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

TL;DR

This work demonstrates that Minkowski Functionals applied to three-dimensional 21 cm maps are highly sensitive to primordial bump-like features sourced by inflation, capturing their amplitude and scale across the cosmic dawn and Epoch of Reionization. By coupling semi-numerical simulations with MF analyses of density, spin temperature, neutral hydrogen, and brightness temperature fields, the authors show that these morphology-based statistics can identify inflationary signatures even at the turnover scale where global signals are degenerate with fiducial models, and can disentangle primordial effects from astrophysical parameters by combining information across redshift. The study introduces derived amplitude and shape metrics from MF curves and quantifies their discriminating power via multiple realizations, highlighting redshift windows where primordial features are separable from EoR variations and emphasizing the value of multi-statistic, tomographic analyses for upcoming SKA data. Overall, the MF framework offers a complementary, non-Gaussian probe of inflationary physics with practical pathways toward Bayesian inference and robust inflation constraints from 21 cm cosmology.

Abstract

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.

Figures (14)

• Figure 1: The primordial power spectrum with a bump-like primary feature at $k_{\rm peak} = 1\,{\rm Mpc}^{-1}$ arising from particle production during inflation.
• Figure 2: Redshift dependence of the mean values of different terms that contribute to the mean brightness temperature (see eq. \ref{['eq:Tb_exp']}). The spatially constant quantity $C$ is defined by eq. \ref{['eq:Cdef']}.
• Figure 3: (a) Lightcones of $\delta_b$, $T_S$, $x_{\rm HI}$, and $\delta T_b$ (rows) for the fiducial model; colours indicate the field values as shown by the respective colorbars. (b) Minkowski functionals $V_0$, $V_1$, $V_2$, and $V_3$ (rows) as functions of threshold $\nu$ for the same fields as in panel (a) (columns), shown at selected redshifts indicated by different line styles in the legend.
• Figure 4: The simulated lightcones of all the fields for the fiducial and bump models.
• Figure 5: The Minkowski functionals for the density field $\delta_b(\vec{x})$ for models involving primordial bump-like features. The upper and lower panels show $V_i$ at redshifts $z = 34.68$ and $z=7.57$, respectively. The fiducial model is indicated by solid black line.