Blue-tilted Runnings and the JWST Early Galaxy Tension

TL;DR

This paper proposes a blue-tilted spectrum of density perturbations with positive running as a solution to the JWST high-redshift galaxy abundance tension, supported by CMB data and primordial black hole formation considerations.

Contribution

It introduces a specific blue-tilted primordial power spectrum model that can reconcile JWST galaxy observations with standard cosmology.

Findings

A blue-tilted spectrum with $eta_s \,\simeq\, 0.2$ resolves the tension at 1σ confidence.

The proposed spectrum is consistent with CMB observations.

Such a spectrum may also facilitate primordial black hole formation.

Abstract

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 $\Lambda$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 $\alpha_s \simeq 0.2$ and $\beta_s \simeq 0.2$, a joint analysis with CMB observations shows that the tension can be resolved at the 1$\sigma$ 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.