Why would inflation slow down because of gravity?

[guss]

This is a basic question. If the expansion of the universe's radius is faster than c, and that speed is increasing or staying constant, how would gravity ever slow the expansion down? Gravitational fields propagate at c, so why would anyone even wonder why it's not slowing down due to gravity (if they thinks gravity effects the "edge" of the universe)?

 

[Drakkith]

The inflation is only faster than c when you compare parts of the universe that are billions of light years away from each other. Closer than that and the inflation isn't faster than c for the local area. And at those vast distances the gravity is extremely weak anyways.

 

[Chalnoth]

This is a basic question. If the expansion of the universe's radius is faster than c, and that speed is increasing or staying constant, how would gravity ever slow the expansion down?

Expansion isn't a speed, it's a rate. So expansion cannot be faster (or slower) than the speed of light.

Furthermore, the speed of light is only a local phenomenon. There simply isn't any well-defined way to talk about speeds of far-away objects in General Relativity.

Gravitational fields propagate at c, so why would anyone even wonder why it's not slowing down due to gravity (if they thinks gravity effects the "edge" of the universe)?

Well, if you fill space with a uniform distribution of normal matter, and see what happens given General Relativity (which does fully take into account the finite speed of gravity), you get a universe that necessarily slows down its expansion. If you add radiation, you get a universe whose expansion slows down even more rapidly. To get a universe that speeds up, you need some sort of exotic energy density filling space.

 

[guss]

Thanks guys.

To get a universe that speeds up, you need some sort of exotic energy density filling space.

...which is what our universe is doing, so this is why there must be "dark" energy, correct?

 

[Chalnoth]

Thanks guys.

...which is what our universe is doing, so this is why there must be "dark" energy, correct?

Correct. That or some form of modified gravity. But the theorists are having a very difficult time finding modified gravity theories that both explain the accelerated expansion and also don't clearly contradict solar system experiments.

Dark energy is easy, though: just propose a cosmological constant. It fits all observations, and is only one single parameter in the theory. Plus it's a parameter that is in the theory regardless, because we know of no way for it to be set to zero, so there actually isn't any added complexity to assuming the acceleration is caused by a cosmological constant.

There also turn out to be a number of models from quantum mechanics that mimic a cosmological constant, but they tend to be rather ad-hoc.