I've heard that gravitational waves travel at the speed of light, and have some parallels to electromagnetic waves.
EM waves slow down as they pass through matter (speed of light in glass is slower than in vacuum, for instance). Do gravitational waves also slow down as they pass through matter?
If so, are there any effects like Cherenkov radiation when matter passes through a medium at a speed greater than the velocity of gravity in that medium?
Do large masses like stars or Jupiter act as ball lenses for gravitational waves?