My understanding is that to define the entropy of a system what you have to do is as follows:
- Define the boundaries of your system.
- Define a set of "microstates" of the system.
- Define a partition of microstates of the system where each element of the partition is measurable and known as a "macrostate".
The entropy of the system in some macrostate A is the logarithm of the measure of the microstates contained in mactrostate A.
Now, I often hear cosmologists say things like "the entropy of the early universe was low." What I understand that to mean is that for some specific partition of the microstates of the universe P, the state of the early universe was contained in some "small" element of that partition.
What I am trying to know is: What is this partition P which is being implicitly referred to? (And optionally, what is special about that partition which allows us to not be explicit about it?)