 In those FRW spacetimes that can reasonably be thought to model our cosmos (e.g., those with massive objects), the time parameter t has an absolute lower bound t0. In particular, as t decreases towards t0, the scale parameter S(t) goes to zero. What happens when t reaches t0? In short, these models cannot say what happens, because there are no points of spacetime with time coordinate t0. That is, t0 is an ideal point that is never reached: the universe exists at all times after t0, but not before or at time t0. A spacetime model with this feature is called singular, and the ideal point that is never reached is called a singularity. In other words, the big bang is a singularity in a FRW spacetime.