It might seem obvious that the universe becomes more complex over time. After all, isn't a gas cloud consisting only of hydrogen and helium a few seconds after the big bang simpler than a cloud of hydrogen, helium, carbon, silicon, and oxygen two billion years later? Aren't the dynamics of a system at the level of particle physics simpler than the dynamics of a group of interacting cells?

As intuitive as these proposals are, we don't currently possess an adequate quantitative measure of the increase or decrease in complexity either across cosmic evolution or across scientific disciplines. In the past, many theorists presupposed that the gap between actual entropy and maximum entropy is not permanent, that heat death will win in the end. These theorists were eager to link complexity in a quantitative way to this “entropy gap”, so that the two would rise and fall together. Others now argue that, in an expanding universe, the maximum possible entropy will increase more quickly than actual entropy, casting the “heat death” hypothesis into question.