What it takes to solve the Hubble tension - through Modifications of Cosmological Recombination II: in light of ACT DR6 and DESI DR2

Aug 27, 2026

Data-driven approaches are developed to address the Hubble tension by modifying the cosmological recombination history in a minimal way. The study introduces a time-varying electron mass m_e(z) whose perturbation increases the CMB-inferred H0 toward the SH0ES value while avoiding degradation of the fit. Using Planck and ACT data with lensing, a perturbative m_e(z) alteration is found that fully resolves the Hubble tension and also alleviates the S8 tension, though adding DESI DR2 BAO prevents a full resolution due to consistency limits with late-time observations.

LanguageEnglish

CategoryResearch & Report

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Pages13

Reading Time33 min

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Introduction

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arXiv :2606 .06495v1 [ astro-ph .CO] 4 Jun 2026 What it takes to solve the Hubble tension through Modifications of Cosmological Recombination II: in light of ACT DR6 and DESI DR2 Nanoom Lee 1, ∗ and Tianji Zhou2 1 William H. Miller III Department of Physics & Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA 2 Department of Physics and Astronomy, Haverford College, Haverford, Pennsylvania 19041, USA (Dated: June 5, 2026) We construct data-driven solutions to the Hubble tension, in light of recent data from the Atacama Cosmology Telescope (ACT DR6) and the Dark Energy Spectroscopic Instrument (DESI DR2) . We search for the minimal modification to the recombination history through a time-varying electron mass me (z) that increases the best-fit H0 inferred from CMB data toward the SH0ES value, without worsening the fit to the data. Using Planck and ACT data including lensing, we find a perturbative modification to me (z) that fully resolves the Hubble tension, with the solution sharing the same qualitative oscillatory structure as in previous work using Planck data alone, demonstrating its robustness to the inclusion of more precise and independent CMB data. As a byproduct, the solution also eases the S8 tension. Once DESI DR2 BAO data are added, however, perturbative modifications to me (z) cannot fully resolve the Hubble tension. This reflects the same fundamental limitation: raising H0 by modifying recombination generically lowers Ωm , being inconsistent with late-time cosmological observations. I. INTRODUCTION The Hubble tension—the persistent discrepancy between the value of the Hubble constant inferred from early-Universe observations and that measured using late-time distance indicators—remains one of the most significant open problems in modern cosmology. The most well-known early-Universe measurement is that inferred from the Planck satellite’s cosmic microwave background (CMB) anisotropy data, H0 = 67 .36 ± 0.54 kms −1 Mpc −1 [1], while the local measurement is provided by the SH0ES collaboration, H0 = 73 .04 ± 1.04 kms −1 Mpc −1 [2], recently updated to H0 = 73 . 17± 0.86 kms −1 Mpc −1 with an improved distance ladder calibration [3] . The discrepancy between these two values now exceeds 5σ [4], making it unlikely to be a statistical fluctuation and motivating extensive scrutiny of both systematic uncertainties and extensions to the standard cosmological model [5–141] . Among early-Universe solutions, modifications to the cosmological recombination history constitute a particularly well-motivated possibility. Recombination directly determines both the sound horizon rd and the detailed structure of the CMB anisotropy spectra. Reducing rd through modifications to recombination naturally leads to a larger inferred H0 , in order to keep the angular scale of the sound horizon θs fixed at its precisely measured value. Ref. [59] (hereafter Paper I) introduced a datadriven, model-independent framework to search for extensions to ΛCDM that can reconcile CMB-inferred parameters with external measurements, without degrading the fit to the data, and applied it to modifications of ∗ [nanoom.lee@jhu.edu](nanoom.lee@jhu.edu) the recombination history. Without any model-building effort, this approach uses the Fisher-bias formalism to identify perturbations to the recombination history that achieve a targeted shift in parameters such as H0 .1 When applied to Planck CMB data, this framework demonstrated that modest, perturbative changes to a timevarying electron mass me (z) could, in principle, raise the inferred value of H0 to match local measurements without worsening the fit to CMB anisotropies alone. However, the inclusion of additional late-time datasets— baryon acoustic oscillations (BAO) and uncalibrated supernovae—significantly restricted the viability of such solutions, due to the generic tendency of recombinationbased solutions to lower Ωm . These results highlighted both the potenti

FAQs

What problem does the paper address?

The paper targets the Hubble tension, the persistent discrepancy between the H0 inferred from early-Universe CMB observations and that measured using late-time distance indicators.

How does the proposed solution modify cosmology?

It searches for the minimal modification to the recombination history via a time-varying electron mass m_e(z), aiming to raise the best-fit H0 from CMB data toward the SH0ES value without worsening the data fit.

Do the solutions remain viable after including DESI DR2 BAO data?

No. With DESI DR2 BAO added, perturbative modifications to m_e(z) cannot fully resolve the Hubble tension, reflecting a limitation tied to how raising H0 via recombination changes lowers Ωm and conflicts with late-time observations.

Aug 27, 2026

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