Years ago, my brother and I had an argument where I was trying to convince him that nothing could travel faster than the speed of light. I was pursuing this in the context of Special Relativity. My main thought was to contrast the behaviour of light to the behaviour of baseballs thrown from train-cars, say. To me, this is an interesting and surprising feature, and I thought he'd be interested.
Instead, he turned the argument into "you don't really know that nothing can go faster than light". Over the years, we've gone back and forth where he'll claim that some new result proves that I was wrong. I've mostly been dismissive, since the cites are mostly pop-sci or will say something like there might be some 1 part in 10^17 discrepancy, or something.
So, recently, I sent him the Wikipedia article on Metric Expansion, and he shot back with "see, matter is traveling faster than the speed of light."
Is this an accurate statement, or does it hinge on interpreting "traveling"? I.e. one can make a distinction between traveling through space, and space itself expanding.
Was I wrong to say (again, according to GR not SR) "nothing can travel faster than the speed of light" (ignoring tiny fluctuations, and according to currently known physics)?