When I was a bit younger, I used to think about time travel.
First of all, you cannot create or destroy matter. You need energy to do this, and energy is created by the destruction of matter. So if you move any object from the future into the past, there can only be one instance of that object. For that object to actually move into the past, and exist beside it's former self, you will need to generate enough energy to fill the void it leaves. Understand?
Again. The sum of matter and energy in the universe are constant. If energy goes up, matter goes down. If matter reduces, energy increases. Time is just the movement. If there was absolutely no movement, there would be no time.
Now imagine the entire universe consisted of an equal amount of energy and matter. 1 grain of matter, and so much energy, that it could codense (you know what I mean), into the same grain of matter.
This grain cannnot move, because there is nothing to measure its movement against. So there is no time for this grain. So let us introduce a second grain. The 2nd grain is our fixed position. The 1st grain can move relative to the second. Immediately you bring in the 2nd grain, you have time. You have change in position, and this is time.
So to build a time machine in our universe, you have got to undo every movement in the universe. This will make time go backwards, but your timemachine will also become "uninvented", and your time travel will stop.
The obvious solution is to keep your time machine from being affected by the movement. So you shield it. So in effect, you make all particles do the exact thing that they did, but in a reverse order, yet keep the particles that make up your time machine from doing the same thing.
You see what will happen. You will create a matter void, and matter will immediately collapse into the space you are forcing empty.
By moving back in time, you will change time, and you cannot stop this.
Parallel universes are nonsense. They aren't proven, and there is not even strong theoritical evidence supporting their existence. What will really happen is that you can kill your grandfather without problems. You can move back in time, but it is actually a move forward in time. The universe will perceive it as being a move back, but if you isolate your time machine, then your time machine becomes your point of reference. Everything is relative. So, time, according to your time machine looks this way - your grandfather is born, your father is born, you are born, you build timemachine, you change the universe so that it resembles a previous time, in the resemblance, you kill your grandfather, you modify it again, so that life continues as normal, and you arrive at normal time. What do you see? There is a new you, there is no time machine.
If we use your grandfather as reference, it looks this way: Grandpapa gets born, grandfather is killed by someone who comes in a timemachine. End.