Non-thermal Axion Dark Radiation and Constraints

Understand

The Peccei-Quinn mechanism presents a neat solution to the strong CP problem.

• As a by-product, it provides an ideal dark matter candidate, "the axion", albeit with a tiny mass.
• Axions therefore can act as dark radiation if excited with large momenta after the end of inflation.
• Nevertheless, the recent measurement of relativistic degrees of freedom from cosmic microwave background radiation strictly constrains the abundance of such extra relativistic species.