Let's assume that the our universe's physics allows time travel. This is my main assumption!
Let's further assume that there is an quite intelligent, invulnerable being, that found a way to achieve this and is able to consume any kind of energy source. It could change everything in our universe; no matter, what and when.
This being could behave like a god as long as our universe exists or at least as long as there is a kind of energy source for them to consume.
Doesn't this, in the contrary, mean that if we could once prove, that time travel is not possible in our universe, that, if there is anything that behaves like a god, then it is really a god (or a god-like thing) outside our universe?
Assuming the possibility of Faster-than-light communication is, by Einstein's theory of relativity, equivalent to time travel. So we just need: "Warp speed, Mr. Zulu..."