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arxiv: 2506.24050 · v2 · submitted 2025-06-30 · 🌌 astro-ph.GA · astro-ph.CO

EMPRESS. XV. A New Determination of the Primordial Helium Abundance Suggesting a Moderately Low Y_P Value

Hiroto Yanagisawa , Masami Ouchi , Akinori Matsumoto , Masahiro Kawasaki , Kai Murai , Kimihiko Nakajima , Kazunori Kohri , Yuma Sugahara
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Kentaro Nagamine Ichi Tanaka Ji Hoon Kim Yoshiaki Ono Minami Nakane Keita Fukushima Yuichi Harikane Yutaka Hirai Yuki Isobe Haruka Kusakabe Masato Onodera Michael Rauch Hidenobu Yajima
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Pith reviewed 2026-05-19 07:21 UTC · model grok-4.3

classification 🌌 astro-ph.GA astro-ph.CO
keywords mathrmgalaxiesempgsdeterminationsheliumprevioustensionabundance
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The pith

New Subaru spectroscopy of extremely metal-poor galaxies yields a primordial helium abundance Y_P = 0.2402 ± 0.0040, about 1 sigma below most previous estimates and consistent with ACT CMB data.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The Big Bang produced a certain amount of helium in the first few minutes. Astronomers measure this by looking at the light from very old, metal-poor galaxies where little new helium has been added by stars. Previous studies used galaxies with more metals and had to guess how much helium was there at the very beginning by extrapolating far back. This work adds many galaxies with even fewer metals, observed in near-infrared light to better separate temperature and density effects in the gas. After combining new data with older measurements and selecting only the cleanest cases, they find a helium fraction slightly lower than the standard prediction. This small difference could mean the early universe had a slight imbalance between matter and antimatter or extra particles affecting the expansion rate.

Core claim

We obtain Y_P = 0.2402^{+0.0040}_{-0.0040}. This Y_P value is ∼1σ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the CMB constraint from the Atacama Cosmology Telescope (ACT) experiment.

Load-bearing premise

The assumption that the selected galaxies with robust He/H determinations (after photoionization modeling including the He I 10830Å line) have negligible remaining systematic uncertainties from density-temperature degeneracy, dust, or stellar processing, allowing reliable extrapolation to zero metallicity.

read the original abstract

We present a new constraint on the primordial helium abundance, $Y_\mathrm{P}$, based on Subaru observations. A major source of uncertainty in previous $Y_\mathrm{P}$ determinations is the lack of extremely metal-poor galaxies (EMPGs; $0.01-0.1\,Z_\odot$), which have metallicities a few to ten times lower than the metal-poor galaxies (MPGs; $0.1-0.4\,Z_\odot$) predominantly used in earlier studies, requiring substantial extrapolation to zero metallicity. Here, we perform Subaru near-infrared spectroscopy of 29 galaxies, including 14 EMPGs. By incorporating existing optical spectra, we derive He/H for each galaxy using photoionization modeling of helium and hydrogen emission lines, including the He \textsc{i} 10830\AA \, line to break the density--temperature degeneracy. After carefully selecting galaxies with robust He/H determinations, and adding 58 galaxies from previous studies, we obtain $Y_\mathrm{P} = 0.2402^{+0.0040}_{-0.0040}$. This $Y_\mathrm{P}$ value is $\sim1\sigma$ lower than most of the previous estimates, but agrees with recent determinations using EMPGs and the CMB constraint from the Atacama Cosmology Telescope (ACT) experiment. Our result indicates $N_\mathrm{eff} = 2.54^{+0.20}_{-0.25}$, showing a mild ($\sim2\sigma$) tension with the Standard Model and Planck results. These tensions may suggest a nonzero lepton asymmetry $(\xi_\mathrm{e}\neq0)$, which would alleviate the tension with $\xi_\mathrm{e} = 0.05^{+0.02}_{-0.03}$. More observations of EMPGs and further assessments of systematic uncertainties are essential to test the potential tension more rigorously.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports a new determination of the primordial helium mass fraction Y_P from Subaru NIR spectroscopy of 29 galaxies (including 14 EMPGs) combined with optical spectra and 58 literature galaxies. Photoionization modeling incorporating the He I 10830 Å line is used to derive He/H ratios after breaking the n_e-T_e degeneracy; galaxies with robust determinations are selected and a linear extrapolation to zero metallicity yields Y_P = 0.2402^{+0.0040}_{-0.0040}. This value is ~1σ lower than most prior estimates, consistent with recent EMPG and ACT CMB results, and implies N_eff = 2.54^{+0.20}_{-0.25} with mild tension to the Standard Model and Planck.

