Variable Speed of Light
Cosmic constants are supposed to be constant -- it's what our math is based on. The speed of light (C) is a constant. Until it isn't. We've had experiments and observations where the speed of light just isn't quite as constant as it should be (and the same for some other radio waves). But that kind of blows Einstein theories up, but also just about everything from the size of the universe, timing of the Big Bang, and so on.
Einstein did propose the idea as well, and a few have offered equations for the variability, but he never formalized/released it as part of his other theories.