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

Abstract

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.

Figures (5)

• Figure 1: The integrated lensing potentials $C_{\ell}^{\psi}$ (\ref{['fig:lensk']}) and patchy-kSZ power spectra (\ref{['fig:kszk']}) for the cases of $\Lambda$CDM (blue solid) and modified $\Lambda$CDM (orange dot-dashed). The deviation between $\Lambda$CDM and modified cosmology increases at higher multipoles $\ell > 800$ as can be seen in the residual plot in the lower panel. The detectors SO and CMB-S4 will be able to estimate the lensing and patchy-kSZ power spectra at these high multipoles, which can be used to probe small-scale modifications in $\Lambda$CDM, as well as the underlying astrophysics which affects the electron density bias. The high redshifts ($z > 6$) contribute to these multipoles at scales $k > 0.1 \, \rm{Mpc^{-1}}$, where JWST shows an excess of massive galaxies. Also plotted in \ref{['fig:kszk']} is the unlensed CMB power spectrum (green-dashed) which dominates in power up to multipoles $\ell >3000$. The patchy-kSZ signal will also increase or decrease if the electron density bias $b_e$ is higher or lower.
• Figure 2: The fits obtained using our phenomenological model at redshifts 7, 8 and 10. Also plotted are the Schechter fits from harikane2025jwst with $1\sigma$ variations on them (dashed).
• Figure 3: JWST-observed massive galaxies can be explained by modified stellar astrophysics (increased ionized patchiness or UV luminosities) or modified $\Lambda$CDM cosmology (increased small-scale power enhancing halo abundance). Both affect the patchy-kSZ signal, but CMB lensing—sensitive only to the matter distribution—constrains $b_{\delta}$, allowing separation of cosmological ($b_{\delta}$) and astrophysical ($b_e$) effects.
• Figure 4: The constraints obtained on modification parameter ($b_{\delta z}$), and electron density bias ($b_e$) from SO and CMB-S4. We show the inference for the case of only-lensing estimation (left) and for lensing + kSZ estimation (right). The parameters $b_{\delta z}$ and $b_e$ are anti-correlated and can be decoupled using both CMB-lensing and patchy-kSZ observations. The blue lines represent true injected values for JWST best-fit cosmology.Lensing is able to constrain cosmological modifications, while kSZ can constrain modifications in the understanding of galactic astrophysics as well.
• Figure 5: The patchy-kSZ and lensing potential variance (with varying $\ell_{\rm{max}}$ and $L_{\rm{max}}$) for $\Lambda$CDM $(b_{\delta z} = 0)$ and different modified $\Lambda$CDM cases using parameter $(b_{\delta z} > 0)$. The error bars from SO and CMB-S4 for the corresponding angular resolutions are shown for the cases of $\Lambda$CDM $(b_{\delta z} =0 )$ and the modified cosmology from JWST fit $(b_{\delta z} =1.15 )$. Here, the minimum multipole for lensing power spectra is taken as $L_{\rm{min}} = 50$ and for patchy-kSZ is taken as $\ell_{\rm{min}} = 2500$.