Accelerated cosmic expansion, mass creation, and the QCD axion

TL;DR

This paper introduces a novel mechanism where continuous energy density creation, linked to a hidden gauge group axion, drives cosmic acceleration and addresses the coincidence problem.

Contribution

It proposes a new model connecting QCD axions and a hidden gauge group to explain dark energy and cosmic acceleration through energy creation.

Findings

Modified Friedmann equations for energy creation

Identification of a hidden gauge group axion

Solution to the coincidence puzzle

Abstract

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 $\varphi_b$. We accordingly modify Einstein equation, derive modified Friedmann equations and analyze the regimes in which cosmic acceleration occurs. The creation process requires $\rho_b\neq 0$ as initial condition, which we enforce by identifying $\varphi_b$ with the axion of a hidden gauge group that confined in recent cosmological times, leading to a level crossing between $\varphi_b$ and the QCD axion, which is assumed to comprise dark matter. The conversion of a small fraction of QCD axions into $\varphi_b$ 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.