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Comment on the recent claims of H0 measurements with the ZTF DR2 SN Ia sample (arXiv:2610.36051)

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ReviewThe objections and verdict, as JSON Paper breakdownThe authors' claims, conditions and measurements, as JSON Signed receiptProof this review has not been altered
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Mickael Rigault, Brodie Popovic, Ariel Goobar, Suhail Dhawan, Marie Aubert, Marc Betoule, Umut Burgaz, Georgios Dimitriadis
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Lluís Galbany, Tomás E. Müller Bravo, Antoine Gilles Lordet, Madeleine Ginolin, Young-Lo Kim, Marek Kowalski, Jakob Nordin, Nicolas Regnault, Mathew Smith, Jesper Sollerman, Jacco H. Terwel, Alice Townsend

Cosmology and Nongalactic Astrophysics · Submitted 5 Nov 2026 · 12 references

Pith's verdict: A well-argued correction that nails the calibration requirements for H0, but it's a commentary piece, not new science.

T0 · AI review

How this review was made: An AI referee (deepseek/deepseek-v4-flash) read the paper on November 6, 2026 and answered a fixed list of questions about it. T0 is the starting level every paper gets. The levels describe how deeply Pith has checked the paper, never the authors.

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  4. T3 Formal statement The claims written out in a formal math language a computer can read.
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arXiv ID2611.07794 (version 1)
Pith IDUA3ACCIN, Pith's permanent record of this paper and its review
CitationAlice Townsend, Antoine Gilles Lordet, Ariel Goobar, Brodie Popovic, Georgios Dimitriadis, Jacco H. Terwel, Jakob Nordin, Jesper Sollerman, Llu\'is Galbany, Madeleine Ginolin, Marc Betoule, Marek Kowalski, Marie Aubert, Mathew Smith, Mickael Rigault, Nicolas Regnault, Suhail Dhawan, Tom\'as E. M\"uller Bravo, Umut Burgaz, Young-Lo Kim. "Comment on the recent claims of H0 measurements with the ZTF DR2 SN Ia sample (arXiv:2610.36051)." arXiv:2611.07794 (2026). https://pith.science/paper/UA3ACCIN
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The paper

The paper claims

ZTF SN Ia DR2 calibration is insufficient for the cross-survey Hubble constant measurement claimed by Karchev et al.

The reading

Karchev et al. (2026) reported a measurement of the Hubble constant H0 by combining the ZTF SN Ia DR2 sample with the SH0ES calibrator sample. The ZTF collaboration argues this measurement is invalid because the absolute photometric calibration of the ZTF SN Ia DR2 data is not established to the necessary precision. The relative zeropoint offset between the ZTF and SH0ES photometric systems is unknown and cannot be determined from publicly available data, and this offset is fully degenerate with H0. The photometric corrections applied by Karchev et al. do not address the global calibration offset. Therefore, the claimed H0 value is not supported by the available calibration information.

What carries the argument

The distance-ladder method for H0 requires that the calibrator supernovae and the Hubble-flow supernovae are measured on exactly the same photometric magnitude scale. The unknown relative zeropoint offset between the ZTF and SH0ES systems is fully degenerate with H0: any unaccounted magnitude difference directly biases H0 by ΔH0/H0 = -ln(10)/5 × Δ(mag). The ZTF DR2 data release explicitly warned that the absolute calibration was not ready, and the comment details three specific missing pieces: (1) stellar magnitudes needed to anchor the photometric system, (2) a 60-90 mmag offset between forced-photometry and scene-modeling photometry within ZTF, and (3) detector nonlinearities (pocket effect) that vary across the focal plane and are not corrected in the public data.

Extended reading notes

Core claim

The ZTF SN Ia DR2 dataset, while internally self-consistent, lacks the absolute photometric calibration needed to combine it with external samples such as SH0ES. The missing calibration products include stellar magnitudes tied to a standard system (e.g., Pan-STARRS), updated filter bandpasses, and corrections for detector nonlinearities such as the pocket effect and the brighter-fatter effect. Without these, any cross-survey H0 determination carries an unknown systematic offset that can be as large as 0.1 mag, corresponding to ~3.5 km/s/Mpc on H0. Karchev et al. did not account for this offset, so their result does not constitute a valid cosmological measurement.

Load-bearing premise

The comment assumes that the known calibration problems (missing stellar magnitudes, FP-SMP offset, pocket effect, brighter-fatter effect) are the only systemic issues preventing an accurate cross-survey H0 measurement, and that Karchev et al. had no access to additional unpublished calibration data that could resolve them.

