Last time I started talking about **entropy** and a puzzle it presents in cosmology. To understand the puzzle we have to understand entropy, which is a crucial part of our view of physics. In fact, we consider entropy to be a (statistical) *law* about the behavior of reality. That law says: *Entropy always increases.*

There are some nuances to this, though. For example we can *decrease* entropy in a system by expending energy. But expending that energy increases the entropy in some other system. *Overall*, entropy does always increase.

This time we’ll see how **Roger Penrose**, in his 2010 book * Cycles of Time*, addresses the puzzle entropy creates in cosmology.