Cosmological Axion and neutrino mass constraints from Planck 2015 temperature and polarization data

Understand

Axions currently provide the most compelling solution to the strong CP problem.

• These particles may be copiously produced in the early universe, including via thermal processes.
• Therefore, relic axions constitute a hot dark matter component and their masses are strongly degenerate with those of the three active neutrinos, as they leave identical signatures in the different cosmological observables.
• In addition, thermal axions, while still relativistic states, also contribute to the relativistic degrees of freedom, parameterised via $N_{eff}$.