Minkowski Functionals of the 21 cm Signal as a Probe of Primordial Features

The redshifted 21 cm signal from the cosmic dawn and Epoch of Reionization (EoR) encodes important information about both astrophysical processes and primordial physics, such as inflation. In this work, we use morphological statistics to explore the sensitivity of the 21 cm signal to inflationary features and EoR dynamics simultaneously. Focusing on primordial features from particle production during inflation we generate semi-numerical simulations of the 21 cm signal across redshifts 5 < z < 35, incorporating these features. Using Minkowski Functionals (MFs), we analyze the morphology of 21 cm fields: density, neutral hydrogen fraction, spin temperature, and brightness temperature. We demonstrate that MFs are highly sensitive to both the amplitude and scale of primordial features, capturing rich morphological information. In particular, we show that MFs can robustly identify inflationary features and distinguish them from the standard model. We further explore various EoR scenarios, and demonstrate that combining MFs across redshifts can disentangle the signatures of primordial features from EoR effects. This approach opens new avenues for probing inflation with upcoming 21 cm surveys.

💡 Research Summary

This paper investigates how localized “bump‑like” features in the primordial power spectrum—produced by particle production events during inflation—manifest in the three‑dimensional 21 cm signal from cosmic dawn through the Epoch of Reionization (EoR). Using the semi‑numerical code 21cmFAST v3, the authors generate realizations of the matter density field (δ b), neutral hydrogen fraction (x HI), spin temperature (T S), and the resulting brightness temperature (δT b) over the redshift range 5 ≤ z ≤ 35. The baseline (fiducial) model assumes a simple power‑law primordial spectrum, while the “bump” models introduce an extra term parameterized by an amplitude A I (10⁻⁹–10⁻⁸) and a peak wavenumber k peak (0.4–0.6 Mpc⁻¹). The latter range deliberately includes the “turn‑over” scale (k turn ≈ 0.5 Mpc⁻¹) where previous studies showed the global 21 cm signal to be insensitive to such features.

To capture non‑Gaussian morphology, the authors compute the four Minkowski Functionals (MFs) in three dimensions—volume fraction (V₀), surface area (V₁), mean curvature (V₂), and Euler characteristic (V₃)—for each field as a function of the threshold ν. All fields are smoothed with a 6 Mpc Gaussian kernel before MF evaluation. The analysis proceeds in three stages: (i) quantifying the dependence of each MF on A I and k peak, (ii) comparing MF signatures of bump models against variations in key astrophysical EoR parameters (ionizing efficiency ζ, minimum virial temperature T vir, soft X‑ray luminosity L_X), and (iii) performing a multi‑redshift joint analysis to disentangle primordial from astrophysical effects.

Key findings include: (1) MF amplitudes increase monotonically with A I; the Euler characteristic V₃ shows the strongest response, reflecting the creation or destruction of isolated ionized/neutral regions. (2) When k peak lies near the turn‑over scale, the mean brightness temperature remains virtually unchanged, yet the MFs exhibit distinct deviations, especially in V₂ and V₃, demonstrating that morphological statistics retain sensitivity to scale‑specific power enhancements that are invisible to global averages. (3) The spin‑temperature field contributes uniquely to V₂ at high redshift (z ≈ 15–20), where Ly α coupling and X‑ray heating drive rapid temperature evolution; this makes V₂ a valuable probe of early heating physics. (4) Varying ζ, T vir, or L_X produces MF trends that differ qualitatively from those induced by primordial bumps—for example, increasing ζ uniformly suppresses V₁ and V₃ across thresholds, whereas a bump generates a pronounced peak in V₃ at intermediate ν. (5) A joint likelihood analysis across several redshift slices (z = 8, 12, 16, 20) shows that the degeneracy between bump parameters and astrophysical parameters can be broken: low‑z MFs are dominated by reionization topology, while high‑z MFs retain the imprint of the primordial feature. Bayesian model‑selection tests indicate that, with realistic SKA‑like noise levels, the presence of a bump with A I ≈ 10⁻⁸ can be detected at >3σ confidence even when its global signal is indistinguishable.

The authors conclude that Minkowski Functionals provide a powerful, complementary avenue for probing inflationary physics with upcoming 21 cm surveys. By exploiting the full three‑dimensional morphology of the brightness temperature field, MFs can reveal subtle primordial signatures that evade traditional power‑spectrum or global‑signal analyses, opening a new window onto the physics of particle production during inflation.