Cosmology: a measure for the multiverse

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The multiverse has developed rapidly from a being merely a speculative idea to a theory verging on respectability. There are good reasons why. Several strands of theoretical physics - quantum mechanics, string theory and cosmic inflation - seem to converge on the idea that our universe is only one among an infinite and ever-growing assemblage of disconnected bubble universes.
Raphael Bousso of the University of California, Berkeley, has also been grappling with the multiverse, and in the past few months he has found a way round the troubling problem of unobservable universes. At a stroke, he has transformed the multiverse from a theory so problematical that it threatens to subvert science, into one that promises predictions we can test. His insights are steering physicists along the path to their ultimate goal of uniting quantum mechanics and gravity into one neat theory of everything.

This stuff really interests me. If multiple universes exist, which I believe they do, then we aren't 'alone'. Also, I'd find it rather comforting that somewhere out there there's another Kip who didn't make some of the same odd decisions I did.

We don't need multiverses for not being alones, only one infinite universe is more that enough. I think that rather that multiverses you must refer to paralel universe; Personally I don't like paralel universes theory...

I think universe or infinite universe or multiverse are much more likely than parallel universes (that are exactly the same as ours)...

and if there really are parallel universes, who says the other kip(s) in the other universe(s) doesn't/don't make the same decisions kip makes?

if we create universes every time we make a decision, then are we all gods?...

I just came across this science news article about the multiverse - a theory saying that our universe is just one among an infinite number of universes:

NewScientist - A measure for the multiverse

(See the link above for the rest of the text).


Interesting stuff, I think.

"That was really stunning," says Bousso. "It was amazing to me when I realised that the two measures reproduce the exact same probabilities."

Their equivalence turns out to be extremely useful, as weaknesses in one measure are strengths in the other, and vice versa. "They are like two people on crutches holding one another up," Bousso says.

So while in the causal patch measure your answers depend strongly on the universe in which your observers start out, the global measure does not suffer from this ambiguity. In the multiverse, bubbles beget bubbles beget bubbles, so that initial conditions are quickly lost in the crowd and no longer matter when it comes to calculating probabilities. In fact the global picture actually defines what the starting vacuum for the causal patch approach should be.

On the other hand, while the global picture suffers from the problem of "duplicate information" (see "What black holes can teach us"), Bousso's causal patch measure successfully circumvents this.

The implications might be immense. The two equivalent measures have not only provided a prediction for dark energy in our own universe that closely matches observations, they were both inspired in different ways by the holographic principle. This suggests that the holographic principle is profoundly significant, and could lead us to a theory of quantum gravity - the long-sought theory of everything that mirrors the dynamics of the multiverse. "By thinking about the measure problem, we seem to be learning, perhaps unexpectedly, about another, equally deep mystery, namely how to formulate the quantum gravity theory of the multiverse," says Bousso.

Even Ellis is impressed by Bousso's results, if not exactly sold on the multiverse. "It is a useful and intriguing kind of consistency test based in fascinating but speculative physics," he says. And there is another far-reaching consequence. If Bousso's equivalence holds, then not only can the resulting measure be used to make real, testable predictions, they can also make calculations in the multiverse without ever referring to unobservable universes lurking beyond our cosmic horizon. Everything we need to know about the multiverse might be right here in our own universe.

Tegmark's sole postulate is: All structures that exist mathematically also exist physically. This is in the sense that "in those complex enough to contain self-aware substructures (SASs), these SASs will subjectively perceive themselves as existing in a physically 'real' world". [2] The MUH can be considered a physico-mathematical expression of the philosophy known as modal realism, which treats physical reality as indexical, or self-referent, rather than absolute. The MUH suggests that not only should worlds corresponding to different sets of initial conditions or to different physical constants be considered real, but also worlds ruled by altogether different equations.

Tegmark claims that the MUH has no free parameters and is not observationally ruled out, and is therefore to be preferred over all other TOE's by Occam's Razor. He envisages conscious experience as taking the form of "self-aware substructures" of mathematical structures, which will subjectively perceive themselves as existing in a physically "real" world.

The MUH is related to the anthropic principle, to theories hypothesizing a multiverse, and to Jürgen Schmidhuber's ultimate ensemble of all computable universes