The multiverse is a good
philosophical alternative to the "design" interpretation of the fine tuning problem but a "falsifiable scientific theory" it is not and nor is it ever likely to become one.
In principle, the multiverse propsal is almost indistinguishable from the "
Deist God" in its appeal to a logically sound and elegant idea with great explanatory power but which is forever unobservable and devoid of any testable predictions.
Simply put, if your best response to the argument that "
the finely tuned and seemingly arbitrary initial conditions, laws and constants of the universe requires belief in the invisible, immaterial agency of a supreme being who exists outside the universe, to be adequately explained", is to posit the existence of something else outside our universe (a "multiverse") which is equally invisible to our observation and equally unprovable as a result...then you are essentially giving ground to the theist notion that, as it stands, there is no naturalistic explanation for fine tuning within the parameters of the observable universe (the only universe we know to exist and which we can study with scientific tools) and that answers need to be sought in unfalsifiable metaphysical realities beyond it.
Umm, I do hope that you see the "elephant in the room", so to speak, with such a bizarrely self-defeating rebuttal. To actually defeat the theistic argument, it would be far more logical to start from the premise that the universe we live in (it being the only one we actually know exists and therefore the only one we can study and scrutinize using the scientific method), is indeed the only universe and that the apparent fine tuning of this universe can be explained on its own terms, by its own "inner logic", such that we don't need to imagine or envision any greater, metaphysical realities beyond what we can actually observe and test in our search after an explanation for why the universe is the way that it is, exceedingly improbable as this would seem to be if mere chance alone is to blame, with the life-permitting physical laws (and numerical values thereof) that it has.
If you are unable to do this: which is to say, if the presumption or assumption that fine tuning has an explanation from within the parameters of our universe that does not compel one to invoke or make reference to something beyond the univetsr that we can't actually see or prove, is found to be unsatisfactory as an explanation accounting for the sheer improbability of the data; then you simply cannot dismiss the "
Intelligent Design fine tuning argument" out-of-hand as a viable and philosophically sound answer to the problem.
It is certainly a logical possibility that we live in an ever expanding megaverse of unlimited physical possibilities but unfortunately no one will ever be able to provide empirical evidence based upon scientifically robust tools of observation and falsifiable testing that is indicative of the "eternal inflationary multiverse" model (on account of the particle horizon and the speed of light i.e. inflation causes the universe(s) to expand at a rate exceeding the speed of light, meaning that universes are causally disconnected at practically infinite distances making communication between them or observation of them impossible for all eternity, among other factors).
The "multiverse" is consequently a very poor answer to the theist fine tuning argument: seeking to refute an untestable but philosophically compelling notion ("the Deist God") with an equally untestable but philosophically compelling notion (an infinite multiverse of inflationary bubble universes each with randomly assigned physical laws and constants).
Multiverse propositions are therefore
the ultimate in circular reasoning for an atheist-naturalist minded person to employ in countering theistic arguments based on fine tuning - amounting to intellectual disarmament of the highest order.
Also, something else merits attention here: the inflationary multiverse doesn't even get by "fine tuning" in the first instance since the "multiverse generator" and "cosmic inflation" itself (upon which the very possibility of a multiverse in space-time is predicated), would both require incredible fine tuning to operate and for the mathematical equations to work. The cure is thus worse than the disease and this has been noted by none other than
Paul Steinhardt, one of the original pioneers of the "inflationary multiverse" hypothesis in the 1980s (who has since turned against it and is far more critical of the idea than I am, denying that it is even "explanatory"):
https://blogs.scientificamerican.com/cross-check/physicist-slams-cosmic-theory-he-helped-conceive/
Physicist Slams Cosmic Theory He Helped Conceive
Paul Steinhardt, Albert Einstein Professor in Science and Director of the Center for Theoretical Science at Princeton University
Steinhardt: From the very beginning, even as I was writing my first paper on inflation in 1982, I was concerned that the inflationary picture only works if you finely tune the constants that control the inflationary period. Andy Albrecht and I (and, independently, Andrei Linde) had just discovered the way of having an extended period of inflation end in a graceful exit to a universe filled with hot matter and radiation, the paradigm for all inflationary models since. But the exit came at a cost -- fine-tuning. The whole point of inflation was to get rid of fine-tuning – to explain features of the original big bang model that must be fine-tuned to match observations. The fact that we had to introduce one fine-tuning to remove another was worrisome. This problem has never been resolved.
