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

TL;DR

This paper proposes a dark-sector phase transition catalyzed by domain walls that directly forms supermassive primordial black holes, explaining high-redshift compact objects and predicting a detectable gravitational-wave background.

Contribution

It introduces a novel mechanism for SMBH seed formation via catalyzed dark phase transition, avoiding common cosmological tensions and linking to gravitational wave signals.

Findings

SMPBH masses up to 10^10 solar masses can form naturally.

The mechanism accounts for the observed population of high-redshift LRDs.

Predicts an ultra-low-frequency gravitational-wave background detectable by pulsar timing arrays.

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, $\Delta N_{\rm eff}$, 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.