Id just first like to say that Im only a third year physics major so I might be wrong, misinformed, mistaken, or any combination of the three about any of this. I dont understand why people are trying to unify the four fundamental forces. electromagnetic, weak, and strong have already been shown to be the same force, but from what Ive read gravity hasnt been. Isnt it possible that gravity is just a different type of force than the quantum forces? For example, in my head at least, gravity can be explained by an accelerating universe and inertia. Say the universe is a hypersphere expanding at an accelerating rate. Anything with mass has inertia, and therefore accelerates slower than the surface of the hypersphere. It would be like accelerating a rubber sheet in space with a marble on it. The marble gradually creates a larger and larger "dent" in the sheet. Anything with mass would cause a curvature in spacetime, because of its inertia. The other effect of an accelerating universe would be to cause an inward force on all massive objects. This would explain why massive objects attract each other, because they are both forced inward on a curve. If this "theory" were the case, gravity would simply be different from the other 3 forces. There would be no particle exchange, and it would just be a fundamentally different force.