However, Robin Collins raises an interesting counterargument in "The Fine-tuning of the Cosmos: A Fresh Look at its Implications":
Therefore, even if we could explain why, in our own universe, Boltzmann Brains are more rare than non-Boltzmann Brains, non-fine-tuned multiverses will still lead Boltzmann Brains to preponderate by cosmic orders of magnitude.Because [Boltzmann Brains] BBs have a finite probability of occurring in any region of space-time with a positive energy density, in a sufficiently large universe that has some lower-bound to its mass-energy density, many BBs are almost certain to occur. This is true even if the constants and parameters of physics are not fine-tuned – for example, if the dark energy density is too large for galaxies and stars to form. Furthermore, in standard versions of the multiverse – such as promoted by Stephen Hawking (reference_, Alexander Velnkin (reference), and Leonard Susskiind (reference), the bubble universes that are produced are infinite in size and meet this minimal mass-energy density condition. Such infinite universes will have an infinite number of BBs, whether or not their constants are fine-tuned. [pp. 6-7]
However, we have reasons to think we're not Boltzmann Brains:
- Empirical: Our experience is ordered. For almost all brain fluctuations, even a momentary experience is chaotic.
- Prudential: Only if we're not Boltzmann Brains will our actions have lasting impacts on the universe in terms of reducing suffering, etc. So we should act as though we're not Boltzmann Brains.
As a disclaimer, I should say that I find most of the remainder of this article by Collins unconvincing. His god hypothesis -- even if it can be made coherent and explanatory of anything -- is far more extravagant than what's needed. Or else, if one defines "god" as merely certain principles of physics that makes fine-tuning true, then the relation of this "god" to ordinary religion crumbles away. That said, it does seem to me at first glance that the specific portion of Collins's argument cited above weakens the conventional multiverse explanation for fine-tuning and therefore strengthens the suggestion of some more targeted principle of nature that gives rise to orderly observers.