I'm sure Ockham will be rolling in his grave. — Wayfarer

That's okay. There should a universe where he is not. :D

In current cosmology, the big debate on whether 'the universe' is 'only one' of a possibly infinite number of 'multiverses', which might forever be undetectable, even in principle. — Wayfarer

How is this hypothesis backed up? Because if the other universes are undetectable, then I am guessing that it was not brought up from empirical data. Then was it deduced somehow?

How is this hypothesis backed up? Because if the other universes are undetectable, then I am guessing that it was not brought up from empirical data. Then was it deduced somehow? — Samuel Lacrampe

That question - 'how is it backed up' - is the subject of a very large and even acrimonious debate between many big time scientists. It is being called a crisis in cosmology ( here are some references.)

You and I are plainly never going to resolve this problem (or even understand it, for that matter.) But the point I was making, whenever anyone refers to 'Ockham's Razor' in defense of 'naturalism' with regards to cosmology, then just look at where naturalism is at right now. It is far more extravagant and a good deal stranger than anything in medieval metaphysics.

How is this hypothesis backed up? Because if the other universes are undetectable, then I am guessing that it was not brought up from empirical data. Then was it deduced somehow? — Samuel Lacrampe

It apparently fell out of some interpretations of quantum mechanics, and later some string theories.
Quantum mechanics is well-established, string theories aren't.

How is this hypothesis backed up? Because if the other universes are undetectable, then I am guessing that it was not brought up from empirical data. Then was it deduced somehow?
— Samuel Lacrampe

It apparently fell out of some interpretations of quantum mechanics, and later some string theories.
Quantum mechanics is well-established, string theories aren't. — jorndoe

The theory in question is a cosmological one (theory of big things explaining what we see in telescopes), the other end of the scale from QM interpretations (explanations of observed behavior of little things). Oddly, the two are sometimes related, especially in the realm of string theory.
These other universes are not QM worlds, but other spacetimes with different physics and numbers of dimensions and such. They're more an answer to the teleological argument than the cosmological one. The view doesn't answer the first-cause issue, it just puts it further behind our big bang.

@noAxioms, you're right, I was thinking more generally in terms of those uhm "larger-world" hypotheses. Something like ...

• modal realism (possible worlds)
• many worlds (quantum mechanics)
• multiverse (e.g. ensemble, M-theory, brane collisions)

... or whatever they all are.

I think the one relevant to this thread is eternal-inflation theory, where other worlds have different physics.
Modal realism pretty much covers any of them. I don't think of think of that as any kind of metaphysical stance, but rather a set of tools for describing them and an assertion that there is no preferred world that is more actual.
For instance, a universe that is undetectable just because it is outside the Hubble-sphere is another world just like the worlds of inflation, QM, or whatever. Is it not real? Some say not.

Yeah you are probably right about this topic being too convoluted. I usually walk away whenever someone pulls the Quantum Mechanics card into a philosophical debate.

- "Nothing can come from nothing."
- "Actually, in QM, particles pop in and out of existence ..."

Fishery,

The contemporary Kalam cosmological argument as it is presented and defended by academics like the Christian theologian/philosopher William Craig, has its roots in the thought of the 12th century Muslim scholar, Al-Ghazali, who argued that the notion of a beginningless universe was absurd. He formulated his argument very simply in the following statement:

"Every being which begins has a cause for its beginning, now the world is a being which begins, therefore, it possesses a cause for its beginning."

Al-Ghazali recognised that if the universe was "past-eternal" and never began to exist in time there must have been an infinite number of (cause -and -effect) events prior to today, but he denied that an infinite number of things could actually exist. He realised that if an actual infinite number of things could really exist then various absurdities would result like, for instance, "Hibert's Hotel" as illustrated in David Hibert's famous thought experiment. If "Hibert's Hotel" could really exist, then it would have a sign posted outside saying " No Vacancy ( guests welcome) which is patently absurd.Briefly, It is clearly unreasonable, I think, to expect that such a Hotel could actually exist. And since nothing depends on Hibert's illustration involving a Hotel, the argument can be generalised to show that that the existence of of an actual infinite number of (any) things - such as an infinite number of past events prior the existence of universe as we observe it is today is frankly absurd.

Your repeated objection is that developments in modern mathematical set theory have seen the use of actually infinite sets become commonplace, such, for example, as the set of negative integers ( 0, -1, -2, -3, -4...) which has ( in set theory) an actually infinite number of numbers in it. You suggest that this invalidates or at least undermines premise (2) of the Kalam argument, but does it ? In my opinion the answer is "No, it does not.". I fully accept that if , as a mathematician, you adopt certain axioms and rules, then you can TALK about actually infinite collections in a way that is consistent and without contradicting yourself - BUT, insn't it true that all this ultimately achieves is showing how one can set about constructing a particular universe of discourse for talking consistently about actual infinities. That is, It does absolutely nothing to show that an actually infinite number of things can really exist. So, in short, if Al-Ghazali is right, ( and I believe he is) we can dismiss this universe of mathematical discourse as a merely fictional realm, like the world of Sherlock Holmes or Harry Potter, or something that exists only in your imagination. The onus is on you to show that this is not the case.

Another objection to the Kalam argument you forward is that premise 2, namely the proposition that that the universe did have a temporal beginning, makes things too easy for a natural theologian like Craig because it patently "bakes in" ( as you put it ) the necessity of a creating cause of the origin of the universe. Even if this were true and Craig was being "disingenuous", the fact remains that overwhelming majority ordinary lay persons (people, that is who are not employed as professional astrophysicists , cosmologists or philosophers etc) ,would agree with the Kalam argument's conclusion that there MUST be a creating cause of the universe. When, for example, the fundamental principles of the "Big Bang" theory is first explained to most non-expert adults, their very first question is: "What caused the Big Bang ?" This question evinces a pre-philosophical/pre-scientific intuition that whatever begins to exist has a cause, and that things do not somehow simply "pop" into existence ( ex nihilo) without having a distinct cause. In my view, this is a perfectly rational and reasonable intuition and therefore offers strong prima facile justification for thinking that if the universe did begin to exist in time and that it's origination must therefore have been the effect of some transcendent cause ?

What do you think?

Regards

John