Cosmological implications of $n_s\approx 1$ in light of the Hubble tension

TL;DR

This paper explores the implications of a scale-invariant primordial spectrum ($n_s=1$) in light of the Hubble tension, proposing that axions in the early universe can naturally produce such a spectrum.

Contribution

It demonstrates that axiverse models with axions acting as curvatons can produce an $n_s$ close to 1, offering a novel explanation consistent with recent cosmological observations.

Findings

Axion curvatons can generate $n_s \,\approx\, 1$ during inflation.

Models with $n_s=1$ are compatible with current cosmological data.

Implications for early universe physics and the Hubble tension are discussed.

Abstract

Recently, a low-$z$ measurement of the Hubble constant, $H_0 = 73.04 \pm 1.04 {\rm ~km/s/Mpc}$, was reported by the SH0ES Team. The long-standing Hubble tension, i.e. the difference between the Hubble constant from the local measurements and that inferred from the cosmic microwave background data based on the $\Lambda$CDM model, was further strengthened. There are many cosmological models modifying the cosmology after and around the recombination era to alleviate this tension. In fact, some of the models alter the small-scale fluctuation amplitude relative to larger scales, and thus require a significant modification of the primordial density perturbation, especially the scalar spectral index, $n_s$. In certain promising models, $n_s$ is favored to be larger than the $\Lambda$CDM prediction, and even the scale-invariant one, $n_s=1$, is allowed. In this Letter, we focus on the very early Universe models to study the implication of such unusual $n_s$. In particular, we find that an axiverse with an axion in the equilibrium distribution during inflation can be easily consistent with $n_s = 1$. This is because the axion behaves as a curvaton with mass much smaller than the inflationary Hubble parameter. We also discuss other explanations of $n_s$ different from that obtained based on the $\Lambda$CDM.