Distinguishing early dark energy from baryonic physics as the origin of the JWST high-redshift galaxy excesses

Early JWST observations revealed an excess of ultraviolet-bright, massive, and morphologically mature galaxies at redshifts $z\gtrsim7$–14 relative to canonical galaxy-formation predictions in the standard $\Lambda$CDM cosmology. Shen et al. (2025) showed, via the first large-volume cosmological hydrodynamic simulations of an early dark energy (EDE) cosmology, that EDE reproduces the observed UV luminosity and stellar mass functions without recalibrating the galaxy-formation model. However, alternative baryonic prescriptions — top-heavy initial mass functions, boosted star-formation efficiency, bursty star formation, modified dust and nebular assumptions — can also enhance the predicted abundances, while the allowed EDE parameter space itself remains contested by CMB and large-scale-structure data, and both EDE and $\Lambda$CDM runs underpredict massive quenched galaxies at $z\gtrsim3$ by an order of magnitude, exposing supermassive-black-hole (SMBH) feedback model limitations. The open problem is to determine, with quantified statistical confidence, whether the high-redshift galaxy population requires early-universe physics that enhances small-scale matter power or is attainable within $\Lambda$CDM with baryonic physics, and to identify and exploit observables — galaxy clustering and bias, the reionization history and ionization topology, and Lyman-alpha forest and 21-cm small-scale power — that break the degeneracy between these two families of explanations.

Background

The concordance cosmological model ($\Lambda$CDM: a cosmological constant $\Lambda$ governing late-time acceleration plus cold dark matter) fixes the expansion history and the primordial density-fluctuation power spectrum, from which galaxies form by hierarchical growth of dark-matter halos. The James Webb Space Telescope (JWST) found more ultraviolet (UV)-bright and massive galaxies at $z\gtrsim7$ than pre-JWST models predicted, and, surprisingly early, rotating gaseous and stellar disks and quenched (star-formation-suppressed) massive galaxies. A standard summary statistic is the UV luminosity function (UVLF), the comoving number density of galaxies versus absolute UV magnitude $M_{\rm UV}$; the stellar mass function (SMF) plays the same role for stellar mass. In $\Lambda$CDM the bright-end abundance is limited by the rarity of massive halos and by the star-formation efficiency (SFE, the fraction of a halo's baryons converted to stars), so an observed excess can be blamed on either the cosmology (more early halos) or the baryonic physics (more light per halo).

Early dark energy (EDE) is a transient additional energy component, typically an axion-like scalar field, that contributes a fraction $f_{\rm EDE}$ of the cosmic energy budget around a critical epoch $z_{\rm c}\sim10^{3.5}$ before recombination and then dilutes away; it was originally proposed to ease the Hubble tension (the mismatch between the Hubble constant inferred from the cosmic microwave background (CMB) and measured locally). A generic consequence of EDE-like pre-recombination modifications is a reduced sound horizon compensated by a tilted primordial spectrum, which enhances the small-scale matter power spectrum and accelerates early structure formation without disturbing late-time cosmology. Whether EDE itself remains allowed is unsettled: analyses combining Planck, ACT DR6 (data release 6 of the Atacama Cosmology Telescope), SPT-3G (the third-generation South Pole Telescope CMB camera), DESI (the Dark Energy Spectroscopic Instrument) and weak-lensing (the gravitational distortion of background galaxy shapes by intervening matter) data variously find $f_{\rm EDE}$ constrained at the 0.05–0.15 level with dataset-dependent conclusions, from preference for $\Lambda$CDM to mild preference for EDE (McDonough et al. 2023; Poulin et al. 2025; Khalife et al. 2025).

