Only the fact that we have only ever seen it expand. I'm not saying that in the smug "duh" sense. If we found out tomorrow that it oscillates expanding and contracting in very slow cycles and just started contracting back to a finite point, I'd believe it. The behavior would be very wavelike and physics loves waves. 

 Yeah, that's why I asked. 
I just assume everything is wavy in some way, until proven otherwise. Endless positive feedback loops are unlikely.