Orateur
Description
When polarized electromagnetic radiation propagates through a region of space permeated by a pseudo-scalar field, the plane of linear polarization undergoes a rotation. The rotation angle depends on the field's values when the radiation is emitted and when it is detected and does not depend on the frequency of the radiation. This phenomenon is called Cosmic Birefringence (CB) and is usually studied for CMB photons in the context of axion dark matter and axion dark energy.
We propose a new explanation in terms of a stiff (w=1) pseudo-scalar field that constitutes a now-subdominant component of the total energy density of the Universe. We find a very narrow range for the axion-photon coupling and we predict a very distinctive dependence of the birefringence angle on the redshift. We show that our model can produce signatures in the stochastic gravitational-wave background for which the Pulsar Timing Array (PTA) collaborations found first evidence in 2023.