[JC] CMB Birefringence from Axion String Networks Calibrated to an AMR Simulation
by
CTPU seminar room
IBS
In this talk, I will review the recent paper (2608.24456), where the authors studied CMB birefringence from axion string networks and obtained a strong constraint on the axion-to-photon-photon anomaly coefficient for an ultra-light axion (mass < 10^-33 eV). I will briefly introduce birefringence induced by an axion background and the evolution of the axion string network. Then, I will explain how the authors model the axion string evolution with parameters informed by the AMR simulation (2412.08699) and obtain the constraint. For your information, I leave the abstract of the paper below.
Abstract of the paper: A cosmological network of axion strings may exist in the Universe today. If axion-like particles couple to electromagnetism, such a network induces spatially varying birefringence in the polarization of the cosmic microwave background (CMB), which can be probed by current and next-generation CMB experiments. We calibrate a loop-crossing model against a large-scale adaptive-mesh-refinement (AMR) simulation of axion-string network dynamics in the early Universe and use the calibrated model to predict CMB birefringence from recombination to today. We find that the non-detection of anisotropic birefringence in CMB observations places a strong upper bound on the electromagnetic anomaly coefficient A that enters the axion-photon coupling gaγγ = −Aαem/πfa. A joint analysis of available anisotropic birefringence measurements constrains |A| < 0.24 at 95% C.L., which is independent of the Peccei-Quinn scale fa, assuming that the axions are hyperlight so that the network survives until today. This limit strongly restricts the high-energy embedding of hyperlight axions, excluding the minimal Grand Unified Theory prediction for the electromagnetic anomaly coefficient at high significance. In addition, we discuss implications for an axion string origin of the recently reported evidence for isotropic birefringence.