Time in cosmology

The notion of time in cosmology is revealed through an examination of trans ition matrix elements of radiative processes occurring in the cosmos. To be gin with, the very concept of time is delineated in classical physics in te rms of correlations between the succession of configurations which describe a process and a standard trajectory called the clock. The total is an isol ated system of fixed energy. This is relevant for cosmology in that the uni verse is an isolated system which we take to be homogeneous and isotropic. Furthermore, in virtue of the constraint which arises from reparametrizatio n invariance of time, it has zero total energy. Therefore the momentum of t he scale factor is determined from the energy of matter. In the quantum the ory this is exploited through the use of WKB approximation for the wave fun ction of the scale factor, justified for a large universe. The formalism th en gives rise to matrix elements describing matter processes. These are sho wn to take on the form of usual time dependent quantum amplitudes wherein t he temporal dependence is given by a background which is once more fixed by the total energy of matter.