^Well, the "Boltzmann brain" idea doesn't really hold up to scrutiny. It proposes that quantum indeterminacy could cause a sentient brain to spontaneously arise by chance in deep space if the universe is old enough (i.e. with enough chances, a really improbable thing becomes inevitable), but it doesn't address the question of what happens next -- i.e. how the brain survives, sustains itself, etc. Without an environment amenable to its existence, it'd pretty much cease to exist as soon as it spontaneously arose.
So it doesn't really work as a synonym for sci-fi "energy beings," particularly those that evolved from corporeal ancestors. It's just an abstract and kind of silly thought experiment.