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Impact of primordial black holes on the formation of the first stars and galaxies

This study combines semi-analytical analysis and cosmological simulations to demonstrate that while stellar-mass primordial black holes likely have a minor impact on primordial star formation, supermassive primordial black holes could act as seeds for the early formation of massive galaxies, addressing recent observational anomalies.

Original authors: Boyuan Liu, Volker Bromm

Published 2026-04-24

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CC BY 4.0

Original authors: Boyuan Liu, Volker Bromm

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Ghosts in the Machine

Imagine the early universe as a giant, empty construction site just after the Big Bang. In the standard story (the one we usually tell), the builders are invisible "dark matter" particles. They slowly clump together, creating invisible scaffolding. Once the scaffolding is strong enough, regular gas falls in, cools down, and lights up as the first stars.

But this paper asks a "what if" question: What if some of that dark matter isn't made of tiny particles, but of tiny, ancient black holes?

These are called Primordial Black Holes (PBHs). They formed in the very first split-second of the universe. The authors, Boyuan Liu and Volker Bromm, want to know: If these black holes exist, how do they change the construction of the first stars and galaxies?

They look at two main scenarios:

Stellar-mass PBHs: Black holes about the size of our Sun (10 to 100 times heavier).
Supermassive PBHs: Black holes millions or billions of times heavier than the Sun.

Scenario 1: The Tiny Black Holes (Stellar-Mass)

Think of the early universe as a quiet forest. In the standard model, the trees (stars) grow slowly because the soil (dark matter) is spread out evenly.

The Two Opposing Forces
If we sprinkle tiny black holes into this forest, two things happen at the same time, fighting against each other:

The "Seed" Effect (Accelerator):
Imagine dropping a heavy rock into soft mud. The mud immediately piles up around the rock. Similarly, a tiny black hole acts like a gravity magnet. It pulls gas and dark matter toward it faster than usual. This should help stars form sooner and in more places. It's like having a head start in a race.
The "Heater" Effect (Brake):
But here's the catch: As gas falls into these black holes, it gets superheated, like water boiling in a pot. This heat pushes the gas away. To form a star, gas needs to be cold and calm so it can collapse. If the black holes heat the gas too much, the gas stays puffy and refuses to collapse. It's like trying to build a sandcastle while someone is blowing a leaf blower at it.

The Verdict: A Tie
The authors ran complex computer simulations to see which force wins.

The Result: For the tiny black holes allowed by current observations (which can only make up a tiny fraction, less than 1%, of all dark matter), the two effects cancel each other out.
The Analogy: It's like adding a turbocharger to a car (the seed effect) but also putting a heavy brake on the wheels (the heating effect). The car doesn't go much faster or slower than normal.
Conclusion: The first stars (Population III) still form, but their numbers and properties don't change drastically. The universe looks mostly the same as the standard model predicts.

One Cool Twist:
While they don't change the number of stars much, these black holes might act as "incubators" for the first supermassive black holes. If a tiny black hole sits right in the middle of a collapsing gas cloud, it might eat its way up to become a giant black hole seed very quickly.

Scenario 2: The Giant Black Holes (Supermassive)

Now, imagine the construction site has giant, pre-built concrete pillars (Supermassive PBHs) instead of just scattered rocks.

The "Seed" Effect on Steroids
If these giant black holes existed, they wouldn't just be a head start; they would be the foundation of entire skyscrapers.

The Analogy: In the standard model, you have to build a skyscraper brick by brick from the ground up. With Supermassive PBHs, you drop a massive foundation in the ground, and the building grows around it instantly.
The Problem: Recent telescopes (like the James Webb Space Telescope, or JWST) have found galaxies that are surprisingly massive and old, appearing very early in the universe's history. Standard physics says these shouldn't exist yet; they should still be under construction.
The Solution: If Supermassive PBHs existed, they could explain these "early giants." They would act as seeds that allowed massive galaxies to form much faster than the standard model allows.

The Catch:
There is a price to pay. If there are too many of these giant black holes, they would mess up other parts of the universe (like the cosmic microwave background radiation). The authors found a "Goldilocks zone"—a specific range of size and number where these black holes could explain the JWST findings without breaking the laws of physics. However, this requires some very exotic ways for the universe to have started (non-standard scenarios).

Summary: What Does This Mean for Us?

