A theory has been presented that if a time traveller leaves his own time, then the universe would become unstable, and ultimately be destroyed. This just isn't possible. Why? well look here. Time itself is something that humans have made up. Seconds, minutes, and hours, even days and years, are all things that humans have made up. The universe therefore, has no concept of time. The wrong theory suggests that the universe needs to have a certain mass, and a certain amount of matter at all times, other wise it would be destroyed. It also describes the fact that if the traveller leaves his own time, the universe in his time will be destroyed because of the lack of matter. When the traveller arrives in the new time, there will be too much matter and the universe will also be destroyed. If the universe has no concept of time, then it cannot distinguish, how much matter is existing in a certain time or not. If it has no concept of time, then it will not be affected by a time traveller. The universe will only think that the traveller is only moving from place to place, not from time to time, because as i said before, the universe knows nothing of time. If the matter is only moving about, then it is never lost or gained, its still there just in a different place. I have never heard of anyone disproving the Time Travel Universe Destruction Theory, but this was the closest i could get in my first attempt.