Jim Al-Khalili's Book Review of Paul Davies' The Goldilocks Enigma (“Life in the universal porridge” Nature 444, 423–424; 2006) explains with great clarity why the anthropic principle is an incomplete answer to the great question of 'Life, the Universe and everything', and why the concept of a Multiverse is more satisfactory.
However, there is an alternative — even if we assume that the only way to make a different universe is to change a few fundamental constants.
Arguments in favour of fine-tuning typically show that some key ingredient of our current Universe, such as atoms or stars, becomes unstable if some physical constant is changed by a relatively small amount and therefore cannot exist in a universe with different constants. But in such circumstances, what is interesting is not the instability of some particular state, but what the system does instead. If conventional atoms or stars become unstable, what other organized forms of matter might arise? In particular, which values of the physical constants permit structures complex enough to resemble intelligent life?
It is not easy to answer this question. At the moment, we cannot rigorously deduce the structure of the helium atom from basic physics, let alone that of a living organism. Our understanding of atomic structure depends on various ad hoc simplifications, justified by comparison with observation. This approach is not available for universes with different constants, so we have no real idea how they might behave.
If — as experience with simpler dynamical systems suggests — large regions of universe-space permit complex structures, then our Universe actually has a reasonable chance of falling into such a region, and the complex structures that arise will necessarily be consistent with the laws of that universe.
The real message of Goldilocks is not that Baby Bear's porridge was 'just right'. It was just right for Goldilocks — but cold porridge was also just right for Mummy Bear and hot porridge was just right for Daddy Bear. We are the product of our own universal porridge, and it will always be just right for us.
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Stewart, I. Life: porridge would be just right for each universe. Nature 444, 1002 (2006). https://doi.org/10.1038/4441002b