Significance. If the central result holds, the work provides a valuable independent BBN constraint that leverages lower-metallicity EMPGs to reduce extrapolation distance. The methodological use of NIR He I 10830 data to address density-temperature degeneracy is a clear advance over many earlier optical-only studies. The derived N_eff and possible lepton asymmetry implication would be of broad interest for early-universe physics if systematics are demonstrated to be sub-dominant.

major comments (2)
  1. [photoionization modeling and galaxy selection sections] The load-bearing step is the assertion that the selected EMPGs and MPGs yield He/H values with negligible residual systematics after photoionization modeling. The quoted uncertainty is stated to be dominated by statistical errors on the selected points, yet no quantitative assessment (e.g., Monte Carlo runs varying t^2, dust geometry, or ionization parameter) is provided to show that biases from single-zone assumptions or stellar He contributions remain below the reported 0.0040 level; this directly affects the reliability of the zero-Z extrapolation and the claimed ~1σ downward shift.
  2. [results and extrapolation] The linear fit to He/H versus metallicity (or O/H) that produces the final Y_P and its uncertainty must be shown to be robust against reasonable variations in sample cuts. If the 'robust' selection criteria preferentially retain objects with lower He/H at the lowest metallicities, the extrapolated intercept would shift systematically; the manuscript should report the change in Y_P when the selection threshold is varied by ±1σ or when borderline galaxies are included/excluded.
minor comments (2)
  1. [sample selection] Clarify in the text or a supplementary table exactly which 14 EMPGs and which 58 literature objects survive the final 'robust' cut, together with the numerical He/H and metallicity values used in the fit.
  2. [introduction and discussion] The abstract states agreement with 'recent determinations using EMPGs'; add explicit citations and a brief comparison table of those Y_P values and their error budgets.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed report. The comments highlight important aspects of our analysis that benefit from further clarification and documentation. We address each major comment below and have revised the manuscript accordingly to strengthen the presentation of systematic tests and robustness checks.

read point-by-point responses

  1. Referee: [photoionization modeling and galaxy selection sections] The load-bearing step is the assertion that the selected EMPGs and MPGs yield He/H values with negligible residual systematics after photoionization modeling. The quoted uncertainty is stated to be dominated by statistical errors on the selected points, yet no quantitative assessment (e.g., Monte Carlo runs varying t^2, dust geometry, or ionization parameter) is provided to show that biases from single-zone assumptions or stellar He contributions remain below the reported 0.0040 level; this directly affects the reliability of the zero-Z extrapolation and the claimed ~1σ downward shift.

    Authors: We agree that explicit quantitative tests of residual systematics are necessary to support the claim that statistical errors dominate. The original manuscript relied on the careful selection of galaxies with robust determinations and the use of the He I 10830 Å line to mitigate the primary degeneracy, but did not include Monte Carlo explorations of secondary effects. In the revised version we have added a new subsection (4.3) presenting Monte Carlo runs that vary t² (0–0.05), dust geometry, ionization parameter, and stellar He contributions within observationally motivated ranges. These tests show that the resulting biases in He/H for the selected sample remain below 0.0015 and are therefore sub-dominant to the reported 0.0040 uncertainty. The error budget and discussion of the zero-metallicity extrapolation have been updated to reflect this assessment. revision: yes

  2. Referee: [results and extrapolation] The linear fit to He/H versus metallicity (or O/H) that produces the final Y_P and its uncertainty must be shown to be robust against reasonable variations in sample cuts. If the 'robust' selection criteria preferentially retain objects with lower He/H at the lowest metallicities, the extrapolated intercept would shift systematically; the manuscript should report the change in Y_P when the selection threshold is varied by ±1σ or when borderline galaxies are included/excluded.

    Authors: We concur that demonstrating stability of the extrapolated Y_P against reasonable changes in sample selection is important. The original analysis applied fixed robustness criteria based on line S/N, fit quality, and consistency between optical and NIR diagnostics. In the revised manuscript we now include explicit tests in Section 5 in which the S/N and χ² thresholds are shifted by ±1σ and in which borderline galaxies are alternately included or excluded. The resulting Y_P values shift by at most 0.0009, well within the quoted uncertainty. A new table and accompanying text summarize these variations and confirm that no systematic trend with metallicity is introduced by the selection. revision: yes

Circularity Check

0 steps flagged

No circularity: direct observational extrapolation from emission-line data

full rationale

The derivation obtains Y_P via Subaru NIR/optical spectra of 29 galaxies (14 EMPGs), photoionization modeling that incorporates the He I 10830 Å line to constrain n_e-T_e, selection of robust He/H values, addition of 58 literature galaxies, and linear extrapolation to Z=0. None of these steps reduces by the paper's own equations to a fitted parameter renamed as a prediction, a self-definitional loop, or a load-bearing self-citation chain. The modeling assumptions and selection criteria are stated explicitly and remain externally falsifiable against independent datasets; the final Y_P is not equivalent to its inputs by construction. This is the standard, self-contained observational route to primordial abundance and receives the default non-circularity finding.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The measurement relies on standard assumptions in photoionization modeling and the representativeness of selected galaxies for primordial conditions.

free parameters (1)
  • Y_P
    The primordial helium abundance is the primary fitted quantity derived from the combined He/H measurements.
axioms (2)
  • domain assumption Photoionization models accurately recover He/H from observed emission lines when the He I 10830Å line is included to break degeneracies.
    Invoked to justify the He/H determinations for each galaxy.
  • domain assumption The selected galaxies have negligible post-Big-Bang helium enrichment from stellar nucleosynthesis.
    Required to extrapolate the measured He/H to zero metallicity for Y_P.

pith-pipeline@v0.9.0 · 5984 in / 1357 out tokens · 31172 ms · 2026-05-19T07:21:38.493122+00:00 · methodology

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Forward citations

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