Early formation and rapid growth of supermassive black holes

Determine the physical mechanisms that enable supermassive black holes to form at early cosmic times and grow to extremely large masses, consistent with current observational constraints.

Background

In discussing the entropy budget of the observable universe, the authors note that supermassive black holes (SMBHs) dominate the non-horizon entropy. Typical SMBHs have masses around 108 solar masses and entropies near 1093, contributing about 10105 to the total entropy.

The existence of very massive SMBHs at early times remains puzzling, and understanding their formation and growth is identified as a major open problem in astrophysics.

References

How SMBHs formed so early and grew so huge is still one of the big open questions in astrophysics.

— Gibbons-Hawking Entropy and BMN Strings  (2511.08213 - Huang, 11 Nov 2025) in Section 2.2 (Other entropies)

A number of physical questions also remain open. One is the strong $z\approx4$ clustering signal of \citet{shen2007}: whether it reflects an observational systematic or a genuine feature to be reconciled with the rapid BH buildup at higher redshift.

— BAQARO: Tracing Stochastic Black Hole Growth Histories and Quasar Lightcurves in a Cosmological Context  (2609.12061 - Pizzati et al., 10 Sep 2026) in Section 6, “Summary”

We caution, however, that our conclusions about the lowest-mass BHs are limited by resolution: because our merger trees do not extend below the resolution limit of $M_\mathrm{halo}\approx10{10.5}\,$, we cannot rule out a more important merger channel in the early growth of low-mass BHs -- exactly the regime where \citet{pacucci2020} report mergers to take over ($M_\mathrm{BH}\lesssim10{5}\,$, $z\gtrsim5.5$) and where merger-driven seed growth is expected in dense, high-$\sigma$ environments \citep{bhowmick2024}.

— BAQARO: Tracing Stochastic Black Hole Growth Histories and Quasar Lightcurves in a Cosmological Context  (2609.12061 - Pizzati et al., 10 Sep 2026) in Section 4.4, “The relative roles of mergers and accretion”

The growth and evolution of black holes in the Universe remain unknown until now.

— Bridging Between Statistical Mechanics and Black Hole Evolution: Theoretical Formalism  (2609.16534 - Mukherjee, 15 Sep 2026) in Section 2, Motivation

One of the open questions in the field is how these massive BHs form and evolve into present-day systems.

— Extreme Enrichment at Cosmic Dawn: Three FeII-Strong Quasars at z > 6 from the JWST Aether survey  (2609.19459 - Gloudemans et al., 16 Sep 2026) in Section 3, subsection “Black hole mass and Eddington ratio”

Although CIELO does not model SMBHs, this potential connection motivates future work exploring whether stellar knots trace the preferential sites for early SMBH growth.

— The properties of central stellar knots embedded in galactic bulges of CIELO simulations  (2609.11465 - Acosta-Tripailao et al., 10 Sep 2026) in Section 5, Conclusions (Section \ref{sec:conc})

The origin of supermassive black holes, especially those already present at high redshift, remains an open problem.

— Formation of Supermassive Black Hole Seeds from Vacuum-Dependent Thermal Dark Matter  (2610.08051 - Kawasaki et al., 6 Oct 2026) in Abstract; Section 1, Introduction

Distinguishing between these possibilities remains a central unresolved problem in modern astrophysics.

— Widespread steep X-ray spectra of luminous $z\simeq5.85-7.5$ quasars and their implications for future X-ray surveys  (2610.10522 - Tortosa et al., 7 Oct 2026) in Section 1, Introduction

This conclusion applies specifically to this massive halo in a $5\sigma$ overdense region, and should not be interpreted as a general result for the MBH population in the early Universe. A statistical study of MBH evolution across a wide range of halo environments is therefore needed to fully understand the impact of seeding prescriptions on the MBH population, which we defer to future work (ACORN II).

— ACORN I. Massive Black Hole Seeding and Tidal Disruption Events from Star Clusters in Cosmological Simulations  (2610.12398 - Zhou et al., 8 Oct 2026) in Section Mass Evolution of the Central MBH