Supermassive Primordial Black Holes from a Catalyzed Dark Phase Transition for Little Red Dots

Jinhui Guo, Jia Liu, Masanori Tanaka, Xiao-Ping Wang, Huangyu Xiao

First listed 2026-04-01 | Last updated 2026-04-01

Abstract

JWST has revealed an abundant population of compact, low-metallicity "Little Red Dots" (LRDs) at high redshift, challenging conventional scenarios in which supermassive black holes (SMBHs) grow from stellar-mass seeds. We consider a scenario in which the SMBHs are instead supermassive primordial black holes (SMPBHs), formed directly in a decoupled, subdominant dark sector undergoing a first-order phase transition. Unlike conventional stochastic phase transitions, our mechanism is based on the catalysis by domain walls (DWs): most of the Universe completes the transition rapidly, while rare long-lived false-vacuum domains survive because of DW statistics and collapse into PBHs. This mechanism naturally yields SMPBH seeds with masses up to $M_{\rm PBH}\sim \mathcal{O}(10^{10}) M_\odot$, whose abundance can account for the observed LRD population. It also avoids the usual tensions with phase transition completion, $ΔN_{\rm eff}$, and large curvature perturbations. The dark phase transition simultaneously generates an ultra-low-frequency stochastic gravitational-wave background peaking near the pulsar-timing-array range, providing a test of this dark-sector origin of LRDs.

Short digest

This paper proposes a catalyzed dark phase transition that can produce supermassive primordial black holes in the mass range relevant to little red dot interpretations. The main result is that the allowed parameter space can overlap with the PBH masses and abundances invoked in LRD-motivated seed scenarios while also making gravitational-wave predictions. The paper matters because it links LRD-inspired black-hole seeding ideas to concrete early-universe model building and external observables.

Key figures to inspect

• Figure 1 is the must-see schematic: it explains the domain-wall-catalyzed phase transition setup and the false-vacuum domains where PBH formation can proceed.
• Figure 3 is the core phenomenology plot: it maps PBH mass and abundance against domain-wall density and shows where the model overlaps with the parameter space favored for LRD-motivated PBH seeds.
• Figure 2 shows the time ordering of the transition, the wall-induced catalysis, and when PBH formation is completed, so it is the best figure for understanding the dynamics of the scenario.
• Figure 4 is useful if you care about an external observational check, because it compares the predicted gravitational-wave background against PTA/SKA sensitivities.