This measure has the advantage of being stationary in the sense that probabilities remain the same over time in the limit of large t . However, it suffers from the youngness paradox, which has the effect of making it exponentially more probable that we'd be in regions of high temperature, in conflict with what we observe; this is because regions that exited inflation later than our region, spent more time than us experiencing runaway inflationary exponential growth. For example, observers in a Universe of 13.8 billion years old (our observed age) are outnumbered by observers in a 13.0 billion year old Universe by a factor of 10 10 60 . This lopsidedness continues, until the most numerous observers resembling us are "Boltzmann babies" formed by improbable fluctuations in the hot, very early, Universe. Therefore, physicists reject the simple proper-time cutoff as a failed hypothesis.