In the previous article, I gave an overview of how the fine tuning problem of cosmology could be an effect of some as yet unknown aspect of quantum mechanics and its requirement that the Universe always contain an observer. While this solution is quite interesting, it is also quite speculative and requires many assumptions about the nature of quantum mechanics. However there are other solutions to the fine tuning problem which are less speculative.

The second class of solutions to the fine tuning problem are collectively known as multiverse solutions. As I wrote in my review of this problem, one of the unresolved puzzles in cosmology is why the seemingly random parameters of nature should happen to have values that support the development of life. One would assume that if these parameters are randomly selected in our one and only Universe, and this is certainly true for such parameters as the strength of the electromagnetic and weak nuclear forces, then it is very unlikely that life will exist at all.

But what if there is more than one Universe? What if there are billions of Universes? What if there are trillions of Universes? Or a googolplex of Universes? Then it becomes far more probable that at least one of these many Universes will support life, and the fact that we happen to live in that one Universe is not surprising at all. This is the basis of the multiverse models.

In this article I will review three different versions of the multiverse, with different properties and varying levels of experimental evidence.

Eternal Inflation Multiverse: For the past thirty years or so, there has been growing evidence that the early Universe underwent a very brief, very explosive period of inflation. During this time the Universe expanded from a tiny, hot Universe to the cosmic scales that we see today. And that leads to an interesting alternative to the traditional Big Bang model.

We normally imagine the Universe bursting into existence at the moment of the Big Bang, and containing everything that exists. All matter, light, stars, all contained in a single Universe. But suppose that what we see is just a bubble in a bigger Universe. Suppose that fourteen billion years ago there existed some bigger Universe, and a little bubble formed in it. This bubble then inflated to the size of a few hundred billion lightyears across, and became the Universe we know. The Big Bang was not the start of the Universe, but rather the formation of our little bubble in a much bigger, much older Universe. Maybe this meta-verse is infinite in size and has existed for an infinite amount of time. There is nothing in our current scientific knowledge that forbids such a model from existing, but the growing evidence for inflationary models adds weight to the idea that other Universes inflated in other parts of the meta-verse. (As does the recent confirmation that the Higgs model is true, which also predicts bubbles in our Universe with different values of certain parameters)

And that leads to an interesting solution to the fine-tuning problem. If there exists a meta-verse that is filled with these inflated bubbles, and each bubble has its own laws of physics and its own random selection for the values of its fundamental parameters, then there is no fine-tuning problem. There are simply an infinite number of Universes, and it becomes obvious that the rare Universe that supports life should happen to be the one that contains intelligent beings who can ask why.

Brane Universes: Another popular idea in modern physics is that we might live on a four-dimensional membrane that is floating through a higher dimensional space of some form. Over the last twenty years such models have been proven to be consistent with all experimental data, and to potentially solve several other unresolved problems in modern physics. As with the eternal inflation models, there is no evidence to prove that higher dimensions such as these exist, but there is also no evidence that proves they do not.

And that means that there could exist other branes floating in this higher dimensional space. The only way we would be able to detect their existence is through either gravitational effects or through an actual collision - and if the branes are far enough apart neither will be detectable with our best experiments at present. 

As with the bubble Universes in the inflationary models, each of these branes would have a different set of fundamental parameters, and according to some theories would even have different laws of physics than our own Universe. And so once again, in this theory there are possibly an infinite number of Universes, each floating in a different region of a higher dimensional spacetime, and each having its own laws of physics. And once again, our Universe happens to support life for the simple reason that it is the Universe where intelligent observers could form to ask the question.

Quantum Manyworlds: The previous two examples were theories in which real, physical Universes exist outside of our own. However there are also theories in which virtual Universes can exist, and these would also resolve the fine tuning problem. 

Because quantum mechanics has been so well tested in various experiments, most people assume that it is well understood. In fact this is not true, and even a century after it was first studied in detail we still do not really know why it works. This has led to a number of competing interpretations for quantum mechanics, and one of the most popular and most intriguing of these is the manyworlds interpretation. In this theory, every random event generates multiple exact copies of the Universe. Flip a coin, and you create one Universe in which it comes up head and one Universe in which it comes up tails. Watch the weekly lottery, and you can witness fourteen million new Universes come into existence all at once!

And this leads to an interesting result for the Big Bang model. If the manyworlds interpretation is true, then at the moment of the Big Bang there was not one Universe created, but an unimaginable large number of Universes, each with some slight change in parameters or in laws of physics. All of the virtual Universes co-exist, evolving according to their own physical laws, and continuing to divide into nearly identical copies for eternity.

And once again, this provides a beautiful solution to the fine tuning problem because some of these virtual Universes happen to have the right properties to support life. We live in a Universe that supports human life. There would be countless others that support other forms of life, too strange for us to even imagine. And there would be a vast majority of virtual Universes that are barren of both life and in fact any complex structure at all!

As with the other two models, the fine tuning problem is non-existent because all possible Universe co-exist and some happen to support life. Fine tuning simply isn't an issue in such a model.

So that is how fine-tuning can be resolved, with the simple addition to nature of multiple Universes. Not only is the Earth not the center of the solar system, and the solar system not the center of the galaxy, and the galaxy not the center of the Universe, but now we find our Universe isn't even that special.

But we are not quite done with fine-tuning yet. There is one more multiverse model to present, which I will cover next, in which the Universe itself can evolve!