Primordial Black Holes Significantly Impact Global 21-cm Signal at Redshift Z ~ 10.4

The search for information about the Universe’s first luminous sources has long focused on early star-forming galaxies, but recent observations from the James Webb Space Telescope suggest active galactic nuclei (AGNs) existed as early as redshift 10-10.4. Atrideb Chatterjee from the Kapteyn Astronomical Institute, University of Groningen, and colleagues explore the potential contribution of these AGNs to the 21-cm global signal, a key indicator of the early Universe. Their research investigates the possibility that these AGNs originate from primordial black holes (PBHs), utilising an analytical model constrained by existing cosmological data. This work demonstrates that PBHs could significantly alter the observed evolution of the 21-cm signal, offering a new avenue for understanding the conditions and sources present in the very early Universe.

Early AGNs and the 21cm Signal

The 21-cm global signal, a crucial source of information regarding the earliest luminous objects in the Universe, has long been interpreted using models that primarily feature star-forming galaxies0.4, prompting a reassessment of prevailing cosmological assumptions. This discovery necessitates investigation into the potential contribution of these AGNs to the observed 21-cm signal, particularly considering alternative formation pathways beyond standard galactic evolution. Scientists demonstrate that these early AGNs, potentially originating from primordial black holes (PBHs), can significantly alter the predicted evolution of the global signal.

The research team employed an analytical model of PBHs, carefully constrained by existing cosmological and astrophysical data, to explore this possibility. By assuming that a portion of these early AGNs are seeded by PBHs, the study unveils a mechanism through which these exotic objects can exert a substantial influence on the thermal and ionization state of the intergalactic medium (IGM) at very early epochs. This effect is particularly pronounced when galaxy formation is still in its nascent stages, a scenario not typically accounted for in current 21-cm signal forecasts. Experiments show that incorporating PBH-seeded galaxies into models of the 21-cm signal provides a more complete picture of the early Universe.

The study establishes a theoretical framework comprising both standard star-forming galaxies and a population of PBH-seeded systems. A semi-analytical model for star-forming galaxies, consistent with recent high-redshift observations, is combined with a model for PBH accretion and growth. This allows scientists to accurately calculate the star formation rate density and the resulting number of photons emitted at various frequencies and redshifts. The research proves that the inclusion of PBH-seeded galaxies can significantly modify the predicted 21-cm signal, potentially offering a new avenue for interpreting observations from ongoing and future experiments.

This work opens exciting possibilities for understanding the nature of the first luminous sources and the conditions of the early Universe. Ongoing experiments such as SARAS-3, LEDA, SCI-HI, BIGHORNS, REACH, and CTP are poised to detect the global 21-cm signal, and the findings presented here provide crucial theoretical guidance for interpreting the data. By considering the contribution of PBH-seeded AGNs, scientists can refine their models and potentially unlock new insights into the formation and evolution of the cosmos. The research adopts a standard ΛCDM cosmological model with specific parameter values: ΩΛ = 0.685, Ωm = 0.315, Ωb = 0.049, H0 = 67km s−1 Mpc−1, ns = 0.96, and σ8 = 0.81, ensuring consistency with the latest Planck Collaboration data.

Primordial Black Holes and the 21-cm Signal

The study investigates the contribution of Active Galactic Nuclei (AGNs), potentially seeded by Primordial Black Holes (PBHs), to the 21-cm global signal, a key indicator of the Universe’s first luminous sources0.4. The work employs an analytical model of PBHs, constrained by existing cosmological and astrophysical limits, to assess their impact on the evolution of the 21-cm signal at different redshifts. This approach allows scientists to explore a previously under-considered influence on the early Universe’s IGM thermal and ionization state.

Scientists developed a semi-analytical model incorporating both standard star-forming galaxies and PBH-seeded systems to determine the overall effect on the 21-cm signal. The star-forming galaxy component utilises a Star Formation Rate Density (SFRD) calculated from halo mass functions, specifically the Tinker et al. (2008) function implemented via the colossus package. This model assumes star formation occurs over the dynamical timescale, tdyn(z), and establishes a minimum halo mass, Mmin(z), corresponding to a virial temperature of 104 K, consistent with atomic cooling scenarios. The team then incorporated a star formation efficiency, f⋆(Mh), defined by parameters {0.16, 1011.7, 0.9, 0.65} to align predicted UV Luminosity Functions with high-redshift JWST observations.

