P.123 - §3 We do not know the actual mechanism of space respiration; we merely observe that all space alternately contracts and expands. This respiration affects both the horizontal extension of pervaded space and the vertical extensions of unpervaded space which exist in the vast space reservoirs above and below Paradise. In attempting to imagine the volume outlines of these space reservoirs, you might think of an hourglass.
P.124 - §0 next they contract. Pervaded space is now approaching the mid-point of the expanding phase, while unpervaded space nears the mid-point of the contracting phase, and we are informed that the outermost limits of both space extensions are, theoretically, now approximately equidistant from Paradise. The unpervaded-space reservoirs now extend vertically above upper Paradise and below nether Paradise just as far as the pervaded space of the universe extends horizontally outward from peripheral Paradise to and even beyond the fourth outer space level.
P.124 - §1 For a billion years of Urantia time the space reservoirs contract while the master universe and the force activities of all horizontal space expand. It thus requires a little over two billion Urantia years to complete the entire expansion-contraction cycle.