Signatures of Primordial Energy Injection from Axion Strings

TL;DR

This paper investigates axion strings, revealing they emit high-energy particles that impact early universe signals, with constraints from CMB data and potential for future detection.

Contribution

It demonstrates, through simulations and analysis, that axion strings produce high-energy Standard Model particles affecting cosmological observations, a novel insight into their signatures.

Findings

CMB data constrains axion decay constants to below 10^{12} GeV

Axion strings emit high-energy SM particles affecting BBN and gamma-ray observations

Future CMB surveys could detect lower decay constant axion strings

Abstract

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 $f_a \lesssim 10^{12}$ GeV, up to model dependence on the ultraviolet completion, for axion masses $m_a \lesssim 10^{-29}$ eV; future CMB surveys could find striking evidence of axion strings with lower decay constants.