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My understanding is that gravitational waves are only radiated when there's a dipole moment: two (or more, I guess) objects in rotation around each other. A black hole just sitting on its own doesn't radiate any gravitational waves at all. According to http://www.ast.cam.ac.uk/public/ask/2519 , the Earth and Moon should also be radiating gravitational waves (and therefore losing a little bit of energy) but this is surely dwarfed by other effects and not measurable.

Important pedantic point: There are no gravitational dipoles, as there is no repulsive gravitational interaction.

A dumbbell-shaped mass is the sum of a gravitational monopole and a gravitational quadrupole. That gravitational radiation originates only from quadrupolar sources (which are far less-efficient radiators than an equal-sized dipole) is one reason that gravitational waves are hard to detect.

But in a nonstatic, expanding universe any two objects, no matter how far apart, are gravitationally related and have dipole moment. Isn't it true?

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