Accelerating massive galaxy formation with primordial black hole seed nuclei

This paper proposes that if massive primordial black holes constitute a fraction of dark matter, they can act as high-density seeds to dramatically accelerate galaxy formation within 100 Myr and potentially explain the existence of low surface brightness galaxies as residues of failed accretion.

The Gist

Here is an explanation of the paper "Accelerating massive galaxy formation with primordial black hole seed nuclei," translated into simple, everyday language with some creative analogies.

The Big Mystery: Galaxies That Shouldn't Exist

Imagine you are baking a giant cake. According to the standard recipe (our current understanding of the universe, called ΛCDM), you need to mix ingredients slowly over a long time to get a big, fluffy cake.

However, the James Webb Space Telescope (JWST) has recently found "cakes" (massive, bright galaxies) that are fully baked and ready to eat when the universe was only a toddler. These galaxies are too big and too bright to have formed so quickly using the standard recipe. It's like finding a fully grown oak tree in a garden that was planted yesterday.

The Proposed Solution: The "Super-Seed"

Jeremy Mould, the author of this paper, suggests a new ingredient for our cosmic recipe: Primordial Black Holes (PBHs).

Think of these PBHs as giant, pre-made "seed" nuclei that appeared right at the very beginning of the universe (before stars even existed).

• Standard Theory: Galaxies form like a snowball rolling down a hill. It starts small, gathers snow (gas and dust), and slowly grows bigger over billions of years.
• Mould's Theory: Imagine dropping a massive boulder (the PBH) right at the top of the hill. The snow doesn't just roll; it gets sucked into the boulder immediately. The PBH acts as a gravitational magnet, pulling in gas and dust so fast that a massive galaxy can form in just 100 million years instead of billions.

How It Works: The Cosmic Vacuum Cleaner

The paper argues that if these massive black holes exist, they act as gravitational anchors.

1. The Anchor: A PBH with the mass of millions of suns sits in the center of a cloud of gas.
2. The Suction: Because it's so heavy, it creates a deep "gravity well." Gas rushes in to fill it.
3. The Explosion: Once the gas is packed tight, it ignites into stars at a furious rate. This explains why JWST sees so many bright, massive galaxies so early in the universe's history.

The "Leftovers": Diffuse Galaxies

If this theory is true, what happens to the gas clouds that didn't get a giant black hole seed?

• The Analogy: Imagine a bakery where some bakers have a giant, powerful mixer (the PBH), and others only have a small hand whisk.
• The Result: The bakers with the mixers make huge, dense cakes (normal galaxies). The bakers with the whisks make small, messy, scattered dough balls that never quite come together.
• The Real World: Mould suggests these "messy dough balls" are Ultra Diffuse Galaxies (UDGs). These are faint, ghost-like galaxies that look like they are falling apart. They are the "failed" galaxies that never got a massive black hole seed to pull them together.

The Evidence and the Caveats

The paper runs computer simulations to test this idea.

• The Simulation: They created a digital universe with 100,000 "particles" representing dark matter. When they added a heavy "seed" in the middle, the particles clumped together much faster and formed denser structures, just like the theory predicts.
• The Catch: We haven't actually seen these massive primordial black holes yet. They are hypothetical. The paper admits that if they don't exist, we might need to tweak the standard recipe (like changing how fast stars form) to explain the JWST discoveries.

Summary in a Nutshell

• The Problem: The universe has too many big galaxies too early.
• The Idea: Maybe the universe started with giant "black hole seeds" that acted as super-fast assembly lines for galaxies.
• The Bonus: This theory also explains why some galaxies are faint and scattered (they missed the seed).
• The Goal: This isn't just about black holes; it's about solving the mystery of how the universe grew up so fast.

The Bottom Line: If massive primordial black holes exist, they are the "secret sauce" that allowed the universe to build its biggest structures in record time. If they don't exist, we have to rewrite the history books of how the cosmos grew up.

Technical Summary

Here is a detailed technical summary of the paper "Accelerating massive galaxy formation with primordial black hole seed nuclei" by Jeremy Mould (Draft March 6, 2026).

1. Problem Statement

The paper addresses a significant tension in modern cosmology: the James Webb Space Telescope (JWST) has discovered an unexpected abundance of UV-bright, massive galaxies at very high redshifts ($z > 8$).

• The Conflict: Standard $\Lambda$CDM cosmology posits that structure forms hierarchically, requiring substantial time to build massive objects. The existence of these early massive galaxies challenges this timeline, as standard models struggle to explain how such structures could form so quickly.
• The Proposed Solution: The author hypothesizes that if a fraction of Dark Matter (DM) consists of Primordial Black Holes (PBHs), specifically in the mass range of $10^5$to$10^8 M_\odot$, they could act as pre-existing, high-density seeds. These seeds would dramatically accelerate the assembly of galaxy halos, shortening the formation time to as little as 100 Myr.

