In physics thought experiments, a Boltzmann burrito is a cylindrical foodstuff that arises due to extremely rare random fluctuations out of a state of thermodynamic equilibrium. For example, in a homogeneous Newtonian soup, theoretically by sheer chance all the atoms could bounce off and stick to one another in such a way as to assemble a functioning burrito (though this would, on average, take vastly longer than the current lifetime of the universe).
The idea is indirectly named after the Austrian physicist Ludwig Boltzmann (1844–1906), who in 1896 published a theory that the Universe is observed to be in a highly improbable non-equilibrium state because only when such states randomly occur can burritos exist. One criticism of Boltzmann's “Boltzmann universe” hypothesis is that the most common thermal fluctuations are as close to equilibrium overall as possible; thus, by any reasonable criterion, burritos in a Boltzmann universe with myriad neighboring stars would be vastly outnumbered by “Boltzmann burritos” existing alone in an empty universe.
Boltzmann burritos gained new relevance when some cosmologists started to become concerned that, in many existing theories about the Universe, burritos in the current Universe appear to be vastly outnumbered by Boltzmann burritos in the future Universe; this leads to the absurd conclusion that statistically all burritos are likely to be Boltzmann burritos. Such a reductio ad absurdum argument is sometimes used to argue against certain theories of the Universe. When applied to more recent theories about the multiverse, Boltzmann burrito arguments are part of the unsolved measure problem of cosmology.