We have not explained any feature of the universe by introducing inflation after all. Instead we have just shifted the problem of the original big bang model (how can we explain our simple universe when there is a nearly infinite variety of possibilities that could emerge from the big bang?) to the inflationary model (how can we explain our simple universe when there is a nearly infinite variety of possibilities could emerge in a multiverse?).
I have to admit that I did not take the multiverse problem seriously at first even though I had been involved in uncovering it. I thought someone would figure out a resolution once the problem was revealed. That was 1983. I was wrong.
To me, the accidental universe idea (the notion that the features of the observable universe are accidental: consequences of living in this particular region of the multiverse rather than another), is scientifically meaningless because it explains nothing and predicts nothing. Also, it misses the most salient fact we have learned about large-scale structure of the universe: its extraordinary simplicity when averaged over large scales. In order to explain the one simple universe we can see, the inflationary multiverse and accidental universe hypotheses posit an infinite variety of universes with arbitrary amounts of complexity that we cannot see. Variations on the accidental universe, such as those employing the anthropic principle, do nothing to help the situation.
Scientific ideas should be simple, explanatory, predictive. The inflationary multiverse as currently understood appears to have none of those properties.
If some atheists feel comfortable with the idea of explaining the problem of fine tuning and thereby quashing the suggestion of some Supremely Intelligent Designer, by having recourse to a greater, unseen reality beyond our observable universe (an infinite multiverse of inflationary bubble universes, endlessly popping into existence and where every conceivable set of physical laws and constants is realised somewhere)...a reality that is unfalsifiable and imperceptible to any instrument of measurement or analysis which could be created and utilized by humankind, putting it beyond the reach of the scientific method....then be my guest but please tell me how this doesn't amount to some kind of "faith" as described by the Book of Hebrews?
Hebrews 11: 1-3
Now faith is the substance of things hoped for, the evidence of things not seen.
Through faith we understand that the worlds were framed by the word of God, so that things which are seen were not made of things which do appear.
So far as I can read the situation: we appear to be at an inescapable impasse with the fine tuning problem whereby our best explanations to account for it (an inflationary multiverse or God) are essentially untestable philosophical answers arising from an unspoken recognition on both sides that our best chance of addressing fine tuning is to envisage a greater, unobserved reality outside the universe because on its own terms our universe looks far too improbable, unnatural and inexplicably fortunate to exist.
That's not science, that's metaphysics (even if dressed up in sciency-sounding terminology with respect to "cosmic inflation", "the multiverse" and "string theory"). But perhaps we're left with just precisely that because these are questions for which science has no answers, limited as it is to the study of the observable, testable universe.
If the various fine tunings cannot be explained from first principles using the evidence of our own observable universe - as brute facts, out of some as-of-yet unknown physical necessity or as supreme good luck - then we may never pass beyond this great impasse.
During a recent Ted Talk, Harry Cliff, a particle physicist at the European Organization for Nuclear Research (CERN), warned: “
The next few years may tell us whether we'll be able to continue to increase our understanding of nature or whether maybe, for the first time in the history of science, we could be facing questions that we cannot answer.” He added that scientists are approaching this limit as “
the laws of physics forbid” further understanding of the Universe.
Mr Cliff says that his theory is based on two numbers - the finely tuned strength of the Higgs Boson Field and the cosmological constant (the energy of the vacuum of space that causes the universe to expand) - which account for everything that the Universe is made up of, and if these numbers are slightly off, then “
there would be no physical structure in the Universe.”
Mr Cliff concluded his Ted Talk by saying: “
We may be entering a new era in physics. An era where there are weird features in the universe that we cannot explain. An era where we have hints that we live in a multiverse that lies frustratingly beyond our [scientific] reach. An era where we will never be able to answer the question why is there something rather than nothing.” One could equally paraphrase Mr Cliff to say: "
An era where we have hints that we live in a designed universe courtesy of a Creator who lies frustratingly beyond our [scientific] reach." Hence the impasse.