On the Constraints on Superconducting Cosmic Strings from 21-cm Cosmology

In our latest work, “On the Constraints on Superconducting Cosmic Strings from 21-cm Cosmology,” led by group member T. Gessey-Jones, we delve into one of the most exciting frontiers of cosmology: using faint radio signals from the early Universe to search for new, fundamental physics. This study investigates whether current 21-cm observations can constrain the existence of superconducting cosmic strings, hypothetical relics from a symmetry-breaking phase transition in the very early Universe.

Probing Exotic Physics with Cosmic Dawn Signals

Cosmic strings, if they exist, are one-dimensional topological defects that would stretch across the cosmos. A particularly interesting subclass of these objects are superconducting strings, which can carry enormous electrical currents. As detailed in models like that of Thériault et al. (2021), these strings would lose energy by emitting electromagnetic radiation, creating a pervasive, low-frequency radio background that permeates the early Universe.

This excess radio background would have a profound impact on the 21-cm signal from the “Cosmic Dawn” — the era when the first stars and galaxies were forming. The 21-cm signal is observed as an absorption feature when neutral hydrogen gas is cooler than the background radiation. An extra radio background from cosmic strings would enhance the total radiation temperature, potentially making this 21-cm absorption trough significantly deeper and providing a tantalizing signature of physics beyond the Standard Model.

A Robust Bayesian Analysis of Combined Datasets

The primary challenge in detecting such a signal is distinguishing it from both astrophysical sources and instrumental foregrounds. Our paper addresses this head-on by performing the first joint Bayesian analysis to constrain superconducting cosmic strings using a comprehensive suite of modern cosmological data. We combine:

• The latest 21-cm power spectrum upper limits from the HERA experiment (10.3847/1538-4357/acaf50).
• Global 21-cm signal data from the SARAS 3 experiment (10.1038/s41550-022-01610-5).
• Synergistic constraints from the unresolved cosmic X-ray background.

This work, a companion paper to the astrophysical analysis in Pochinda et al. (2023), leverages our group’s 21cmSPACE simulation code and advanced neural network emulators. This allows us to rigorously account for the complex degeneracies between exotic physics, the uncertain astrophysics of the first galaxies, and foreground contamination.

The Crucial Role of Astrophysical Uncertainties

Our analysis leads to a significant conclusion: in contrast to some previous works (e.g., Brandenberger et al. 2019), we find that current 21-cm measurements are not yet able to place strong constraints on superconducting cosmic strings.

The reason lies in a critical degeneracy with the X-ray emission from the first galaxies. Our models show that if early galaxies were highly efficient X-ray emitters (with an efficiency $f_X \gtrsim 3 \times 10^{40}$ erg s$^{-1}$ M$_{\odot}^{-1}$ yr), the resulting X-ray heating of the intergalactic medium would suppress the 21-cm absorption signal. This astrophysical heating can effectively mimic or mask the signature of cosmic strings, making it impossible to disentangle the two effects with current data. While we find that high string-induced radio backgrounds are disfavoured, no models can be confidently ruled out.

An Optimistic Outlook

Despite this finding, our work charts a clear and optimistic path forward. The discovery of this degeneracy highlights exactly where future observational efforts should be focused. As next-generation experiments like the SKA come online, their increased sensitivity will help break these degeneracies. Furthermore, recent theoretical developments, such as the proposed “soft photon heating” mechanism (Acharya et al. 2023), suggest that cosmic strings may have additional, unique thermal signatures. Such effects could provide the “smoking gun” needed to distinguish them from standard astrophysics, turning a current analytical challenge into a future discovery opportunity.

This research, co-authored by group members and collaborators including H. T. J. Bevins, A. Fialkov, W. J. Handley, and E. de Lera Acedo, exemplifies our commitment to using principled statistical methods to probe the frontiers of cosmology and fundamental physics.

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