When physicists are asked about “parallel worlds” or ideas along those lines, they have to be careful to distinguish among different interpretations of that idea. There is the “multiverse” of inflationary cosmology, the “many worlds” or “branches of the wave function” of quantum mechanics, and “parallel branes” of string theory. Increasingly, however, people are wondering whether the first two concepts might actually represent the same underlying idea. ...This is nonsense. Woit comments here. Complementarity is just a buzzword to ignore the obvious contradictions.
There are two ideas that fit together to make this crazy-sounding proposal into something sensible. The first is quantum vacuum decay.
When particle physicists say “vacuum,” they don’t mean “empty space,” they mean “a state of a theory that has the lowest energy of all similar-looking states.” ...
Keep that in mind, and now let’s introduce the second key ingredient: horizon complementarity.
The idea of horizon complementarity is a generalization of the idea of black hole complementarity, which in turn is a play on the idea of quantum complementarity. (Confused yet?) Complementarity was introduced by Niels Bohr, as a way of basically saying “you can think of an electron as a particle, or as a wave, but not as both at the same time.” That is, there are different but equally valid ways of describing something, but ways that you can’t invoke simultaneously.
For black holes, complementarity was taken to roughly mean “you can talk about what’s going on inside the black hole, or outside, but not both at the same time.” It is a way of escaping the paradox of information loss as black holes evaporate.
Carroll wrote a 2005 essay on Why (Almost All) Cosmologists are Atheists. I expected arguments against the existence of God, or examples of how religious thinking has misled scientists in the past. Instead he gives this:
The basic scientific assumption is that there is exists a complete and coherent description of how the world works. ...He follows this with explanations for dark matter, and why one argument is preferred over another.
In particular, how do we go about deciding whether a theory is more or less likely to be consistent with a single coherent description of nature? It is at this point that the judgment of the individual scientist necessarily plays a crucial role; the process is irreducibly non-algorithmic.
I don't see where Carroll makes any scientific arguments. He may turn out to be right about dark matter, but as a scientific question, we don't know.
Update: NewScientist adds to this nonsense with When the multiverse and many-worlds collide:
The problem is the observability of our universe. While most of us simply take it for granted that we should be able to observe our universe, it is a different story for cosmologists. When they apply quantum mechanics - which successfully describes the behaviour of very small objects like atoms - to the entire cosmos, the equations imply that it must exist in many different states simultaneously, a phenomenon called a superposition. Yet that is clearly not what we observe.It has an accompanying editorial, God deserves a cosmological explanation, says that "cosmologists claim to have found a way to rid themselves of the need for a God-like observer." Peter Woit criticizes it here.