On the one hand, gravity waves travel at c, but on the other hand, spatial expansion has no speed limit; ie, it can happen at FTL speeds. At first one might think, "So what? Gravity waves are gravity waves and spatial expansion is spatial expansion." But what is a gravity wave other than the cyclical expanding and contracting of a region of space for some duration? Seems to me like they shouldn't have a speed limit either. But if they can't propagate faster than c, then doesn't that mean Alcubierre drives are impossible? Since a vessel using an Alcubierre drive is really just a surfboard riding a gravity wave of it's own design and creation?