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

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.

Figures (9)

• Figure 1: Schematic of a DW-catalyzed phase transition. Black curves denote DWs, and green circles denote nucleation bubbles. The red dashed circle denotes a FVD not penetrated by any DW, where no bubble nucleation occurs.
• Figure 2: Time evolution of the energy densities for FOPTs catalyzed by DWs for benchmark A in Eq. \ref{['eq:BP_A']}. Each energy density is normalized to the initial SM radiation energy density $\rho_{{\rm R},n}$. In the red-shaded region, PBH formation has been completed in the FVDs when $\Delta V\gtrsim \rho_{{\rm R}}(t_{\rm PBH})$.
• Figure 3: Prediction for the PBH mass and fraction with different DW densities (green dashed lines) for benchmark A. The orange band indicates $T_n \in [0.2\,{\rm MeV},\,2\,{\rm MeV}]$, which yields $\mathcal{O}(10^{6}-10^{8})\,M_\odot$ PBHs relevant to LRD simulations in Ref. Zhang:2025oyl. Above the red line, corresponding to $n_{\rm DW} H_n^{-3} \leq 0.05$, the PT completion condition in Eq. \ref{['eq:Nbubble_DW']} cannot be satisfied. The blue region shows the LRD-favored PBH parameter space DeLuca:2025nao, while the gray region is excluded by existing constraints from cosmic structures Carr:2018ridCarr:2020erq. For benchmark A, the cosmological constraints do not visibly cut into the parameter space, but become relevant for larger $\Delta V$ and smaller $T_n$, as shown in App. \ref{['sec:details-plots']}.
• Figure 4: Prediction for the GW spectrum for two $T_{n}$ values with $n_{\rm DW}H_{n}^{-3} = 0.09$ in benchmark A. For the bubble-wall velocity, we take $v_{w} = 0.95$. The red and blue solid lines are the expected sensitivity curves of SKA Carilli:2004nx and IPTA Hobbs:2009yy. The gray shaded region indicates the signal observed by NANOGrav NANOGrav:2023gorNANOGrav:2023hvm.
• Figure 5: PBH formation probability $P_{\rm PBH}$ contours plots as functions of $\beta/H_n$ and $t_s/H_n^{-1}$ for each $\alpha$ value. We take $\rho_{{\rm DR},n}/\rho_{{\rm R},n} = 2\times 10^{-5}$ in this plot.