See article at Astrobites.
As I was saying all along:
1. Gravity bends light, which is why we get gravitational lensing of light.
2. Gravity waves travel at speed of light. This was measured twenty years ago, but it was a sensible assumption all along.
3. LIGO tells us that gravity waves and light waves from the same event arrive at the same time (give or take a fraction of a millisecond), therefore they must have taken the same path to get here.
4. If the light waves were bent before they got here, then so were the gravity waves.
5. Therefore, there must also be gravitational lensing of gravity, as shown in the second diagram down. Gravity gets focused along the plane of a spiral galaxy. There's less than you'd expect above and below the plane, and more than you would expect around the plane.
For calculation purposes, you can take the common sense approach and say that matter in a Galaxy distorts/creates gravity and shapes the gravitational field. Or the more esoteric view that a galaxy is a giant gravitational field with a certain shape and distribution, with stars trapped in it to balance it all out.
Debate Magazine
