Physicist and popular author Brian Greene gave a TED talk on "Why is our universe fine-tuned for life?" Alas, IMHO the talk from a scientific point of view was near worthless as it just piled on speculation upon speculation upon speculation to arrive at a definite answer.
His central thesis seems to be that there is in fact a Multiverse and that explains why this particular Universe, our Universe, is a Goldilocks Universe just right for life. Our Universe appears to be designed and fine-tuned for life on the grounds that if you have zillions of universes all with different hands being dealt from the deck of physics, sooner or later one will be by complete chance a Goldilocks Universe and of course we have no choice but to find ourselves a Goldilocks Universe.
However, IMHO, the concept of a Multiverse to explain apparent design / fine-tuning is a last-ditch desperate attempt to get around the very concept of a designed and fine-tuned Universe, since in scientific circles these concepts have too many theological implications.
In the first part of his thesis, Greene suggests that Dark Energy and the Accelerating Universe is ultimately explainable by a Multiverse.
We note that in 1929 it was discovered that our Universe was expanding. Common sense predicted that this expansion should be decelerating thus slowing down under the ever present pull of gravity. But oops, in 1998 cosmologists discover that contrary to common sense, the expansion rate of the Universe is actually accelerating (in apparent violation of normally accepted conservation laws), and to account for this they pull out of their cosmological hat the concept of Dark Energy. Apparently gravity can also push things apart and "invisible energy" (i.e. - Dark Energy) can generate in space repulsive gravity. Galaxies can push against all other galaxies and this idea is called in cosmological circles great progress in understanding our Universe. IMHO this is just pure speculation.
However, just giving a name to something is not in and of itself an explanation. In fact there is no explanation, and the corresponding Nobel Prize awarded for Dark Energy was in fact for the discovery, not for any explanation. Dark Energy (whatever that actually is) is just pure speculation for what is driving the observation of an apparently accelerating expanding Universe.
In fact there is one massive fly in this cosmological ointment. The amount of Dark Energy 'observed' versus the amount of Dark Energy required is many, many, many orders of magnitude apart. So, Greene brings in his beloved String Theory to the rescue.
In the second part of his thesis Greene suggests that String Theory (the subject matter of his bread-and-butter day job) predicts a Multiverse.
String Theory replaces the Standard Model of Particle Physics by theoretically replacing vibrating strings for actual particles. This is pure speculation. Differing vibrations equate to differing types of 'particles' - also speculation. Alas, four decades on, there's not the slightest experimental evidence that this theory has the slightest validity. The one thing going for String Theory is that it can provide a unified theory, a Theory of Everything (TOE) which unites the micro (quantum realm) with the macro (relativity realm). In order words, you get a speculative theory of quantum gravity, a unification that's eluded traditional quantum and relativity physicists.
Alas, to achieve this String Theory version of quantum gravity, one requires an extra (undetected) six or so spatial dimensions and there are lots of possible candidate shapes for all combinations of these extra dimensions (coupled with the three spatial dimensions and one temporal dimension we already have to have to navigate our way around in). That's another two speculations piled on to already existing speculations. Now the upshot of all of this is that extra dimensionality quantum gravity String Theory gives rise to a possibility of some ten to the five-hundredth power (that's one followed by 500 zeros) differing configurations in the laws, principles and relationships that normally rule the physics roost - pure speculation. That includes massively differing values for any and all corresponding Dark Energy strengths, so that discrepancy is accounted for - more speculation. Each 'shape' or landscape of course results in a universe and all of these vast and vastly differing String Theory universes form a Multiverse, one of which is a Goldilocks String Theory Universe (our Goldilocks String Theory Universe). So we are in just the right Universe with just the right amount of Dark Energy - not too much which would blow any universe apart in nanoseconds and not too little which would give gravity the upper hand and collapse any universe back into a Big Crunch well before any real complexity could have emerged to ultimately form life.
In the third and final part of his thesis, Greene suggests that the cosmological concept of Inflation pulls all of his various threads together.
But how do you actually generate these other universes? The answer my friend is blowing in the cosmological winds and is called Inflation, or actually Eternal Inflation (yet more speculation). The 'fuel' required is the quantum field of quantum fluctuations. You have an inflating quantum field Cosmos and like bubbles coming into existence in a bottle of soda pop, little buds bud off this inflating Cosmos which expand creating more little buds which bud off each of which bud off more little buds which all expand - eternally. Note: this too is pure speculation. The concept of Inflation apparently solves various cosmological anomalies, but nobody really has a clue what started Inflation off or how buds (new universes), bud off or how Inflation ceases, if Inflation ever ceases and runs out of puff. Anyway, all of these little buds (baby universes), when they first bud (which are Big Bang events, ours being one of countless zillions) hence budding off from the 'parent' are collectively forming the Multiverse one of which will be a Goldilocks Universe, etc.
So Brian Greene sums it all up by saying that (speculative) String Theory and (speculative) Inflation equals a (speculative) Multiverse and we live in THE one Goldilocks Universe because it has just the right amount of (speculative) Dark Energy. The only 'proof' he suggests is that maybe one day extremely subtle temperature variations in the (actually detected) Cosmic Microwave Background Radiation might give the game away in favour of his endlessly speculating about our fine-tuning via his Multiverse concept.
Discussion: Even if these is a Multiverse, whether via the Many Worlds Interpretation of quantum mechanics; String Theory landscapes; or Eternal Inflation, that's no absolute guarantee that there will be differing laws, principles and relationships in the physics of each and every one. Each possible universe is a speculative Multiverse might be a Type-42 universe because only a Type-42 universe is ever possible. If each universe in the Multiverse is a standard Type-42 universe, then the whole Multiverse concept as an attempt to explain fine-tuning is null-and-void. The Multiverse is just another moot point in a cosmological sea of moot points.
However, even the concept of a Multiverse is pure pie-in-the-sky conjecture with no observational evidence in support. By that criteria, especially the criteria sworn to by many scientists that extraordinary claims require extraordinary evidence, the UFO extraterrestrial hypothesis (ETH) is light-years ahead of the Multiverse concept in terms of validity, if validity is according to the evidence.