REVIEW 3 major objections 5 minor 1 cited by
Calibrating DESI's 100,000-galaxy peculiar-velocity sample with type Ia supernovae yields H0 = 73.7 ± 1.1 km/s/Mpc, with a statistical error smaller than any previous Hubble-constant sample.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-03 18:48 UTC pith:YBYRE5EZ
load-bearing objection Careful, transparent calibration of the largest PV sample to date gives H0 = 73.7 ± 1.1 but inherits the SH0ES zero-point; worth a serious referee. the 3 major comments →
The DESI DR1 Peculiar Velocity Survey: global zero-point and H₀ constraints
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using the DESI DR1 Fundamental Plane and Tully-Fisher distances—roughly 104,000 galaxies—the authors measure the global zero-point by matching to external calibrators, chiefly type Ia supernovae lying in the same galaxy groups. The zero-point shift in log-distance ratio is η_zp = 0.139 ± 0.007, applied to the full PV sample; the resulting Hubble diagram gives H0 = 73.7 ± 0.06 (stat) ± 1.1 (syst) km/s/Mpc. Calibration via surface brightness fluctuations gives 74.1 ± 1.1 km/s/Mpc, consistent with the supernova value. The paper claims this is the lowest statistical H0 uncertainty achieved to date, with the systematic error dominated by the external calibrators' distance-ladder uncertainties rat
What carries the argument
The central objects are the Fundamental Plane (an empirical relation between elliptical-galaxy size, velocity dispersion, and surface brightness) and the Tully-Fisher relation (spiral-galaxy rotation speed versus luminosity), both of which yield relative distances. The argument is carried by a group-based zero-point: galaxies from several external group catalogs are merged so that any galaxy sharing a group with a calibrated distance can serve as a calibrator, increasing the calibrator overlap roughly tenfold. This enables a global zero-point η_zp = ⟨η_PV⟩ − ⟨η_cal⟩ computed with a full covariance matrix, including the zero-point uncertainty as a block; the relation Δμ = −5η links the two di
Load-bearing premise
The result hinges on a correction that shifts the mean distance of galaxies in groups back to the full-sample mean; the physical mechanism behind this offset is unconfirmed, and removing the correction is the second-largest systematic in the analysis.
What would settle it
Build the zero-point using only the galaxies that directly host type Ia supernovae, without group matching, and compare H0; if the difference exceeds the quoted systematic, the group correction is not fully understood. Alternatively, re-run with a deeper, DESI-native group catalog: if the correction vanishes, it was a selection artifact.
If this is right
- The statistical uncertainty on H0 drops to 0.06 km/s/Mpc, so any further precision gains must come from reducing calibration systematics, not from adding more peculiar-velocity galaxies.
- If future DESI releases can overlap with Cepheid or tip-of-the-red-giant-branch calibrators, the group-based approach could produce a percent-level H0 from a shorter distance ladder independent of supernovae.
- The consistency between the supernova and surface-brightness-fluctuation zero-points (0.139 versus 0.134, differing by 0.005) suggests the Cepheid-calibrated distance scale anchors the DESI sample robustly.
- The low-redshift (z < 0.01) disagreement between DESI and the calibrators means future analyses must either model local volume effects or continue to exclude that region from zero-point fits.
Where Pith is reading between the lines
- If the group correction (δFP ≈ −0.057 mag) is indeed a selection effect from shallower external group catalogs, a DESI-native group catalog should make the correction vanish; this is directly testable in the next data release.
- The z < 0.01 discrepancy could be turned into a probe of local large-scale structure: a peculiar-velocity sample this size may map the local velocity field in detail, converting a nuisance into a measurement.
- The method's dependence on supernova calibration means it cannot settle the Hubble tension by itself, but a future two-rung DESI ladder calibrated directly to Cepheids or the tip of the red giant branch would provide a largely independent check on both the supernova distance ladder and the cosmic-microwave-background-inferred H0.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper calibrates the zero-point of the DESI DR1 Fundamental Plane and Tully-Fisher peculiar-velocity distance sample. Using a galaxy-group catalog to expand the overlap with external distance anchors, the authors align the TF and FP samples, correct a statistically significant offset between grouped and ungrouped FP galaxies (δFP, Eq. 1), apply a similar group correction for SN hosts (δSN, §5.5), and compute a global zero-point relative to SH0ES/Pantheon+ SNe Ia. The calibrated sample yields H0 = 73.7 ± 0.06 (stat) ± 1.1 (syst) km/s/Mpc, consistent with an SBF-based calibration (74.1 ± 1.1). The paper reports extensive systematics tests, with the largest shifts coming from the z>0.01 calibrator cut and the group corrections.
Significance. If the zero-point methodology holds, this work demonstrates that a >100,000-galaxy PV sample reaches a statistical precision well below the systematic floor, and it provides a consistency check of the SH0ES/Pantheon+ distance ladder through an independent set of distance tracers. The transparent use of the full SN covariance matrix and the detailed systematics budget are commendable. However, the H0 value is not an independent Hubble constant: the zero-point is defined by matching to SNe Ia that carry the SH0ES calibration, so the central value will track SH0ES by construction. The scientific value lies in the calibration infrastructure and the SBF/maser cross-checks, not in a new local H0 measurement.
major comments (3)
- [§4.1, §5.5, Eq. (12)] The central zero-point rests on the group-selection corrections δFP = −0.057 ± 0.010 mag (Eq. 1) and δSN (§5.5), whose physical origin is explicitly unconfirmed (§7). The systematics tests in §6.1.5 remove these corrections, but all variants share the same grouping scheme and correction philosophy; removing a correction is not equivalent to an independent calibration. I request a direct-match-only zero-point (using only galaxies with direct SN/SBF matches, no group-expanded distances) as a cross-check, and explicit propagation of the δFP and δSN uncertainties into the covariance matrix of Eq. (12). The current matrix includes σ_ηzp and σ_δη but not the uncertainties in the group corrections themselves.
