Revisiting constraints on superconducting cosmic strings in light of Dark Ages global 21-cm signal

TL;DR

The study investigates energy injection from decaying superconducting cosmic strings (SCS) into the intergalactic medium during the dark ages, accounting for both nonthermal radio and ionizing photons. By modeling the resulting heating and ionization with modified Peebles equations and a detailed energy-deposition framework, it derives an upper bound on the decay efficiency $g(I,G\mu_s) \lesssim 5.1\times10^{14}\ \mathrm{GeV^2}$ from the $z\sim89$ 21-cm absorption level. The work shows that exploiting the astrophysically clean dark ages signal can yield stronger, less model-dependent constraints on SCS than previous analyses that focused on low-frequency radiation or CMB distortions, while highlighting a degeneracy between loop current $I$ and string tension $G\mu_s$. It also discusses the implications for future lunar/space-based 21-cm experiments and the importance of accounting for cosmological parameter uncertainties and foregrounds in deriving robust bounds. Overall, the paper reinforces the dark ages 21-cm signal as a powerful probe of exotic physics, notably SCS, and maps the viable SCS parameter space under current observational capabilities.

Abstract

The Superconducting Cosmic Strings (SCS) are a special case of cosmic strings that have a core carrying a charged field. When SCS passes through magnetized regions, the charged particles in the string experience a Lorentz force, which can produce radiation on the entire electromagnetic spectrum. This radiation can inject energy into the surrounding plasma, resulting in a modification of the thermal and ionization evolution of the intergalactic medium (IGM) and, subsequently, the global 21-cm signal. The signatures of SCS in the post-recombination era have been primarily studied in the low-frequency (radio) regime, which does not impact the state of the IGM. In this work, we study the effect of decaying SCS on the dark ages global 21-cm signal $(δT_b)$, considering both the ionizing and radio radiation. The dark ages signal can provide pristine cosmological information free from astrophysical uncertainties, as the universe was primarily homogeneous during this era in the absence of baryonic structure formation. Considering a change in the $δT_b$ at redshift $z\sim 89$ from the $Λ\rm CDM$ framework, we derive an upper bound on the decay efficiency parameter, $g\equiv g(I,~Gμ_s)$, to be $\lesssim 5.1\times10^{14}\, \rm GeV^2$, where, $I$ and $Gμ_s$ represent the loop current and string tension of SCS, respectively.

Figures (4)

• Figure 1: Represents the effect of decaying superconducting cosmic string on IGM temperature ($T_{gas}$) with redshift ($z$). The grey and orange dashed lines depict the CMB temperature and IGM gas temperature evolution without X-ray heating in the absence of SCS radiation, while the solid black line represents the IGM gas temperature evolution with X-ray heating in the absence of SCS radiation. In the left panel [Fig. (\ref{['p2a']})], we vary the dimensionless string tension for a fixed string loop current $I=10^{4}$ GeV. In the right panel [Fig. (\ref{['p2b']})], we fixed $G\mu_s=10^{-15}$ and vary the cosmic string current.
• Figure 2: The evolution of the differential brightness temperature $\delta T_{b}$ in the presence of superconducting cosmic strings. In both the figures, the black solid line represents $\delta T_b$ from the standard $\Lambda\rm CDM$ framework. In Fig. (\ref{['t21a']}) we consider strings with loop current $I= 10^3$ GeV and $G\mu_s = 5\times 10^{-16}$ and in Fig. (\ref{['t21b']}) we consider strings with loop current $I= 10^6$ GeV and $G\mu_s = 5\times 10^{-16}$. The cyan-dashed and blue solid lines represent $\delta T_b$ when we separately consider radio photons (due to first term on the RHS of Eq. \ref{['eq: cs_tot_energy']}) and ionizing photons from SCS radiation (due to third term on the RHS of Eq. \ref{['eq: cs_tot_energy']}), respectively. The orange dashed line represents $\delta T_b$ evolution on considering the entire photon spectrum emitted from the strings simultaneously (due to the first and third term on the RHS of Eq. \ref{['eq: cs_tot_energy']})).
• Figure 3: Constraints on the parameter $g$ as a function of the 21-cm differential brightness temperature $\delta T_b$ at $z\sim89$. Here the parameter $g= I^2 (\Gamma~G\mu_s)^{-7/6}$. The parameter value above $g\gtrsim5.1\times10^{14}\, \rm GeV^2$ is excluded due to emission signal at cosmic dawn, and below $g\lesssim2.2\times10^{12}\, \rm GeV^2$, the effect of decaying SCS on 21-cm signal is no longer detectable.
• Figure 4: Constraints on the cosmic string parameter space from the 21-cm signal of the dark ages. The dashed orange line indicates the critical current corresponding to $G\mu_s$. The black and brown dashed lines are the constraints from Gessey-Jones et al. (2024) at $1\sigma$ and $2\sigma$, respectively 10.1093/mnras/stae512. The grey-shaded regions indicate the excluded region by Gessey-Jones et al. (2024) 10.1093/mnras/stae512. The sky-blue shaded region indicates the excluded region by the pulsar timing constraint from gravitational radiation obtained by Ref. Miyamoto:2012ck. The cyan and green shaded regions were obtained by the COBE/FIRAS and PIXIE measurements at the $2\sigma$ limit Cyr:2023iwu.