Cosmic Reionization and 21 cm Signal Analysis

Cosmic reionization marks the epoch in the early universe when the first stars, galaxies and accreting black holes emitted ultraviolet photons that ionised the surrounding hydrogen, transforming the intergalactic medium from a neutral to an ionised state. The redshifted 21 cm line of neutral hydrogen provides a direct probe of this phase transition by encoding fluctuations in brightness temperature that trace both the density field and the history of ionisation and heating. Analyses of the 21 cm signal encompass measurements of the global (sky-averaged) signal and its spatial power spectrum, revealing the timing, morphology and topology of ionised regions. These studies face formidable challenges, including the need to suppress bright astrophysical foregrounds, to calibrate instrument response with high fidelity, and to recover faint spectral features in the presence of radio-frequency interference and data gaps. Progress in simulation and analytical modelling has refined predictions of the 21 cm power spectrum as a function of scale and redshift, while ongoing and planned radio interferometers aim to translate these predictions into robust constraints on the properties of the first luminous sources, the evolution of large-scale structure and fundamental cosmological parameters.

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Cosmic Reionization and 21 cm Signal Analysis publication trend

The graph below shows the total number of articles in cosmic reionization and 21 cm signal analysis across all publications each year (not limited to Nature Index journals).

Technical terms

Epoch of Reionization: The period when the first astrophysical sources ionised the neutral hydrogen in the intergalactic medium.

21 cm line: The hyperfine transition of neutral hydrogen at a rest wavelength of 21 cm, redshifted to low radio frequencies by cosmic expansion.

Power spectrum: A statistical measure of the amplitude of spatial fluctuations in brightness temperature as a function of scale.

Foregrounds: Astrophysical radio emissions, such as Galactic synchrotron and extragalactic point sources, that are orders of magnitude brighter than the 21 cm signal.

Gaussian-constrained realisation: A technique for reconstructing missing or uncertain data by imposing statistical properties consistent with Gaussian processes.

Gibbs sampling: A Markov chain Monte Carlo algorithm used to sample from complex joint probability distributions, enabling marginalisation over nuisance parameters.

Bayesian inference: A statistical approach that updates prior knowledge of model parameters through the likelihood of observed data to derive posterior constraints.

References

1. Statistical Recovery of 21 cm Visibilities and Their Power Spectra with Gaussian-constrained Realizations and Gibbs Sampling. The Astrophysical Journal Supplement Series (2023).
2. Hydrogen Epoch of Reionization Array (HERA) Phase II Deployment and Commissioning. Publications of the Astronomical Society of the Pacific (2024).
3. First upper limits on the 21 cm signal power spectrum from cosmic dawn from one night of observations with NenuFAR. Astronomy & Astrophysics (2024).