2611.12861v1 · First listed 2026-11-10
Little Red Dots from Primordial Black Holes: Non-Gaussianity and Accretion Evade $μ$-Distortion Constraints
Digest
This paper asks whether primordial black holes can provide the massive seeds needed for the JWST little red dot population without violating COBE/FIRAS limits on CMB μ-distortions. Using MCMC fits for broad lognormal and cut-power-law primordial power-spectrum enhancements, including local non-Gaussianity, nonlinear density mapping, and post-formation accretion, the authors find viable LRD-seeding models with |fNL| of order unity and peak power A of order 10^-3 under standard Eddington-limited growth. Both positive and negative fNL branches can reproduce the inferred LRD abundance, but the negative branch is accretion-history dependent and disappears for low black-hole duty cycles (fduty ≲ 0.1). The result reopens a primordial-seed route to early SMBHs and makes future μ-distortion experiments a direct test of that scenario.
Key figures to inspect
- Figure 1. This overview connects the allowed small-scale curvature-power enhancements to the PBH mass range relevant for JWST LRDs, while showing why direct Gaussian seed formation is excluded without either efficient accretion or non-Gaussianity. It also places COBE/FIRAS and projected FOSSIL sensitivity alongside the viable templates.
- Figure 2. This figure gives the clearest direct demonstration of the paper's central non-Gaussianity result: both positive and negative local fNL values can intersect the observed LRD number-density band for fixed broad-spectrum templates.
- Figure 4. This posterior plot isolates the bimodal positive- and negative-fNL solutions in the lognormal model, making clear that the two branches are distinct statistical pathways to the required seed abundance rather than a single broad Gaussian-like solution.
- Figure 7. This duty-cycle comparison is the key caveat figure because it shows how the joint μ-distortion-plus-LRD constraints change with accretion efficiency, including the loss of the negative-fNL preference when fduty becomes small.
- Figure 9. This late synthesis figure compares the lognormal and cut-power-law MCMC posteriors under the same μ+LRD likelihood, showing that the main viable-seed conclusion is not tied to one specific broad power-spectrum parameterization.
Abstract
Recent JWST observations of "Little Red Dots" (LRDs) at $z \gtrsim 4$ suggest the existence of early supermassive black holes (SMBHs) with masses $\gtrsim 10^6\,M_\odot$. Their rapid emergence challenges conventional seeding and growth scenarios based on accretion and mergers of Population III stellar remnants, motivating renewed interest in primordial black holes (PBHs) formed from primordial perturbations. Explaining such massive seeds requires power spectrum enhancements that are constrained by cosmic microwave background (CMB) spectral distortions. In this work we perform the first comprehensive Bayesian exploration of non-Gaussian primordial perturbations capable of producing SMBH seeds while remaining consistent with CMB spectral distortion constraints, and examine the role of accretion history in constraining these models. We perform a suite of Markov chain Monte Carlo analyses of power spectrum enhancements (lognormal and cut-power-law) with varying local non-Gaussianity and accretion histories, satisfying the COBE/FIRAS $μ$-distortion constraint with and without the requirement that the model produces SMBH seeds. We find that both models can seed LRDs and evade $μ$-distortion bounds for $|f_{\rm NL}|= O(1)$ and peak amplitude $A= O(10^{-3})$, when standard Eddington-limited accretion is also incorporated. Interestingly, the results exhibit a preference for both positive and negative values of $f_{\rm NL}$. The preference for negative $f_{\rm NL}$ is strongly dependent on the accretion history, and is removed if the black holes have a small duty cycle, $f_{\rm duty}\lesssim 0.1$. These results demonstrate that the JWST high-$z$ LRD population can be seeded by primordial perturbations while remaining compatible with CMB data, and highlight the importance of next-generation CMB spectral distortion missions to test this hypothesis.
Discussion
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