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Message: <blockquote data-quote="freyar" data-source="post: 6684203" data-attributes="member: 40227">Back before that sidetrack on nomenclature, we were talking about the multiverse and such...We can start with another nomenclature issue, I guess.  There are some people who use "multiverse" to refer to different types of particles living on different branes in different parts of the extra dimensions, but that doesn't fit the usual definition of multiverse because these different particles interact with each other gravitationally.  While you're also right that there can be cyclical models of cosmology, I'm not aware of one that really qualifies as a multiverse.  The typical definition of multiverse is a "large" universe where there are different (separated) regions with different physical laws (ie, different field/particle content, though basic principles of quantum mechanics still apply everywhere), large parts of which are out of (causal) contact with each other.In any case, the "string theory multiverse" I talked about before is of this eternal inflation type that fuindordm describes.  What he describes is a multiverse/universe where most of space keeps expanding exponentially fast with "pockets" of normal expansion, but all those pockets have the same physics, in particular the same vacuum energy.  If we want to explain the small value of the vacuum energy, though, we need pockets of normal expansion with different physics than each other.  In other words, the overarching theory needs (very very very) many states with different values of vacuum energy in each one.  You certainly don't need string theory to have this structure --- this is a complicated but reasonable model just to make up from scratch --- but any model you do make up should eventually come from some theory of quantum gravity.  The fact that string theory, which is a theory of quantum gravity (and everything else), apparently does give you just this structure of many states of varying vacuum energies is a strong motivation and is what led to the recent interest in these multiverse/anthropic models.There is one other small difference compared to what fuindordm describes.  In modern/stringy multiverse models, the transitions are not just from inflationary expansion to normal expansion, but mostly between regions with inflation at different rates.  Of course, there are additional technical details --- for example, in this type of model, our pocket of the universe is created as a bubble in the eternal inflation that then undergoes a more standard type of inflation that heats up.  But now, we see that the expansion of space in our region is starting to accelerate, so we might be entering another phase of inflation (just a very slow one).  So we also experienced a transition from inflation to inflation, just with some more complicated intermediate stage in between.</blockquote>

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