Inflation and Invisible Axions

An era of inflation in the hot big bang can lead to a dilution of axions. Such a dilution removes the upper bound on $f_A$, the symmetry-breaking scale at which axions are produced. The dilution may occur during or just after inflation, but in either case there is a restriction on the reheat temperature of the Universe after inflation. Hence, such a scenario for an invisible axion requires baryogenesis to occur at a low temperature.