The passage of time is a real, physical dimension just like height, width and depth. Any three of those four dimensions can 'expand' into the fourth. The cone shaped cross section of a hypersphere is usually shown as a 3D representation of an event horizon boundary. This is not exactly the scenario the universe seems to have taken, but rather it is an approximation. Four spatial dimensions are not observed, three dimensions and the passage of time are observed.
Actual observations carry a lot of weight when testing a hypothesis.
For all its similarity to a two dimensional object, the surface of a ball IS three dimensional with a topology very different from a true 2D connected space. There are two separate and distinct segments formed in the higher dimensional space, the inside and the outside. Since this corresponds well with our concepts, the past and the future, time is a likely candidate for that unseen fourth dimension. Assumed that Chris was talking about a 4 spatial dimension hypersphere expanding wrt time. I'm sorry to have jargonized our conversation with the undefined term, hypersphere, and I was unaware of Sagan's alternate definition.