Inflating Boltzmann’s Brain
Thermodynamics lets us make engines, refrigerators and much more. But why does it work? The usual answer is that physical changes are deterministic (i.e., one-to-one), and the early universe was highly ordered (i.e., flat). But why was the early universe so ordered? Various new fundamental principles have been proposed to explain early order, but so far these have not been fruitful.
A century ago Boltzmann suggested that the order we see (billions of light years of flat space) is a rare random fluctuation in a much larger universe. One might hope that observer selection could explain why we see such a rare event; only if there is a big fluctuation can there be observers to see it. But observer selection predicts a fluctuation just big enough to make one observer. This is the “Boltzmann’s brain paradox;” the order we see is much larger than is needed to explain just your brain.
Andreas Albrecht explains that while technical problems remain, it now seems hopeful that inflation is the missing key here (along with assuming the universe is large). Since early order is required to create inflation, inflation cannot by itself explain the order we see. But inflation can eliminate the difference between brain-sized and visible-universe-sized fluctuations. A fluctuation that creates inflation is more likely than one that just makes a brain, and any fluctuation big enough to make inflation creates order on the scale we see.
Perhaps we now just need ask: why is the universe so big?