Question: Do gravitational waves need a medium to travel. Do they slow down when passing through a medium? – Michael
Answer: Gravitational waves are best described as distortions in the fabric of space that are due to objects with mass. These distortions can be thought of as propagating like a wave, and thus also transporting energy in the form of gravitational radiation. This description of the wave transport of gravitational energy is very analogous to how light propagates and transports electromagnetic energy. I believe that gravitational waves travel at the speed of light independent of the medium that they are travelling through.