Past exam of the mathematics course of the University of Cambridge / 2022 / ii / Paper 1 / 38D / b / Solution

For an FLRW spacetime,With $a=t$ and $k=-1$, both terms vanish, soIndeed the vacuum equations give a vanishing Ricci tensor, and conformal flatness of an FLRW metric then makes the whole Riemann tensor vanish. The apparent singularity at $t=0$ is therefore only a coordinate singularity.