What are Boltzmann brains, and what challenge do they pose to the multiverse hypothesis?

I thought I would turn to the atheist theoretical physicist Sean Carroll, who has previously debated William Lane Craig, to explain to us what a Boltzmann brain is, and what threat it posts to the multiverse hypothesis. Here is Sean Caroll, quoted by About.com: Ludwig Boltzmann was one of the founders of the field of … Continue reading What are Boltzmann brains, and what challenge do they pose to the multiverse hypothesis?