal equilibrium, there should have been other relativistic particles present, with comparable abundances. The entropy in a comoving volume provides a very useful fiducial quantity during the expansion of the Universe.
