Modified Cosmology or Modified Galaxy Astrophysics is Driving the z > 6 JWST Results? Cosmic Microwave Background 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 ΛCDM cosmological model to explain these observations, or both. We show that high-resolution cosmic microwave background (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 an 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 10.4 σ (and 29.8 σ ) from the Simons Observatory (and CMB-S4), which will enable a conclusive understanding on what physical process is driving the high-redshift observations of JWST.