The multiverse theory is probably the single best argument against the apparent fine tuning of our universe’s physical constants that many theists like William Lane Craig say implies a designer. That means it naturally has its religious critics because they see the existence of a multiverse as an undermining threat to what theists see as something god best explains.
Since the existence of a potentially infinite number of universes comprising a vast multiverse puts a damper into the argument from fine tuning, let me address the so called “Boltzmann Brain” dilemma rebuttal that theists are using. On his website, William Lane Craig made the following objections to the idea of a multiverse:
If we were just a random member of a World Ensemble, then we ought to be observing a very different universe. Roger Penrose has calculated that the odds of our solar system’s forming instantaneously through the random collision of particles is incomprehensibly more probable that the universe’s being fine-tuned, as it is. So if we were a random member of a World Ensemble, we should be observing a patch of order no larger than our solar system in a sea of chaos.
In order to be observable the patch of order needn’t be even as large as the solar system. The most probable observable world would be one in which a single brain fluctuates into existence out of the quantum vacuum and observes its otherwise empty world.
Now a similar problem afflicts the contemporary appeal to the multiverse to explain away fine-tuning. Roger Penrose of Oxford University has calculated that the odds of our universe’s low entropy condition obtaining by chance alone are on the order of 1:10^10(123), an inconceivable number. If our universe were but one member of a multiverse of randomly ordered worlds, then it is vastly more probable that we should be observing a much smaller universe. For example, the odds of our solar system’s being formed instantly by the random collision of particles is about 1:10^10(60), a vast number, but inconceivably smaller than 10^10(123).
First, a little background knowledge if you’re not familiar with a Boltzmann Brain.
Boltzmann Brains are a term made by Austrian Physicist Ludwig Boltzmann. In short, he lived in the late 1800s at a time when the steady state theory of the universe prevailed. He and many others believed the universe was timeless and eternal, and Boltzmann, who was an atomist, (a theory still controversial at the time since the atom hadn't yet been discovered) theorized that all the matter and structure in the universe could be the product of a random fluctuation of matter. Although this would be highly improbable, the infinite history of the universe he thought would give it the time necessary to happen. But, an entire universe fluctuating out of random collisions is more improbable than just a single solar system, and just a single brain would be more probable than a solar system. So any sentient being in the universe is much more likely to be just a single conscious brain that suddenly materializes from random particles colliding, rather than be a person with a whole body, living in a vast and orderly universe full of galaxies. Hence the term, Boltzmann Brain.
The Boltzmann Brain concept is interesting and I’m only a little familiar with it. One argument I heard against it goes like this. Imagine estimating the probability that if you were born as a form of life on earth, what would be the chances that you’d be born human. Since the number of insects on earth dwarfs the number of human beings overwhelmingly, there is a much higher probability that we should have been born as insects. There are an estimated 10^18 insects on earth compared to a relatively small 7 billion human beings (up from just 1.5 billion 100 years ago). That means that there are about 150 million insects for every one human being on earth. But you obviously weren’t born as an insect despite the overwhelming odds against it. So just because there is a much greater probability of something, it doesn’t mean that it will happen. Rare events happen all the time. In fact, every single event that ever happens in our universe is a rare event because the chances of that event not happening and some other event happening instead are always probabilistically more likely.
So we know we aren’t Boltzmann Brains given that they exist for a tiny amount of time, look around (with no eyes!) and then disappear back into quantum foam. I suppose chance again can explain this dilemma, in the same way that it does for how our universe’s physical constants could fall in the life permitting range. In the multiverse scenario, eternal inflation involves there being an unlimited number of rolls of the dice, and given an eternal future, a universe like ours is inevitable. Does that mean that hyperspace is littered with Boltzmann Brains amongst the universes? I don’t know, but it seems the chance hypothesis and the multiverse still has a chance.