The Second Law of Thermodynamics sayd that entropy will increase. Just to be clear, entropy is the disorder in the system. So the more entropy something has, the more disorganized it is. Yes, things have a tendency to decay. Large atoms will break apart into multiple smaller particles.
More particles mean more entropy because they can rearrange themselves in more ways.
As for light, light is a different story. Light does have an absolute speed. The problem is that slowing down actually decreases entropy. Light does not slow down. Other things can slow down because their energy goes elsewhere to increase entropy elsewhere, but light doesn't have to worry about this.