In "A Brief History of Time" Hawking questions why the expansion of the universe is accelerating. I must be missing something. If the universe is expanding, its volume is constantly increasing. If its volume is increasing its density is decreasing, so there is more space between bodies/particles and less gravitational force. The farther these particles get from each other, the less resistance there will be on the expansion of the universe...therefore causing it to expand faster and faster. What would have to happen to cause a Big Crunch? Would the rate of the creation of new particles have to be greater than the rate at which the universe was expanding? Help me Out.