"...the best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."
-- Stephen Hawking, Travis, 1992, p. 180.
Time Travel Paradoxes and 'Multiverse' theories.
Thinking about time travel confuses people. Traveling back and changing the past, seems to be impossible and to lead unavoidably to a paradox. The apparent existence of such paradoxes has triggered the arrival of many new theories like multiple world and multiple universe (Multiverse) theories, that avoids such paradoxes. Most of the confusion is related to physical time travel theories which would allow travelers to 'really' go back to the past, having more copies of themselves, or even killing their ancestors.
One of the basic objections against time travel is, that it is against our intuition that time is a single straight line without break or branching point and against our common-sense belief that a thing is occupying a single place at a moment in time and it cannot be in two places at the same time. Thinking about physical time travel almost unavoidable leads to the dreaded "Grandfather Paradox":
You go back in time and kill your grandfather before your parents were born. Thus, if your parents are never born, you could never be born. But if you were never born, you couldn't have killed your grandfather. But if you hadn't killed your grandfather, you would've been born. And so on.
Bottom line, however, is that paradoxes do not exist: they are the product of our mind. In our case, it might be the consequence of trying to apply time travel to the wrong theory on time. Our current theory does not allow for time travel. If we however insist using the classic model and imagine the consequences of traveling to the past, paradoxes will immediately pop up.
The multiverse theory presumes that every change in the past, creates a new universe and is one of the theories that evades the obvious time travel paradox. However since paradoxes do not exist they do not need not to be avoided.
A multiple universe theory, allowing travelers to alter the past, without risk of endangering their own history time line, would be an open invitation for an invasion of 'experts from the future' who know it all better, without them having to bear the consequences of their own actions. There is no evidence in our own history that something similar ever happened.