The Riddle of Early Galaxies: A Non-Standard Perturbation Spectrum Explains JWST's Findings ✛ ✎ ✦
Context
The standard cosmological Big Bang model with cold dark matter (ΛCDM) successfully describes large-scale structure but runs into difficulties on galaxy scales. After the launch of the James Webb telescope, it turned out that at z>10 there are roughly 10–100 times more massive galaxies than predicted. This forces a reexamination either of the properties of dark matter or of the shape of the primordial perturbation spectrum inherited from the inflationary epoch. Satisfying three conditions simultaneously—the galaxy excess, consistency with the reionization epoch from cosmic microwave background data, and a matching UV luminosity function—becomes a decisive test for new theories.
Methods
The authors parameterized the density perturbation power spectrum as the standard ΛCDM spectrum multiplied by a ‘bump’ in the form of a Gaussian peak with amplitude A=20 and a characteristic scale k0=4.69 Mpc⁻¹, and added an exponential suppression on scales smaller than Mfs=10⁹ M⊙ (mimicking the free-streaming effect of warm dark matter particles). Within the extended Press–Schechter (EPS) formalism, halo mass functions and conditional merger probabilities were computed. To verify the analytical accretion rate formulas, N-body simulations were run with GADGET-2 for spectra with a bump. The UV luminosity function was obtained by linking the star formation rate triggered by gas disk collisions during mergers to the accretion of small halos onto larger ones, taking into account the low (~3%) probability of a direct disk collision within a dynamical time. Photometric data from JWST and spectroscopic confirmations were used for comparison.
Results
The model with a ‘bump’ and cutoff yields at z~9 an increase in the number of massive halos (M~10¹⁰ M⊙) by about an order of magnitude, and at z~12 by two orders of magnitude compared to ΛCDM, which directly resolves the overabundance problem of galaxies. At the same time, the suppression of small-scale perturbations prevents overly early star formation and ionization: as shown in Figure 3, the ionization curve x(z) practically coincides with the standard one, and 50% ionization is reached at z~10, close to the value of 7.68±0.79 from Planck CMB data. The analytical formula for the halo mass growth rate distribution, derived from EPS with a dynamical time correction, agrees well with simulations (Fig. 4). Based on this distribution, the calculated UV luminosity functions (Fig. 6) at z=7, 12, and 17 reproduce the observational data points with the star formation efficiency parameters given in the table. The model predicts a noticeably higher fraction of galaxies forming monolithically, and that the first stars (Population IV) are born directly in massive halos with masses on the order of 10⁸ M⊙ and above, bypassing the stage of prior metal enrichment.
Implications
The work demonstrates that introducing two features into the initial perturbation spectrum—an enhancement on the protogalaxy scale and a cutoff on smaller scales—can resolve several observational tensions of ΛCDM in one fell swoop: the excess of early massive galaxies, the missing satellite problem, and the shift in the reionization epoch. This strengthens the motivation to search for deviations from the simple power-law spectrum predicted by the simplest inflation models, and opens the way to probing the physics of inflation and the properties of dark matter through observations of the distant Universe. The ‘Population IV’ concept underscores that chemical evolution may have begun immediately in massive systems, which is important for interpreting data on the very first supermassive black holes.
Future development
A natural development of the idea will be the construction of self-consistent inflationary models that produce a spectrum with a bump and cutoff, as well as detailed hydrodynamical simulations of galaxy formation in such cosmologies. Of particular interest is the study of the joint formation of the first stars and the seeds of supermassive black holes in monolithically collapsing clouds. The next generation of JWST surveys and future instruments (e.g., the Roman Space Telescope) will allow refining the luminosity function at z>12 and testing the predictions of fewer dwarf satellite galaxies, which will serve as an independent test of the model with a spectrum cutoff.
Impact
The described scenario directly impacts the interpretation of JWST and Planck data, imposes new constraints on dark matter models (warm vs cold) and inflation, as well as on the theory of chemical evolution of the Universe.
Next steps
Specialized large-volume N-body simulations with a spectrum containing both a bump and a cutoff are required to refine merger rates and the spatial distribution of halos. In parallel, it is necessary to investigate the impact of feedback from Population IV stars and accretion onto black holes in early massive galaxies.
Key open problems
The proposed model directly tackles unsolved problems in physics: the nature of dark matter (particles with keV-scale masses or a non-standard inflationary potential), the mechanism that generates seed fluctuations during inflation, and the explanation for the rapid growth of supermassive black holes at z>6. Moreover, it offers a unified perspective on ‘monolithic’ and hierarchical formation of galaxies.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors parameterized the density perturbation power spectrum by adding a Gaussian peak and exponential suppression on small scales to explain the excess of massive galaxies at z>10 as observed by JWST. They used the extended Press–Schechter formalism and N-body simulations to compute halo mass functions and accretion rates, then linked the star formation rate during mergers to the UV luminosity function. The model reproduces the observational data points at z=7, 12, and 17 and predicts a more monolithic formation of galaxies.
Where its strength is
This work unifies three key constraints—the galaxy excess, reionization, and the UV luminosity function—within a single modification of the power spectrum, providing a consistent explanation for JWST observations.
Where it could go next
- It might be interesting to compare the model's predictions with the characteristics of compact objects in the early universe discussed in a neighboring paper: how does the proposed perturbation spectrum affect their formation and properties?based on: Red Lanterns of the Cosmos: What Compact Objects from the Early Universe Conceal arXiv:2606.03522v1
- A related paper examines the connection between galaxy color and evolution; one could explore how the modified power spectrum and the associated more monolithic galaxy formation are reflected in color distributions at high z.based on: Why Some Galaxies Are Blue and Others Red arXiv:2607.19471
- Another neighboring analysis considers the role of black holes in forming massive galaxies; it may be worth evaluating how the proposed scenario aligns with their contribution to luminosity at early stages.based on: Tiny Black Holes Build Giants arXiv:2607.09201
Worth trying
- Test whether the model with a bump and cutoff yields predictions for the abundance and masses of compact objects similar to those described in 2606.03522v1.
- Assess how merger rates and hence galaxy morphology change under such a spectrum, and compare with observed fractions of disk and spheroidal systems at z~10.
- Consider the combined effect of the modified spectrum and black hole feedback on the luminosity function to refine star formation efficiency parameters.
Nearby works in our archive
- Why Some Galaxies Are Blue and Others Redsimilarity 0.70
- The Mystery of Massive Early Galaxies: A Solution in Motionsimilarity 0.68
- Tiny Black Holes Build Giantssimilarity 0.67
- Red Monsters and Blue Giants at the Dawn of the Universesimilarity 0.66
- A Dark Deal: How the Universe Reconciles Its Clockssimilarity 0.66
- Where Dwarfs Ignite Starssimilarity 0.66
- Red Lanterns of the Cosmos: What Compact Objects from the Early Universe Concealsimilarity 0.65
The archive is crowded around this work: 9 reviews nearby, closest at 0.70. This ground is well walked — and there is plenty to compare against.
The term ‘Population IV stars’ was introduced by the authors to denote a hypothetical first generation of stars born in massive halos with primordial chemical composition — unlike the classical Population III stars, which form in mini-halos with masses of ~10⁵–10⁶ M⊙. If such stars existed, they could have been bright but short-lived, and their explosions would have rapidly enriched the intergalactic medium with metals.
Key numbers
- bump amplitude: 20
- bump scale k0: 4.69 Mpc⁻¹ (corresponds to halo mass ~10¹⁰ M⊙)
- cutoff mass Mfs: 10⁹ M⊙
- galaxy excess at z~9: about 10 times
- galaxy excess at z~12: about 100 times