P. P. Avelino

In one hospital ward it's +40°C, in another +10°C, and on average it's +25°C. If temperature affected something nonlinearly (say, the rate of bacterial growth) the average would give a wrong prediction. The same problem arises when cosmologists describe a universe where galaxies are packed into clusters and voids yawn between them.

After the Big Bang, the Universe went through a series of phase transitions, like water turning into ice, but with unpredictable patterns.