“Entropy shakes its angry fist at you for being clever enough to organize the world.” -Brandon Sanderson
The universe was born hot, dense, expanding, full of matter, antimatter and radiation… and in a low-entropy state. If entropy is a measure of disorder, though, that sure does sound like an awfully high-entropy state, not a low-entropy one. So why, when we talk about the Universe, do we say that the early Universe had such low entropy?
Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and evolution, and continues to do so. Image credit: NASA / CXC / M. Weiss.
The answer has much more to do with a comparison to the entropy of the Universe today, as well as in the far future. Today, the entropy is some quadrillion (or 10^15) times larger than it was around 13.8 billion years ago. And if we extrapolate into the very far future, it will be another 100 quintillion (10^20) times bigger than it is today. But what’s responsible for the difference?
A system set up in the initial conditions on the left and let to evolve will become the system on the right spontaneously, ...