For the First Stars: If the universe is filled with small primordial black holes (the kind we might detect via gravitational waves), they probably didn't change the story of the first stars much. They tried to speed things up, but they also tried to heat things up, and the result was a wash.
For the First Galaxies: If the universe is filled with giant primordial black holes, they could be the secret ingredient that allowed the massive, ancient galaxies we are seeing with JWST to exist.
The Future: The authors are calling for more detailed computer simulations. We need to understand exactly how these black holes interact with gas, dust, and light.

The Bottom Line:
Primordial black holes are like hidden variables in the universe's recipe. If they are small, they barely change the taste of the dish. But if they are huge, they might be the reason the universe looks so different from what we expected, solving the mystery of why we see "grown-up" galaxies when the universe was still a toddler.

1. Problem Statement

Recent gravitational wave (GW) observations of binary black hole mergers and the stochastic gravitational wave background (SGWB) have renewed interest in Primordial Black Holes (PBHs) as a potential component of Dark Matter (DM). While PBHs in the stellar-mass ( $\sim 10-100 M_\odot$ ) and supermassive ( $\sim 10^7-10^{11} M_\odot$ ) regimes could explain these observations even if they constitute only a small fraction ( $f_{\text{PBH}} \lesssim 1\%$ ) of the total DM, their impact on the early universe remains unclear.

The central problem addressed is how PBHs influence the formation of the first stars (Population III) and galaxies during the Cosmic Dawn ( $z \sim 10-30$ ). This involves balancing two competing physical mechanisms:

Acceleration of Structure Formation: PBHs act as gravitational seeds, accelerating the collapse of matter via "seed" effects (individual halos) and "Poisson" effects (statistical clustering).
Gas Heating via Accretion Feedback: PBHs accrete gas, converting gravitational energy into radiation and heat, which can suppress gas cooling and delay or prevent star formation.

The authors aim to quantify whether these effects significantly alter the cosmic stellar mass density, the internal structure of star-forming halos, or the formation of massive black hole seeds, particularly in light of recent JWST observations of unexpectedly massive high-redshift galaxies.

2. Methodology

The authors employ a hybrid approach combining semi-analytical modeling and cosmological hydrodynamic simulations (using codes like GIZMO and AREPO).

Theoretical Framework (Linear Perturbation Theory):
- They derive the linear power spectrum of DM density fluctuations, incorporating both standard adiabatic perturbations and additional isocurvature perturbations arising from the discrete nature of PBHs.
- They introduce a heuristic correlation term to account for the interplay between adiabatic clustering and Poisson noise, which improves agreement with simulations in the intermediate mass range.
- They utilize the Press-Schechter (PS) formalism to estimate halo abundances, acknowledging its limitations regarding non-Gaussianity in PBH cosmologies.
Initial Conditions for Simulations:
- They develop a numerical scheme to generate initial conditions for $N$ $N$ -body simulations that include PBHs. This involves:
  - Placing PBHs based on a Poisson distribution superimposed on the DM density field.
  - Applying displacement and velocity fields to DM/gas particles to capture both the collective "Poisson" effect (large scales) and the non-linear "seed" effect (small scales near individual PBHs).
  - Implementing truncation prescriptions to handle non-linear dynamics near PBHs while maintaining linearity elsewhere.
Accretion and Feedback Modeling:
- They model PBH accretion using the Bondi-Hoyle-Lyttleton formula.
- They calculate the heating rate of the intergalactic medium (IGM) and interstellar medium (ISM) based on the accretion luminosity, accounting for radiative efficiency transitions (thin disks to advection-dominated flows).
- They evaluate the impact of this heating on the molecular hydrogen ( $H_2$ ) cooling threshold ( $M_{\text{mol}}$ ) required for Pop III star formation.
Simulation Scope:
- Stellar-mass PBHs: Focus on $m_{\text{PBH}} = 33 M_\odot$ with fractions $f_{\text{PBH}} \in [10^{-4}, 0.1]$ .
- Supermassive PBHs (SMPBHs): Analyze the "seed" effect for $m_{\text{PBH}} \sim 10^{10} M_\odot$ to explain JWST galaxy candidates.