2. Methodology

The paper employs a combination of analytical scaling arguments and $N$-body simulations to test the PBH seed hypothesis.

• Simulation Framework:

  • The author utilizes a DM-only $N$-body code (Mould & Hurley 2025) to simulate the mutual gravitational attraction of PBH particles.
  • Initial Conditions: A spatially uniform random distribution of particles within a sphere, with a central massive nucleus (the PBH seed) at rest.
  • Mass Spectrum: Simulations explore both monochromatic masses and power-law Initial Mass Functions (IMF) where $dN/dM \propto M^{-1}$.
  • Mass Loss: To simulate Hawking radiation or other mass-loss mechanisms, particles lose 1% of their mass per time step (scale-free approach).
  • Parameters: The study varies particle numbers ($10^4$to$2.5 \times 10^5$), mass ratios, and nucleus masses ($10^5$to$10^7 M_\odot$).

• Analytical Approach:

  • The paper derives the galaxy formation timescale based on the free-fall time ($t_{ff}$) and the Kelvin-Helmholtz time for gas cooling around a PBH seed.
  • It compares the cooling time ($t_{cool}$) against the free-fall time to determine if the Rees-Ostriker criterion ($t_{cool} < t_{ff}$) is met, which triggers catastrophic collapse and starbursts.

3. Key Contributions

• PBH as Galaxy Seeds: The paper formalizes the mechanism by which massive PBHs ($>10^5 M_\odot$) act as gravitational anchors, attracting ambient dark matter and baryonic gas much faster than standard CDM fluctuations.
• Timescale Reduction: It demonstrates that the presence of a $10^8 M_\odot$PBH seed can reduce the galaxy formation time to$\sim 10^8$years (corresponding to$z \approx 30$), providing sufficient "maneuvering room" for models to accommodate JWST's early galaxy discoveries.
• Diffuse Galaxy Hypothesis: The paper proposes a novel explanation for Ultra Diffuse Galaxies (UDGs). It suggests that UDGs may be the "residue" of galaxy formation processes where regions lacked a massive PBH seed, resulting in shallow potential wells that failed to accrete a major baryonic component.
• Structural Evolution: The study analyzes how PBH seeds influence the density profiles of dark matter halos, specifically the transition from cored to cuspy profiles based on seed mass.

4. Results

• Halo Concentration: Simulations show that halos seeded by massive PBHs develop significantly deeper potential wells and are more centrally concentrated compared to standard $\Lambda$CDM halos.
  • Cusps vs. Cores: Seeds with masses $\ge 10^6 M_\odot$ develop steep central density cusps due to gravitational focusing. Seeds with lower masses ($\sim 10^5 M_\odot$) or specific mass gaps in the IMF tend to produce cored profiles.
  • Self-Interacting DM (SIDM): The paper notes that adding self-interaction cross-sections can erase these cusps, but this is a property of SIDM, not PBHs.
• Star Formation Efficiency:
  • The presence of a massive seed allows gas to cool efficiently (via atomic lines and $H_2$) before feedback mechanisms (supernovae) can quench it.
  • Calculations indicate that for a $3 \times 10^7 M_\odot$ seed, the free-fall time is short enough that star formation can proceed at high efficiency (potentially exceeding the Eddington limit temporarily) before the first supernovae occur (approx. 3 Myr).
• Mass Constraints:
  • The paper acknowledges constraints on PBH abundance (e.g., CMB, lensing, dynamical friction). However, it argues that a small fraction of DM ($f_{PBH} \ll 1$) in the $10^5 - 10^7 M_\odot$ range is not ruled out and is sufficient to seed the observed massive galaxies.
  • The "asteroid window" ($10^{17} - 10^{25}$ g) remains a candidate for the bulk of DM, but the focus here is on the supermassive end.

5. Significance

• Resolving the JWST Tension: The paper offers a viable physical mechanism to explain the existence of massive, early galaxies without requiring extreme modifications to the standard star formation efficiency or the initial mass function (IMF) of stars.
• Alternative to Direct Collapse: While Direct Collapse Black Holes (DCBH) are a competing theory, this paper highlights that PBHs provide a distinct, pre-existing seed that does not rely on specific baryonic collapse conditions in the early universe.
• Predictive Power for UDGs: The hypothesis that UDGs are "failed" galaxies lacking PBH seeds provides a testable prediction: UDGs should have very shallow potential wells, potentially lacking globular clusters or hosting only wandering black holes, distinguishing them from standard dwarf galaxies.
• Future Work: The author calls for hydrodynamical simulations incorporating PBH seeds to fully test the interplay between rapid gas accretion, feedback, and star formation, as current $N$-body results are DM-only.

In conclusion, Mould proposes that massive Primordial Black Holes are a potent, albeit unproven, catalyst for early structure formation, capable of reconciling high-redshift galaxy observations with cosmological theory while offering a new perspective on the nature of diffuse galaxies.