Universal Encyclopedia

Suppose you time travel in the past and kill your Grand Father,  Then your father wouldn't have been born. So you wouldn't have been born. So you wouldn't have been to time travel in past. So your Grandfather would not die and so your father. So you are born and can time travel. This continues for infinity

The Ekpyrotic Universe Theory suggests that, unlike the Big Bang that began from singularity, our universe is one of a pair of universes that collided.  It is an alternative to the widely accepted Big Bang theory . This collision resets the universe. From there, it expands for billions of light year until it contracts back to the Big Crunch . The speed and energy of that reduction creates another massive collision and the universe is reborn. and this continues for infinity. And the Mind-Blowing Fact is that if the ekpyrotic scenario is correct, our twin universe is right next to us in another dimension, separated by a distance less than the diameter of an atom. That’s close, even for siblings! 

Multiply their masses. Then, divide it by the square of the distance between them.