Physical Basis for the Symmetries in the Friedmann-Robertson-Walker Metric

Understand

Modern cosmological theory is based on the Friedmann--Robertson--Walker (FRW) metric.

• Often written in terms of co-moving coordinates, this well-known solution to Einstein's equations owes its elegant and highly practical formulation to the cosmological principle and Weyl's postulate, upon which it is founded.
• However, there is physics behind such symmetries, and not all of it has yet been recognized.
• In this paper, we derive the FRW metric coefficients from the general form of the spherically symmetric line element and demonstrate that, because the co-moving frame also happens to be in free fall, the symmetries in FRW are valid only for a medium with zero active mass.