Cosmic birefringence tomography with radio galaxies I: Testing birefringence solutions to the CMB-BAO tension

Abstract

We present an independent test of cosmic birefringence induced by axionlike particles (ALPs) at low redshift using radio galaxies. We focus on models that can alleviate the mild tension between recent baryon acoustic oscillation (BAO) measurements and observations of the cosmic microwave background (CMB). This tension can be explained by a large cosmic birefringence rotation angle generated during the reionization epoch. Such a large rotation can arise if the recent isotropic cosmic birefringence measurements are subject to an $nπ$ phase ambiguity, with $n\in\mathbb{Z}$, or if cold dark matter injects energy into ALPs. We constrain these two scenarios by fitting the cosmic birefringence angles estimated from the jet morphologies and polarization angles of radio galaxies obtained by Battye, Jackson & Browne (2026). For the $nπ$ model, the radio-galaxy data exclude $|n|\geq 1$ at more than $2σ$ for $\log_{10}(m_ϕ/{\rm eV})\lesssim -30.7$. For the energy injection model, the data are consistent with no energy injection, but with a slight relative preference for a nonzero energy injection compared to the no-injection case. Since our samples are limited to $\sim 3000$ and there could still be several astrophysical uncertainties and statistical fluctuations, further exploration of tomographic cosmic birefringence using a large number of low-redshift radio galaxies is important for independent tests of cosmic birefringence.