Cosmology

Wheeler and Feynman tried to work out a consistent scheme for the interactions of particles.... A particle shakes; its influence spreads outward like waves from a stone thrown in a pond. To make their theory symmetrical, they also had to use inward-traveling waves--implying action backward in time. They found that they could avoid unpleasant paradoxes because these normal and time-reversed waves ("retarded" and "advanced") canceled each other out--but only if the universe was arranged so as to guarantee that all radiation would be absorbed somewhere, sometime.
		p. 113
James Gleick	Genius: The Life and Science of Richard Feynman Vintage Books, 1993
Gleick is summarizing Wheeler-Feynman absorber theory. The "guarantee" he mentions is good if the universe is closed.