Axion strings are horizon-size topological defects that may be produced in
the early Universe. Ultra-light axion-like particles may form strings that
persist to temperatures below that of big bang nucleosynthesis. Such strings
have been considered previously as sources of gravitational waves and cosmic
microwave background (CMB) polarization rotation. In this work we show, through
analytic arguments and dedicated adaptive mesh refinement cosmological
simulations, that axion strings deposit a sub-dominant fraction of their energy
into high-energy Standard Model (SM) final states, for example, by the direct
production of heavy radial modes that subsequently decay to SM particles. This
high-energy SM radiation is absorbed by the primordial plasma, leading to novel
signatures in precision big bang nucleosynthesis, the CMB power spectrum, and
gamma-ray surveys. In particular, we show that CMB power spectrum data
constrains axion strings with decay constants fa​≲1012 GeV, up to
model dependence on the ultraviolet completion, for axion masses ma​≲10−29 eV; future CMB surveys could find striking evidence of axion strings
with lower decay constants.Comment: 30 pages, 20 figures, Supplementary Animations at
https://goo.by/qHk9d, Video Abstract at https://youtu.be/Ae4AR46qKn