Just read some of the topics and can see the problems of paradoxes etc. But one thing bothers me about time travel, and especially about going back in time.

What I want to know is this:- if I were able to travel back in time and stepped out of my time machine / worm hole or what ever, what happens when all the atoms that make up me suddenly come face to face with a universe that already contains them? Do I then make the mass of the universe increase by me? Will kids be unable to answer that exam question, Avagadro lives on in all of us - explain?