Cosmic birefringence from a stiff axion field and its cosmological implications

We propose a model in which the observed cosmic microwave background birefringence arises from the interaction of photons with a stiff (w=1) pseudo-scalar field that today contributes a subdominant fraction of the total energy density, distinct from both dark matter and dark energy. Big Bang Nucleosynthesis constrains the cosmological density of the stiff field, leading to a lower bound for its coupling with photons, g ϕγ > 8.9 times 10-14, GeV-1. Photon conversion in cluster-hosted quasars sets an upper bound, g ϕγ < 5.0 times 10-13, GeV-1. Therefore, there is a narrow window for the discovery of this cosmological component that can be explored in the forthcoming years with astrophysical axion searches. Moreover, we show that this scenario leads to gravitational-wave signals that can be tested with planned experiments.