Friedmann, Robertson-Walker (FRW) Models in Cosmology

Friedmann, Robertson-Walker (FRW) models are established on the basis of the assumption that the universe is homogeneous and isotropic in all epochs. Even though the universe is clearly inhomogeneous at the local scales of stars and cluster of stars, it is generally argued that an overall homogeneity will be achieved only at a large enough scale of about 14 billion light years. According to the FRW models, the universe has an encompassing space-time singularity at a finite time in the past. This curvature singularity is called the big bang. FRW singularity must be interpreted as the catastrophic event from which the entire universe emerged, where all the known laws of physics and mathematics breakdown in such a way that we cannot know what was happened during and before the big bang singularity. In these models the three-space is flat and are of positive and negative constant curvature; which incorporate the closed and open FRW models respectively. In this paper an attempt has been made to describe the FRW models with easier mathematical calculations, physical interpretations and diagrams where necessary.