Mikage U. Kobayashi, Gen Chiaki, Kazutaka Kimura, Kazuyuki Akitsu, Kazunori Kohri, Tomo Takahashi, Kazuyuki Omukai

The recent James Webb Space Telescope (JWST) observations reported the unexpectedly large abundance of massive galaxies with stellar masses of $\sim 10^{10}~M_{\odot}$ at high redshifts $z \simeq 6.5 - 9$ compared with the prediction of the standard $Λ$CDM model. As a possible solution to the tension, we consider a blue-tilted spectrum of density perturbations with a positive running. We find that, for $α_s \simeq 0.2$ and $β_s \simeq 0.2$, a joint analysis with CMB observations shows that the tension can be resolved at the 1$σ$ confidence level. Such a blue-tilted spectrum is also plausible from the perspective of primordial black hole formation on much smaller scales in the early Universe.