Thermal Axions as Hot Dark Matter: The Mass Ceiling from Abundance and Radiation Limits Combined

For decoupling at g_* = 61.75, the radiation contribution is 5.55e-02, the mass that saturates the hot-component ceiling is 7.52e-01 eV, and the mass that would account for all the dark matter by abundance is 9.03e+01 eV. At the top of the ladder the corresponding masses rise to 1.30e+00 eV and 1.56e+02 eV. The large gap between the hot ceiling and the accounting value at every epoch is the quantitative statement that thermal axions cannot be the bulk of the dark matter: long before their abundance suffices, their free-streaming and radiation signatures exceed what the data allow. The bound density of 9.72e+54 g per cubic centimetre certifies that the top-of-ladder row is physically realised in this framework. This study is computed from stated assumptions and published comparison values; it is not a measurement.