DESI and the Hubble tension in light of modified recombination

TL;DR

This paper investigates whether modifications to the recombination history, rather than dark energy density changes, can reconcile DESI and CMB data with the Hubble tension, suggesting early recombination and small-scale clumping as key factors.

Contribution

It demonstrates that allowing for a different ionization history can align CMB and DESI data with a higher H0, offering a new perspective on resolving the Hubble tension.

Findings

Preferred early recombination similar to small-scale clumping models

H0 estimate consistent with SH0ES at 2σ

Higher CMB lensing power reduces neutrino mass constraints

Abstract

Recent measurements and analyses from the Dark Energy Spectroscopic Instrument (DESI) Collaboration and supernova surveys combined with cosmic microwave background (CMB) observations, indicate that the dark energy density changes over time. Here we explore the possibility that the dark energy density is constant, but that the cosmological recombination history differs substantially from that in $\Lambda$CDM. When we free up the ionization history, but otherwise assume the standard cosmological model, we find the combination of CMB and DESI data prefer i) early recombination qualitatively similar to models with small-scale clumping, ii) a value of $H_0$ consistent with the estimate from the SH0ES Collaboration at the $2\sigma$ level, and iii) a higher CMB lensing power, which takes pressure off of otherwise tight constraints on the sum of neutrino masses. Our work provides additional motivation for finding physical models that lead to the small-scale clumping that can theoretically explain the ionization history preferred by DESI and CMB data.