[It's so sad to be an armchair cosmologist without a degree in physics but I'll not let that stop me]

If gravity is manifested in the wave that we're presently trying to find, it must travel [it is said that the Earth would still orbit for a period of time if, hypothetically, the Sun were to suddenly be removed]. If it travels, it must expend energy. If it expends energy, it must diminish in strength over time [the moon is increasing its orbit presumably on account of diminished gravity]. If it diminishes in strength, the arms of a spiral galaxy must be the stars there being "let loose" and beginning to travel in a straight line [Newton's law of inertia].

This diminution over time, could explain the expanding Universe. As you radiate more energy, you lose mass and, canonically, when you lose mass, gravity diminishes in strength.

caveats:

a. the moon may be expanding its orbit because of other orbiting bodies "pull" on the moon.
b. diminution in gravity may be just the loss of mass through radiation, but this would also explain the arms of a spiral galaxy but only if black holes also lost sufficient energy over time.