What is exactly is the underlying rationale of multiverse theory?

Theory is simple.

There is a timeline where you didn't fuck up and you aren't miserable.

The issues are many...

Are we on a fixed timeline?
Are we capable if changing lines?
Are we changing lines all the time?
Do we move on the time line individually (consciousness) or as a whole (universe)?
//related to free will and determinism

Is it the cat in the box "dead and alive", or "dead or alive"?

Just here to say that I saw a Rhino in OPs gif. I thought it looked cool, and after staring at it for a while did I realize its a girl.

>There is a timeline where you didn't fuck up and you aren't miserable.

Impossible. Causality ensures things happen the way they have happened, are happening and will happen. Let's have two universes. One is A and the other is B. They both have the EXACT same variables. If A happens one way, B isn't going to go in some nondeterministic manner and be this completely new and wonderful universe, it's going to be the same as A.

I thought it was a fingerprint.

Basically god-hating atheist "scientists" hate god and needed to invent the multiverse to escape the conclusion that the universe we see was specially designed for us by an intelligent creator.

>what is quantum uncertainty

Casualty doesn't work in the quantum level

And the quantum level doesn't factor in how we on a macro level make our decisions.

Quantum Physicists are retarded. QM is neccessarily probabilistic because observing a system also changes it, and we can't account for what changes we make. We use probability as a workaround for this simple fact. However, we now have people taking this to mean that the universe itself is probabilistic. One consequence of this viewpoint is that neither possibility is more real than the other until it is measured to be one or the other. Others argue that each possibility exists in its own universe so that the wavefunction collapses both ways.
Multiverse theory, as far as its relationship to QM, which is the current popsci obsession, is a misinterpretation of a misinterpretation, and only IFLS brainlets are dumb enough to take it seriously.

Multiverse is a hypothesis of certain group of string theorists, which comes from their perspective of how it has been evolving and where it's headed, and they don't seem to be wrong yet (hell, they might even be right). Polchinski has recently written a good rundown on why he specifically joined the multiverse side.
What you speak of is many-worlds interpretation of QM which is actually retarded and comes from severe misunderstanding of both QM and probability. Other popular retarded ideas that plague popsci is bohmian mechanics and anything on entanglement. I suggest anyone interested in QM who doesn't want to make a fool of himself to go through the Landau books. It is impossible to misunderstand or misinterpret him, the best series for undergrads there is.

en.m.wikipedia.org/wiki/Quantum_mind

If I read another piece of bullshit from people mystifying consciousness as something that can't be defined as purely physical then I will get drunk. Luckily as my consciousness is magical quantum energy, my decision making will not be impaired. Who knows, maybe thanks to quantum mortality if I were to die I wake up in a universe where every thing I don't like doesn't exist and I'm rich. Anything can happen in these strange many-worlds ;^)

>The quantum mind or quantum consciousness[1] group of hypotheses propose that classical mechanics cannot explain consciousness. It posits that quantum mechanical phenomena, such as quantum entanglement and superposition, may play an important part in the brain's function and could form the basis of an explanation of consciousness.

no woo to be found here, there's a difference between this hypothesis and mainstream quantum woo

Maybe it does but I doubt it. And It doesn't really explain how consciousness really leads to any decisions, only how it could work.

Ah, I figured the current big Multiverse theory was just the QM many worlds interpretation, didn't realize Multiverse specifically implied the String Theory related multiverses. Just read Our Mathematical Universe by Max Tegmark, and that's just my most recent association with popsci Multiverse shit

Well i don't know about the popsci world, but among theoretical physicists, multiverse is a hypothesis mostly based on the anthropic principle, though each member of the group has his own little twist to it. I think it's important to note that debates on anthropic principle fall under philosophy, not physics.
I've seen MWI being referred to as multiverse more and more often around here and it's pretty annoying to me, so i'm just clarifying that one is crackpottery and the other is a valid hypothesis (though ugly and discouraging any form of scientific endeavour).

Is the atheists explanation of the fine tuned argument.

They simply make pseudoscience BS to avoid to accept the fact fine tuned universe is legit evidence of God.

>Multiverse is a hypothesis of certain group of string theorists
No it isn't, it's connected to eternal inflation

Fine tuning has two explanations, either it's god (what fine-tuned god?) or then there are a gazillion pocket universes. We're like mold, it's no surprise that mold is where there's dampness - doesn't mean that the whole world was designed so that that specific spot would be wet.

I'm not concerning myself with crackpottery like quantum multiverse. The only multiverse theory that has a chance of survival is the one coming from this group of string theorists and it has to do with mostly anthropic principle, though they use branes, holographic principle and other ideas, each in their own way to support their notion of multiverse. The common element is anthropic principle and branes.