I've come across sentences like
...the reaction rates (the number of reactions per particle per unit time) must be larger than the cosmic expansion rate $H(t)$ in order for the particles to maintain equilibrium.
The quote is from the book "Extragalactic Astronomy and Cosmology" by Peter Schneider, page 195. Why does the reaction rate have to be greater than $H(t)$ and not $\dot a$? $\dot a$ feels like it should be the cosmic expansion rate.