We have always heard of time travel one thing we never pay attention to is that the future is fixed, only this thing can make the time travel possible, now let me explain u this in a better way with an example, suppose of a star which is ten light years away from us the light travels from this star to us and we see what the star was ten years ago. now if we reach the star in ten minutes then we will see its future , it means for us the future of star is fixed as we can go to the past of the star and the future as well this must be the case with all the planets and stars meaning the future is fixed.

The Alternate Universe time travel explains a lot. This type of time travel depends upon the existence of alternate universe which are more or less similar to our universe but lagging in time i.e. representing a previous time. If one changes past, in that universe, nothing will happen to our universe and nothing much will be different for that universe either because that universe will evolve taking that change for granted so there will be no grand father paradoxes neither entropy or causality problems

Those systems in which a slight change in the initial condition can create great changes in the final condition are called chaotic systems, they are unpredictable. Similar is the case with time line, a slight change in the past conditions can create great changes in present condition. You can find tons of examples in cartoons and movies of his sort like, if someone kills Adolph Hitler what will happen?
In this way it would be chaotic or it won’t be linear.