Question: What theory proposes that parallel universes are inaccessible?
About the article "Quantum mechanics", section "Philosophical implications".

Candidate passages:
Parallel Universes is a 2001 documentary produced by the BBC's Horizon series.
The documentary has to do with parallel universes, string theory, M theory, supergravity, and other theoretical physics concepts.
Participants include Michio Kaku, Paul Steinhardt, and other physicists.
The program is about explanation of the Big Bang theory through the M theory and that there are many other parallel universes with different laws of physics.
It has been rated 4 star by BBC, History Channel and 4½ by Discovery Networks. 
The title refers to the fact that the story takes place in two parallel universes, a concept loosely based on multiverse theory. 
From 1977, Petit started to build an atypical cosmological model, nowadays called the bigravity theory but formerly known as the "twin universe theory", completed over the years.
This model proposes a radically different vision for the universe, in opposition with the standard cosmology, but shares similarities with a model published before by Andrei Sakharov.
In the bi-gravity theory, there is not only one universe, but two parallel universes with two conjugated Riemannian metrics having their own geodesics, interacting through gravitation.
Whereas Petit claims his theory explains various observational facts that the standard model cannot answer, and despite several publications through peer review, this model has not triggered much interest in the cosmological community throughout the years.
However, in August 2007 Petit incorporated an international club of high-level geometers who take interest in his model and validate his work from its mathematical ground. 
There could be "an ensemble of parallel universes" such that when the traveler kills the grandfather, the act took place in (or resulted in the creation of) a parallel universe where the traveler's counterpart never exists as a result.
However, his prior existence in the original universe is unaltered.
Succinctly, this explanation states that: if time travel is possible, then multiple versions of the future exist in parallel universes.
This theory would also apply if a person went back in time to shoot himself, because in the past he would be dead as in the future he would be alive and well.
Examples of parallel universes postulated in physics are: In quantum mechanics, the many-worlds interpretation suggests that every seemingly random quantum event with a non-zero probability actually occurs in all possible ways in different "worlds", so that history is constantly branching into different alternatives.
The physicist David Deutsch has argued that if backwards time travel is possible, it should result in the traveler ending up in a different branch of history than the one he departed from.
See also quantum suicide and immortality.
M-theory is put forward as a hypothetical master theory that unifies the six superstring theories, although at present it is largely incomplete.
One possible consequence of ideas drawn from M-theory is that multiple universes in the form of 3-Dimensional membranes known as branes could exist side-by-side in a fourth large spatial dimension (which is distinct from the concept of time as a fourth dimension)—see Brane cosmology.
However, there is currently no argument from physics that there would be one brane for each physically possible version of history as in the many-worlds interpretation, nor is there any argument that time travel would take one to a different brane.  
From the back of the book:In this thrilling journey into the mysteries of our cosmos, bestselling author Michio Kaku takes us on a dizzying ride to explore black holes and time machines, multidimensional space and, most tantalizing of all, the possibility that parallel universes may lie alongside our own.
Kaku skillfully guides us through the latest innovations in string theory and its most recent iteration, M-theory, which posits that our universe may be just one in an endless multiverse, a singular bubble floating in a sea of infinite bubble universes.
If M-theory is proven correct, we may perhaps finally find an answer to the question, "What happened before the big bang?"
This is an exciting and unforgettable introduction to the cutting-edge theories of physics and cosmology from one of the preeminent voices in the field.