Spectrum of radiation from global strings and the relic axion density

We discuss key aspects of the nature of radiation from global strings and its impact on the relic axion density. Using a simple model we demonstrate the dependence on the spectrum of radiation emitted by strings. We then study the radiation emitted by perturbed straight strings paying particular attention to the difference between the overall phase of the field and the small perturbations about the string solution which are the axions. We find that a significant correction is required to be sure that one is analyzing the axions and not the self-field of the string. Typically this requires one to excise a sizeable region around the string - something which is not usually done in the case of numerical field theory simulations of string networks. We have measured the spectrum of radiation from these strings and find that it is compatible with an exponential, as predicted by the Nambu-like Kalb-Ramond action, and in particular is not a hard'' spectrum often found in string network simulations. We conclude by attempting to assess the uncertainties on relic density and find that this leads to a range of possible axion masses when compared to the measured density from the Cosmic Microwave Background, albeit that they are typically higher than what is predicted by the Initial Misalignment Mechanism. If the decay is via a soft spectrum’’ from loops produced close to the backreaction scale we find that $m_{\rm a}\approx 160,μ{\rm eV}$ and a detection frequency $f\approx 38,{\rm GHz}$. If axions are emitted directly by the string network, and we use emission spectra reported in field theory simulations, then $m_{\rm a}\approx 4,μ{\rm eV}$ and $f\approx 1,{\rm GHz}$, however this increases to $m_a \approx 125,μ{\rm eV}$ and $f\approx 30,{\rm GHz}$ using our spectra for the case of an oscillating string. In all scenarios there are significant remaining uncertainties that we delineate.

💡 Research Summary

The paper investigates how the spectrum of axions radiated by global cosmic strings influences the relic axion density and the inferred axion mass. After a brief review of axion physics, the authors distinguish two historically proposed scenarios for the string‑radiation spectrum. Scenario A, based on the Kalb‑Ramond action, predicts a “soft” spectrum in which most of the radiated energy is emitted at frequencies comparable to the fundamental mode of string perturbations, leading to an exponential fall‑off at higher frequencies. Scenario B, derived from many field‑theory simulations, assumes a “hard” spectrum with a power‑law tail (q≈1) that extends up to the string core scale, producing a logarithmic divergence in the total emitted power.

The authors then develop a semi‑analytic framework for the relic axion density that explicitly incorporates the spectral shape. They model the long‑string network with the one‑scale model, defining the string energy density ρ∞=ζ µ t⁻², where µ≈π f_a² log(Δ/δ) includes the large logarithm of the ratio between the horizon scale Δ and the string core width δ. The chopping efficiency c and the rms velocity ⟨v²⟩ determine how much energy is transferred from the network to loops or directly to axions.

Two emission channels are considered: (i) axions emitted directly by the long‑string network, and (ii) axions produced by loops that are chopped off the network. Loops are assumed to have size ℓ=α t at formation, radiate power P=κ µ=Γ_a f_a², and decay after a lifetime τ=α/κ t. The loop distribution n(ℓ,t)=ν t⁻³⁄²(ℓ+κt)⁻⁵⁄² is inserted into an integral over time and loop size to obtain the full spectral energy density ∂ρ_a/∂ω. The spectrum is encoded in a normalized function g(z) (z≡ω ℓ/2π) which the authors treat in two forms: a power‑law g(z)∝z⁻ᵠ with a hard cutoff, and an exponential g(z)∝exp