Axion

Physics

Dark matter

Cosmic microwave background

Particle physics

Quantum chromodynamics

Exponential decay

Photon

Inflation (cosmology)

Quantum electrodynamics

Theoretical physics

Astrophysics

Massless particle

Quantum mechanics

Anisotropy

Universe

Journal of High Energy Physics

We present a new mechanism to deplete the energy density of the QCD axion, making decay constants as high as $f_a \simeq 10^{17}\,\rm{GeV}$ viable for generic initial conditions. In our setup, the axion couples to a massless dark photon with a coupling that is moderately stronger than the axion coupling to gluons. Dark photons are produced copiously through a tachyonic instability when the axion field starts oscillating, and an exponential suppression of the axion density can be achieved. For a large part of the parameter space this dark radiation component of the universe can be observable in upcoming CMB experiments. Such dynamical depletion of the axion density ameliorates the isocurvature bound on the scale of inflation. The depletion also amplifies the power spectrum at scales that enter the horizon before particle production begins, potentially leading to axion miniclusters.

Opening up the QCD axion window