The number of photons emitted per unit time at a given frequency and redshift, nν, is then calculated using the SFRD, enabling a quantitative assessment of the 21-cm signal contribution. Crucially, this work diverges from previous studies focused on explaining anomalies detected by the EDGES and SARAS-3 experiments, which have since been refuted. Instead, the research focuses on the fundamental imprint of PBH-seeded galaxies on the global 21-cm signal. The methodology leverages a more fundamental PBH model, based on Dayal & maiolino (2025), and ensures consistency with established cosmological parameters, ΩΛ = 0.685, Ωm = 0.315, Ωb = 0.049, H0 = 67km s−1 Mpc−1, ns = 0.96, and σ8 = 0.81, as defined by the Planck Collaboration (2020). This innovative approach is particularly relevant given the advent of new experiments designed to detect the global 21-cm signal, including SARAS-3, LEDA, SCI-HI, BIGHORNS, REACH, and CTP, all of which will benefit from a more complete understanding of the early Universe’s complex radiative landscape. By meticulously modelling both star-forming galaxies and the potential contribution of PBH-seeded systems, the study pioneers a more nuanced approach to interpreting future 21-cm observations and unlocking the secrets of the first luminous epochs.

Active Galactic Nuclei and the 21-cm Signal

Scientists have achieved a significant breakthrough in modelling the 21-cm global signal, traditionally understood through the lens of early star-forming galaxies0.4, prompting a re-evaluation of their contribution to this crucial cosmological signal. This work investigates the impact of AGNs, hypothesised to be seeded by Primordial Black Holes (PBHs), on the evolution of the 21-cm signal using an analytical PBH model consistent with established cosmological parameters. The research team employed a Hubble constant of H0 = 100h km s−1 Mpc−1 with h = 0.67, a spectral index of ns = 0.96, and a normalisation of σ8 = 0.81, as defined by the Planck Collaboration.

Their theoretical framework incorporates both standard star-forming galaxies and a population of PBH-seeded galaxies, allowing for a detailed comparison of their respective influences on the 21-cm signal. The Star Formation Rate Density (SFRD) was calculated at a given redshift z using a model incorporating halo mass, dynamical time scale, and halo mass function, ultimately determining the number of photons produced per unit time. Experiments revealed that the number of ionising photons produced intrinsically within galaxies at redshift z is computed as nion(z), and the number escaping to reionise the Intergalactic Medium (IGM) is calculated as nSF ion,esc, with an ionizing escape fraction fixed at fesc = 0.2. Measurements confirm that the inclusion of PBH-seeded galaxies, modelled with a log-normal mass function, significantly alters the predicted 21-cm signal.

The team determined a normalizing constant, κ, by matching the observed mass function of 10−5.27cMpc−3 at z ~ 10, assuming an average seed mass of 103.65M⊙ and a standard deviation of σ = 0.7. The breakthrough delivers a bolometric luminosity calculation for PBH-seeded galaxies, Lbol = εrMac BHc2 ∆t L⊙, where εr is the radiative efficiency and Mac BH is the accreted black hole mass. Tests prove that the photon production rate from these galaxies, nPBH ν, is directly linked to the luminosity and mass function of the PBHs, offering a new pathway to interpret the 21-cm signal and potentially constrain the properties of primordial black holes in the early universe. This work opens exciting possibilities for understanding the first luminous sources and the reionisation epoch.

Primordial Black Holes Explain 21-cm Signal Evolution

This research presents a model incorporating primordial black holes alongside standard star-forming galaxies to explain the 21-cm global signal, a key indicator of the early Universe. By considering active galactic nuclei seeded by these primordial black holes, the study demonstrates a potentially significant influence on the observed redshift evolution of the signal, moving beyond traditional models focused solely on early galaxies. The analytical framework employed allows for consistent modelling within existing cosmological and astrophysical constraints, offering a plausible mechanism for understanding the early Universe. The authors acknowledge limitations stemming from the simplified assumptions within their star-forming galaxy model and the data-driven approach to star formation efficiency, noting that a more physically motivated model could refine the results. Future research could focus on incorporating more complex astrophysical processes and exploring a wider range of primordial black hole parameters to further constrain their impact on the 21-cm signal and the early Universe.