ABSTRACT
This chapter sets out various interesting physical consequences of the Friedmann–Robertson–Walker (FRW) metric and the importance of the comoving system that should be regarded as a unique privileged system to describe the behaviour of matter and radiation. It discusses how cosmological redshifts are related to the cosmological expansion. The chapter defines the proper distance of an astronomical object in the expanding universe and how this relates to the luminosity distance. It derives Edwin Hubble's law on pure theoretical grounds. In particular velocities and momenta of massive objects can be unambiguously calculated, from a cosmological point of view, only in the comoving system. In general relativity the dynamics of the gravitational field, described by the metric, is described by A. Einstein's equations. The chapter concludes how A. Friedmann's equation and the fluid equation, that we could only partly justify within a Issac Newtonian approach, can be obtained in a rigorous way from Einstein's equations starting from the Friedmann–Robertson–Walker (FRW) metric.