Shen et al. (2024) first argued with an empirical (halo-occupation-type) model that EDE could jointly explain the Hubble tension and the JWST abundance puzzles. Shen et al. (2025) then ran paired $(100\,{\rm cMpc})^3$ cosmological hydrodynamic simulations ($2\times1200^3$ resolution elements, moving-mesh code with the IllustrisTNG galaxy-formation model) in $\Lambda$CDM and in an EDE realization fitted to ACT DR4-era CMB data with $f_{\rm EDE}(z_{\rm c})=0.179$, using identical subgrid physics (phenomenological prescriptions for star formation, feedback, and black holes operating below the simulation's resolution scale). The EDE run matches the JWST UVLF to $z\simeq14$ and the SMF to $z\simeq9$ while the $\Lambda$CDM run underpredicts them by $\sim0.5$–1 dex; predictions converge to $\Lambda$CDM at $z\lesssim3$, and early disk emergence is enhanced. Yet the authors emphasize that significant degeneracies remain with baryonic solutions (top-heavy initial mass function (IMF), strongly boosted SFE, extreme burstiness — large temporal fluctuations in UV output at fixed halo mass — and dust/nebular-emission assumptions), that both cosmologies underpredict massive quenched galaxies at $z\gtrsim3$ by at least an order of magnitude (pointing to SMBH accretion/feedback subgrid limitations), and that breaking these degeneracies requires larger-volume simulations varying subgrid parameters, on-the-fly radiative transfer (RT, i.e., photon transport computed during the simulation rather than post-processed) for reionization predictions, and exploitation of distinctive observables. Three such observables stand out. Galaxy clustering: in EDE, brighter underlying matter clustering combines with reduced halo bias (galaxies of fixed luminosity occupy less rare, more abundant halos), a signature absent from pure SFE-boost scenarios. Reionization: the epoch when UV photons from the first galaxies ionized the intergalactic medium, whose history and spatial topology respond to the enhanced early galaxy abundance. Small-scale power probes: the Lyman-alpha forest (absorption by neutral hydrogen along quasar sightlines) and the 21-cm signal (hyperfine-line emission/absorption from neutral hydrogen) have mainly been used to constrain models that suppress small-scale power (e.g., warm dark matter); the opposite, enhanced-power regime relevant to EDE remains largely unexploited. The unresolved question is which family of explanations the data actually favor, and which measurements decide it.

Problem Statement

Determine whether the statistical properties of the galaxy population at $z\gtrsim6$–14 revealed by JWST — the bright end of the UV luminosity function, the stellar mass function and its most massive outliers, and the number density of early stellar and gaseous disks — are consistent with standard $\Lambda$CDM combined with plausible baryonic galaxy-formation physics (star-formation efficiency, temporal burstiness, initial mass function, dust attenuation and nebular emission; "plausible" meaning that the adopted prescriptions must be compatible with the broader astrophysical calibrations and observational constraints that exist independently of the high-redshift abundance data being explained), or require pre-recombination early-universe physics that enhances small-scale matter power, with early dark energy as the representative case; and identify which observables decide the question. The abundance of massive quenched galaxies at $z\gtrsim3$ is not itself a discriminating statistic between the two families: both the $\Lambda$CDM and the EDE runs underpredict it by at least an order of magnitude, a shortfall the source attributes to SMBH subgrid limitations common to both cosmologies, making it a linked open issue in its own right rather than evidence for either family. The source's future-work discussion identifies three linked directions that would advance a solution (candidate routes, not mandatory deliverables):

(1) Simulation-based prediction with controlled degeneracies. Cosmological hydrodynamic simulation suites (of sufficient volume and resolution; possible extensions include on-the-fly radiative transfer for reionization-era predictions and zoom-in runs resolving the multiphase interstellar medium), run in $\Lambda$CDM and in enhanced-small-scale-power cosmologies — with EDE parameters propagated from current CMB and large-scale-structure constraints, e.g., $f_{\rm EDE}$ and $n_{\rm s}$ — while systematically varying the galaxy-formation subgrid prescriptions, so that the covariance between cosmological and baryonic variations is quantified for the predicted observables entering the comparison.

(2) Discriminating observables. Quantitative predictions for statistics that differ between the families even when both match the abundance data: galaxy clustering and bias, where EDE predicts enhanced underlying matter clustering combined with reduced halo bias at fixed galaxy properties; the reionization history and ionization topology; and the imprint of enhanced small-scale power on the Lyman-alpha forest and the 21-cm signal — a regime these probes have largely not been used to constrain.

(3) Statistical discrimination on data. A joint statistical comparison with current and foreseeable observations, with quantified significance and with the dominant systematics (cosmic/sample variance, stellar-mass and photometric systematics including Eddington bias (the spurious enhancement of inferred abundances when measurement scatter pushes sources across a steep selection boundary), foreground contamination) propagated, that either demonstrates that $\Lambda$CDM plus baryonic physics suffices at the precision of the data, establishes the requirement for enhanced small-scale power, or quantitatively demonstrates that the available data cannot yet separate the two families.

Completion is a defensible, quantitative discrimination: on the basis of a statistical comparison with the dominant systematics propagated, the answer must state which explanation the data favor, at what confidence, over which observables, and why the competing family is disfavored — or, if the data cannot yet separate the two families, demonstrate that quantitatively and identify which measurements, with what projected sensitivity, would decide the question.

References

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