A Penrose diagram is a diagram (named for Roger Penrose)
that shows the global causal structure of a spacetime.
It is a spacetime diagram in which all light rays are shown as traveling
at 45° and in which, therefore, all particles with mass are shown as
traveling on trajectories that lie within the light
cone which sits at every point in the diagram. A Penrose diagram allows
even infinite spacetime to be represent finitely by means of an appropriate
change of coordinates.
| Penrose diagram of the universe