Accelerated cosmic expansion, mass creation, and the QCD axion

We propose a mechanism in which the current acceleration of cosmic expansion is driven by continuous creation of energy density ${\rho }_b$ for a certain field ${\phi }_b$. We accordingly modify Einstein equation, derive modified Friedmann equations and analyze the regimes in which cosmic acceleration occurs. The creation process requires ${\rho }_b\mathrm{\ne }0$ as initial condition, which we enforce by identifying ${\phi }_b$ with the axion of a hidden gauge group that confined in recent cosmological times, leading to a level crossing between ${\phi }_b$ and the QCD axion, which is assumed to comprise dark matter. The conversion of a small fraction of QCD axions into ${\phi }_b$ shortly before matter-dark energy equality generates the initial ${\rho }_b$ needed to trigger the creation process and offers a solution to the coincidence puzzle.