Editorial extensions

If this is right

  • If the calibration issues are not resolvable with the current public release, then the Karchev et al. H0 value and its claimed 2.7σ tension with Planck are unsupported and should not be used in the Hubble tension debate.
  • The ZTF Cosmology Science Working Group is preparing a fully calibrated data release; only after that release can ZTF SN Ia data be reliably combined with external samples for cosmology.
  • The comment reinforces that any cross-survey distance-ladder measurement must verify that the photometric systems of calibrator and Hubble-flow samples are tied with known accuracy, and that internal calibration offsets (e.g., FP vs SMP) are fully characterized.
  • The specific calibration problems described (missing bandpass updates, pocket effect, FP-SMP offset) provide a checklist for future surveys (e.g., LSST) to avoid similar pitfalls.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the missing calibration data were released and the offset reduced to <0.01 mag, the large ZTF sample (O(3000) SNe) could provide a competitive H0 measurement independent of the Cepheid-anchored SH0ES sample, potentially arbitrating the Hubble tension.
  • The comment implies that other attempts to use ZTF DR2 for cosmology without the official calibration products would face the same fundamental barrier, not just Karchev et al.
  • The detailed description of the pocket effect and its variability suggests that simple a posteriori corrections (as attempted by Karchev et al.) are insufficient; a per-CCD, time-dependent model is likely required, which may be complex to implement.
Original abstract

Karchev et al. (2026) derived a value of H0 by combining the ZTF SN Ia DR2 sample with SH0ES calibrator Type Ia supernovae (SNe Ia; Riess et al. 2022), used to anchor the SN Ia absolute luminosity. The ZTF SN Ia DR2 dataset (Rigault et al., 2025) is the largest (O(3000)) SN Ia sample to date obtained with a single telescope. However, as emphasized in the data release, the absolute photometric calibration of the survey has not yet been established to the precision required for cosmological inference. The relative photometric zeropoint offset between the SH0ES and ZTF SN Ia DR2 samples, which is fully degenerate with H0, is therefore unknown and cannot be determined from publicly available data. The photometric corrections introduced in Karchev et al. (2026) do not address this global calibration offset. Consequently, their reported H0 determination is not supported by the available calibration information. We nonetheless share the excitement of pursuing state-of-the-art cosmology with ZTF and are actively working toward the release of a fully calibrated dataset

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-11-06: 1 request, 5,764 input tokens.

What the abstract itself states, as probabilities: claims first 0.19 · states limitation 0.61 · human participants 0.03 · controlled comparison 0.10 · randomized assignment 0.01 · states quantitative result 0.28 · compares with prior results 0.47 · mentions code or data availability 0.12.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Karchev et al. (2026) derived a value of H0 by combining the ZTF SN Ia DR2 sample with SH0ES calibrator Type Ia supernovae (SNe Ia; Riess et al. 2022), used to anchor the SN Ia absolute luminosity. The ZTF SN Ia DR2 dataset (Rigault et al., 2025) is the largest (O(3000)) SN Ia sample to date obtained with a single telescope. However, as emphasized in the data release, the absolute photometric calibration of the survey has not yet been established to the precision required for cosmological inference. The relative photometric zeropoint offset between the SH0ES and ZTF SN Ia DR2 samples, which is fully degenerate with H0, is therefore unknown and cannot be determined from publicly available data. The photometric corrections introduced in Karchev et al. (2026) do not address this global calibration offset. Consequently, their reported H0 determination is not supported by the available calibration information. We nonetheless share the excitement of pursuing state-of-the-art cosmology with ZTF and are actively working toward the release of a fully calibrated dataset

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Karchev et al. (2026) derived a value of H0 by combining the ZTF SN Ia DR2 sample with SH0ES calibrator Type Ia supernovae (SNe Ia; Riess et al. 2022), used to anchor the SN Ia absolute luminosity. background 0.87 0.05 0.04 asserted 1.00
s1 The ZTF SN Ia DR2 dataset (Rigault et al., 2025) is the largest (O(3000)) SN Ia sample to date obtained with a single telescope. background 0.99 0.06 0.02 asserted 1.00
s2 However, as emphasized in the data release, the absolute photometric calibration of the survey has not yet been established to the precision required for cosmological inference. background 0.47 0.08 0.05 asserted 1.00
s3 The relative photometric zeropoint offset between the SH0ES and ZTF SN Ia DR2 samples, which is fully degenerate with H0, is therefore unknown and cannot be determined from publicly available data. gap 0.76 0.19 0.20 asserted 1.00
s4 The photometric corrections introduced in Karchev et al. (2026) do not address this global calibration offset. interpretation 0.45 0.17 0.08 asserted 1.00
s5 Consequently, their reported H0 determination is not supported by the available calibration information. interpretation 0.75 0.41 0.89 asserted 1.00
s6 We nonetheless share the excitement of pursuing state-of-the-art cosmology with ZTF and are actively working toward the release of a fully calibrated dataset future work 1.00 0.76 0.03 asserted 0.99

What kind of paper the classifier reports this is: position or commentary 1.00 · evidence from argument 0.73.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

References

Discussion

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