3. Key Contributions

Unified Framework: Developed a consistent theoretical and numerical framework that simultaneously treats the gravitational acceleration of structure formation and the thermal feedback from PBH accretion.
Improved Initial Conditions: Proposed a robust method for generating cosmological initial conditions that capture both the "seed" (non-linear) and "Poisson" (statistical) effects of PBHs, addressing a gap in existing simulation setups.
Quantification of Competing Effects: Provided a detailed analysis of how PBH heating competes with the accelerated formation of halos, specifically for Pop III stars.
DCBH Trigger Mechanism: Identified a mechanism where stellar-mass PBHs could trigger Direct Collapse Black Hole (DCBH) formation in atomic-cooling halos via local Lyman-Werner (LW) radiation, bypassing the need for rare external radiation fields.
JWST Interpretation: Offered a back-of-the-envelope derivation linking SMPBHs to the formation of the massive, old galaxy candidate ZF-UDS-7329 observed by JWST.

4. Key Results

A. Stellar-Mass PBHs and Pop III Stars

Global Impact (Minor): For $f_{\text{PBH}}$ $f_{PBH}$ allowed by current observational constraints ( $\lesssim 0.01$ $≲ 0.01$ ), the global impact on the cosmic stellar mass density of Pop III stars is minor (variations within a factor of $\sim 2-3$ $\sim 2 - 3$ ).
- Reason: The heating from PBH accretion raises the minimum halo mass ( $M_{\text{mol}}$ ) required for star formation, suppressing the number of forming stars. However, the "Poisson" effect accelerates the formation of halos, increasing their abundance. These two effects largely cancel each other out.
- Environmental Dominance: The heating effect of PBHs is generally overwhelmed by standard environmental factors like Lyman-Werner (LW) background radiation and baryon-dark matter streaming motions.
Local Structure (Significant): While the global statistics are stable, PBHs alter the internal structure of star-forming halos:
- Gas density profiles become "clumpier" in the outer regions ( $1 \text{ pc} \lesssim r \lesssim 20 \text{ pc}$ ).
- PBHs tend to be excluded from the very center ( $r < 1 \text{ pc}$ ) of the densest gas clumps due to "survivor bias" (strong heating prevents gas condensation near the PBH).
- PBHs do not sink to the center of Pop III protostellar disks within the star formation timescale ( $t_{\text{DF}} \gg t_{\text{ff}}$ ), meaning they do not directly interact with the first stars.
Massive BH Seeds (Potential Trigger): In larger atomic-cooling halos ( $M_h \gtrsim 10^8 M_\odot$ ), the local LW radiation from accreting stellar-mass PBHs can reach the critical intensity ( $J_{21} \gtrsim 1000$ ) required to dissociate $H_2$ . This could suppress fragmentation and trigger Direct Collapse Black Hole (DCBH) formation, potentially explaining the seeds of supermassive black holes.

B. Supermassive PBHs (SMPBHs) and JWST Galaxies

Explanation of Massive Galaxies: The authors demonstrate that SMPBHs ( $m_{\text{PBH}} \sim 10^{10} - 10^{11} M_\odot$ ) can serve as seeds for the unusually massive galaxies observed by JWST at $z \gtrsim 8$ (e.g., ZF-UDS-7329).
Parameter Space: To explain the abundance and mass of these galaxies, SMPBHs would need to constitute a fraction $f_{\text{PBH}} \sim 10^{-7} - 10^{-3}$ of DM.
Constraints: Such models are marginally consistent with constraints from dynamical friction and large-scale structure, provided that the standard CMB $\mu$ -distortion constraints (which usually rule out Gaussian fluctuations at this mass) are bypassed via non-Gaussian primordial fluctuations or non-standard formation scenarios.

5. Significance and Outlook

Theoretical Implications: The study highlights that while stellar-mass PBHs may not drastically change the total number of first stars, they significantly modify the environment in which these stars form and offer a new pathway for massive black hole seed formation.
Observational Relevance: The findings provide a theoretical basis for interpreting JWST data. If JWST continues to find massive galaxies at high redshifts, SMPBHs become a viable (though constrained) explanation for early structure formation.
Future Directions: The authors emphasize the need for:
- More sophisticated cosmological simulations that fully resolve the non-linear interplay between PBHs, particle DM, and baryons.
- Detailed modeling of radiative feedback (LW and X-rays) from PBHs to better constrain the DCBH formation channel.
- Investigations into how PBH-seeded structures affect the 21-cm signal and other high-redshift observables.

In conclusion, the paper establishes that PBHs act as a subtle but potentially transformative agent in the early universe, capable of accelerating structure formation and triggering specific pathways for black hole growth, even if their overall contribution to the cosmic star formation rate remains modest under current constraints.