Boltzmann brains gained new relevance around 2002, when some cosmologists started to become concerned that, in many existing theories about the Universe, human brains in the current universe appear to be vastly outnumbered by Boltzmann brains in the future Universe who, by chance, have exactly the same perceptions that people do; this leads to the conclusion that statistically humans are likely to be Boltzmann brains. 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 brain arguments are part of the unsolved measure problem of cosmology. Boltzmann brains remain a thought experiment; physicists do not believe that humans are actually Boltzmann brains, but rather use the thought experiment as a tool for evaluating competing scientific theories.