An absolute horizon is only defined in an asymptotically flat spacetime – a spacetime which approaches flat space as one moves far away from any massive bodies. Examples of asymptotically flat spacetimes include Schwarzschild and Kerr black holes. The FRW universe – which is believed to be a good model for our universe – is generally not asymptotically flat. Nonetheless, we can think of an isolated object in an FRW universe as being nearly an isolated object in an asymptotically flat universe.