# Is there evidence for the speed of gravity? [duplicate]

Is it an assumption that the speed of propagation of gravitational waves is exactly the same as the speed of light? If the speed of gravity is only slightly less than the speed of light, the effect might not show up over solar or intra galactic distances, but over extragalactic distances could it give the appearance that there is dark matter influencing motion?

• @Hohmannfan Indeed, his answer is over there – Sir Cumference Sep 16 '16 at 2:13

The fact that gravitational waves propagate at the speed of light is not an assumption, but is derived from the assumption that nothing travels faster than light. In General Relativity, you build a metric, which is a tool to be able to measure and locate events in space and time. Building this metric puts constrains on the speed of information, and a basic assumption is that this speed is at most $c$ and is equal to $c$ if the information carrier is mass-less (photons for example).
Gravity waves are small non-static perturbation of this metric in the void. So they are propagating information without any mass and travel at $c$.