  In a general Lorentzian manifold M, the past lightcone of a point is a proper
subset of M that does not carry enough information to determine the rest of M.
That said, if M is a globally hyperbolic Cauchy development of vacuum initial
data on a Cauchy surface S and there is a point whose past lightcone contains
S, then the contents of such a lightcone determines all of M (up to isometry).
We show some results that describe what properties of M guarantee that past
lightcones do indeed determine all or at least significant portions of M. Null
lines and observer horizons, which are well known features of the de-Sitter
spacetime, play a prominent role.
