In terms of early universe cosmology I often stumble over sentences like:
We assume that, at sufficiently early times, reaction rates for particle interactions are much faster than the expansion rate, so that the cosmic fluid is in thermal equilibrium.
The quote is from the book "The Cosmic Microwave Background" by Ruth Durrer, page 14, ( https://www.cambridge.org/core/books/cosmic-microwave-background/10D066B56BBBA899F3B89A29E0B3B78B )
How can I understand this better? Are there some equations where I can explicitly see that the production of a specific particle species goes drastically down when the Hubble rate is larger than the reaction rate? And with reaction rate one means the rate which an observer in the lab would see?