Modified Cosmology or Modified Galaxy Astrophysics is Driving the z>6 JWST Results? CMB Experiments can discover the Origin in the Near Future

The massive and bright galaxies observed by the James Webb Space Telescope (JWST) at high redshifts ($z > 6$) have challenged our understanding of the Universe. This may require revisiting the physics of galaxy formation and evolution, or modifying the $Λ$CDM cosmological model to explain these observations, or both. We show that high-resolution CMB experiments such as the Simons Observatory (or CMB-S4) can measure smoking-gun signatures jointly in weak lensing and kinematic Sunyaev-Zeldovich (kSZ) power spectra, which can shed light on both these scenarios. An increase in the matter power spectrum at small scales will enhance the number density of dark matter halos at high redshifts, thereby increasing the galaxy formation rate. This will cause enhanced weak lensing signal from these redshifts and also lead to enhanced patchy-kSZ signal from the epoch of reionization. However, if only galaxy astrophysics is modified, without any modification in the matter power spectrum, then the patchy-kSZ signal gets altered, while the weak lensing signal remains nearly unaltered. We show that we can measure the modified astrophysical and cosmological scenarios at a statistical significance of $10.4σ$ (and $29.8σ$) from Simons Observatory (and CMB-S4), which will enable a conclusive understanding on what physical process is driving the high-redshift observations of JWST.

💡 Research Summary

The paper addresses the striking discovery by the James Webb Space Telescope (JWST) of unusually massive and luminous galaxies at redshifts z > 6, which appear in tension with predictions from the standard ΛCDM cosmology. The authors propose two broad classes of explanations: (1) a modification of the underlying cosmology that boosts the small‑scale matter power spectrum, thereby increasing the abundance of high‑z dark‑matter halos and accelerating galaxy formation; and (2) a change in the astrophysical modeling of galaxies (e.g., star‑formation efficiency, UV luminosity functions, dust content) that makes galaxies appear brighter without altering the underlying matter distribution.

To discriminate between these scenarios, the authors focus on two high‑resolution Cosmic Microwave Background (CMB) observables that are sensitive to the same underlying physics: weak gravitational lensing of the CMB and the patchy kinetic Sunyaev‑Zel’dovich (kSZ) effect arising from ionized bubbles during reionization. They argue that an enhanced small‑scale matter power spectrum will simultaneously (i) increase the CMB lensing potential at multipoles ℓ > 800 (because more massive halos at z > 6 act as stronger lenses) and (ii) amplify the patchy‑kSZ power spectrum (since more abundant, earlier‑forming galaxies produce larger ionized regions and higher free‑electron density). In contrast, a purely astrophysical modification would affect only the electron‑density bias that enters the kSZ calculation, leaving the lensing signal essentially unchanged.

The authors introduce a phenomenological bias model to parametrize deviations from ΛCDM:
b²_δ(k,z)=b_δ0 + b_δz (z/z₀)(k/k₀)²,
with k₀ = 1 Mpc⁻¹ and z₀ = 100. The modified nonlinear matter power spectrum becomes P_δδ(k,z)=b²_δ(k,z) · \bar{P}_δδ(k,z), where \bar{P}_δδ is the standard ΛCDM prediction. They explore a range of theoretical motivations for such a boost, including axion‑like dark matter with post‑inflationary Peccei‑Quinn symmetry breaking, primordial black holes, transient features in the inflaton potential, dark‑matter self‑interactions, early dark energy, and warm/fuzzy dark matter scenarios. They also note that current COBE/FIRAS μ‑ and y‑distortion limits do not fully exclude many of these possibilities.

Using the Limber approximation, they derive the impact on the CMB lensing potential C_ψℓ and on the patchy‑kSZ power D_ℓ. Figure 1 (described in the text) shows that the modified cosmology (orange dot‑dashed) diverges from the ΛCDM baseline (blue solid) increasingly at ℓ > 800 for lensing and at ℓ > 3000 for kSZ. The authors then perform a Fisher‑matrix forecast for two upcoming CMB experiments: the Simons Observatory (SO) and CMB‑S4. They adopt realistic noise levels, beam sizes (≈1.4′ for SO, ≈1′ for CMB‑S4), and multipole coverage up to ℓ≈3000–5000. The forecast indicates that SO can distinguish the modified‑cosmology scenario from a pure‑astrophysics scenario at 10.4σ, while CMB‑S4 can achieve 29.8σ significance. This high statistical power arises because the two observables respond differently to changes in the matter power spectrum versus changes in the electron‑density bias.

The paper concludes that a joint analysis of small‑scale CMB lensing and patchy‑kSZ measurements will provide a decisive test of whether the JWST high‑z galaxy excess is driven by new cosmological physics or by revised galaxy formation models. It also outlines practical steps for future work: precise measurement of the lensing potential in the ℓ≈800–1500 range, accurate modeling of the ionization history to extract the electron‑bias from kSZ, and combined likelihood analyses that incorporate both probes. If such measurements confirm a boosted small‑scale power spectrum, they would point to new physics in the dark sector or early Universe; if not, the tension would be resolved by updating astrophysical prescriptions for early galaxies. In either case, the synergy between JWST and next‑generation CMB experiments promises to illuminate the physics of the first billion years of cosmic history.