Causal contact

Every object of mass in space, for instance, exerts a field force on all other objects of mass, according to Newton's law of universal gravitation.

The only objects not in causal contact are those for which there is no event in the history of the universe that could have sent a beam of light to both.

Depending on the expansion history of the universe, there may be a time such that there will be no particle horizons: all matter in the universe will be in causal contact.

[1] A good illustration of this principle is the light cone, which is constructed as follows.

, whilst those events that can send a light pulse to

, the light cone classifies all events in spacetime into 5 distinct categories: See the causal structure of Minkowski spacetime for a more detailed discussion.