Making The Multiverse Case As Simple As Possible.

I've tried to make the case for the multiverse in the past, but let's see if I can do a better job.  First a crude definition:

When I mean multiverse, I mean: in addition to our own observable universe, there are several other casually disconnected regions that are each "separate" universes.  (Separate from ours today in a casual sense.)

Here are my arguments:

1.  Inflation:  Inflation is a period of time when the comoving Hubble radius shrank.  This happens when the universe undergoes an accelerated expansion.  That's it.  I have made no reference to scalar fields, branes or anything else people try to pretend the definition of inflation is.

Now, assume this happened. (As the data seems to show really well.)  Look at the top figure.  We've discussed this before.  The universe starts as some patch, then expands such that the Hubble radius shrinks as shown.

This also means that regions that used to be casually connected to us no longer are as they have been "pushed out".  Furthermore, we have no reason to believe, after inflation, that our new casually connected region is the only one that continues to expand with galaxy formation etc... after inflation ends.

This begs the questions:

1. How many casually disconnected regions were created by inflation? (Given a 60+ e-fold expansion, way too many to count.)
2. Are we sure that the only thing that existed before inflation was this one casually connected patch that inflated?  How in the world can you justify that it is all that existed without giving some ad hoc argument?  If our patch existed, by a Copernican Principle type argument, we should expect our initial patch is not some special "only one".
3. If inflation started from some quantum process on our initial patch, as many expect, what would prevent inflation from happening over and over again now that there are many more patches that could potentially inflate? (Quantum processes like radioactive decay happen again and again with a certain probability)

Anyways, given all this, we should expect inflation generically produces a very, very large multiverse which means a structure with many, many casually disconnected separate universes.  Each of these should be like our own with galaxy formation, stars, planets, (life?), etc...  

If you want to know what kind of structure we live in, you need to take what we see today and multiply it again and again.

2.  The CMB itself:

This isn't a separate argument as much as some data related to the above claims.

Well, it turns out that every ~1 square degree piece of our sky of the CMB represents a casually disconnected region.  Look at the middle images.  The orange circles represents casually disconnected regions. (Not drawn to scale)  This means we are receiving CMB photons from ~10^5 casually disconnected regions.

So today, somewhere on the order of 10^5 casually disconnected regions are coming into casual contact for the first time.  Inflation aside, what would lead one to believe these are the only casually disconnected regions?   A billion years ago these regions were not in casual contact.  How many other regions are still expanding and evolving independently without being in casual contact?  How many will never be in casual contact even as time goes to infinity?

Summary:

Just remember, a 60+ e-fold expansion (inflation) generically creates many many casually disconnected regions.  The overwhelmingly vast majority of which have been evolving as "separate universes".  This alone creates a multiverse like structure.  Furthermore, within the CMB itself we are observing regions that, until today, have existed without being in casual contact. (After inflation)  How many "separate universe" type regions exist today?

This means: think of everything you know and love about our observable universe.  Now you need to copy that an enumerable number of times if you want to actually imagine what kind of structure we live in.

Furthermore, when you realize that if inflation happens once, you probably should consider that nothing should stop it from happening again. (Or that it isn't still going on in some regions as is predicted.) This would eventually create even more innumerable separate regions of a "multiverse".  Such a scenario can best be pictured with the bottom figure.