[JohnMakin]

It's just a joke but the average number of years for a spontaneous quantum fluctuation to produce a boltzmann brain was calculated at something like 10^500 years. You're right that the processes involved would still remain barring some kind of big rip event.

[squigz]

Does this mean such an event could produce, say, an entire universe?

If so, does this theoretically mean that a cyclic universe is possible in this way, and that if one were to go far enough - impossibly, unfathomably far - you might find the remnants of other universes?

[immibis]

The theory of Boltzmann brains is that you're way more likely to get just a brain (including false memories of a planet earth and a whole visibile universe around it), than to get a brain, a planet earth, and a whole visible universe around it. So the chance that any of that is real, given that a brain exists to perceive it, is infinitesimal. We are probably just floating human brains that popped into the vastness of space three microseconds ago, complete with false memories of the distant past.

To dispel a misconception: They're not some hypothetical type of brain that exists as pure quantum fluctuations (though those are even more likely). Boltzmann was talking about the probability of actual flesh-and-blood human brains arising spontaneously out of the vacuum.

[simonh]

Wouldn’t the vast majority of those be incoherent broken messes, of various levels of inconsistency? Only a teeny tiny fraction would be coherent. So the expected experience fir any arbitrary Boltzmann brain would be all over the place.