- [Abstract, §3.1, §6.3] Because the zero-point is defined by aligning DESI distances to SH0ES/Pantheon+ SNe Ia, the resulting H0 inherits the entire three-rung calibration and will track the SH0ES value by construction. The paper discloses this clearly in §6.3, but the abstract and title present 'H0 constraints' without this caveat. I recommend reframing the primary result as a calibration of the DESI PV zero-point with an H0 consistency check, and stating in the abstract that the H0 value is not an independent measurement of the Hubble constant.
- [§5.5, §6.1.4] The z>0.01 calibrator cut is the largest systematic in Fig. 10, yet its justification is qualitative: low-redshift SN residuals are attributed to peculiar velocities and volume scattering, while the DESI PV residuals in the same region are described as 'in the opposite direction.' The binary test that removes the cut is useful, but it does not establish that the cut is not absorbing a real redshift-dependent offset. I ask for a sensitivity scan of the cut redshift (e.g., 0.008, 0.012, 0.015) and, if feasible, a joint model of the low-z calibration offset.
minor comments (5)
- [§3.3] 'tip of the red giant branch (TRBG)' should be 'TRGB'; the acronym is correct elsewhere.
- [§5.4] 'between FP and T' should be 'FP and TF'.
- [§5.5/Fig. 7] The text says the average difference is 0.139±0.007, but the caption reads 'PV G′ SN G′: 0.139+/-0.007'; please make the notation consistent.
- [Abstract] The claim of 'lower statistical uncertainty than any previously used to measure H0' is technically true but potentially misleading; consider emphasizing that the total uncertainty is dominated by calibrator systematics.
- [Data Availability] The Zenodo DOI is a placeholder; please provide the final DOI or a clear statement that it will be activated upon acceptance.
Circularity Check
Headline H0 is inherited from the SN calibrator rather than independently predicted: the zero-point is defined to match SH0ES/Pantheon+ distances, so the fitted H0 tracks the input ladder by construction.
specific steps
-
fitted input called prediction
[Sec. 5.5 (Eq. 10) and Sec. 6.3]
"ηzp = ⟨η PV⟩G′ − ⟨η cal⟩G′. ... Since this distance ladder contains systematic uncertainties from all three rungs, our analysis cannot in principle give a H0 constraint tighter than the SH0ES measurement. ... For this reason, our H0 constraints are never tighter than those achieved from the calibrator by itself."
The zero-point is defined as the weighted difference between the DESI PV sample and the SH0ES/Pantheon+ SN distances. Those SN distances already encode the SH0ES H0 (73.04 km/s/Mpc). After shifting the DESI sample by this zero-point, the H0 fit returns the calibrator's H0 up to small relative-distance weighting. The headline H0 = 73.7 ± 1.1 is therefore statistically forced by the input calibration rather than being an independent prediction from the DESI sample. The paper is transparent about this floor, but the numerical central value is still inherited by construction.
full rationale
The paper's central contribution is the zero-point calibration of the DESI FP/TF sample and the demonstration that a >100,000-galaxy PV sample can yield very small statistical uncertainties. The group-matching and selection-correction methodology are independent modeling steps. However, the headline H0 constraint itself is not independent of the SH0ES/Pantheon+ distance ladder: Eq. (10) defines the zero-point as the difference between DESI and SN-calibrator distances, and the paper explicitly acknowledges that the H0 constraint cannot be tighter than the calibrator's own H0 and that the systematic uncertainty is approximately equal to the SH0ES total uncertainty. Thus the central numerical result reduces, by construction, to the input SN calibration. The group corrections (δFP, δSN) are internally estimated and tested, so they are systematic modeling choices rather than circular predictions, but the main H0 value is a calibrated inheritance rather than an independent measurement. This is disclosed, so the circularity is partial rather than hidden, scoring 6.
Axiom & Free-Parameter Ledger
free parameters (6)
- FP group-selection correction δFP =
-0.057 ± 0.010 mag (converted to η)
- SN-host group correction δSN =
consistent with 0 (weighted mean difference applied)
- TF-FP alignment offset δη =
0.0051 ± 0.0041
- TF relation slope/intercept (a, b) =
not quoted in this paper
- FP relation coefficients (a, b, c) =
not quoted in this paper
- q0 in free-fit case =
-0.56 +0.01/-0.02
axioms (7)
- domain assumption TF and FP relations are standard candles/rulers with no significant redshift evolution over the fitted range.
- domain assumption All galaxies within a galaxy group are at the same distance.
- domain assumption The SH0ES/Pantheon+ SN covariance matrix fully captures the statistical and systematic uncertainties of the distance ladder.
- ad hoc to paper The low-redshift (z < 0.01) disagreement between DESI PV and calibrators is due to peculiar-velocity and volume-scattering effects, justifying the redshift cut.
- domain assumption The 2M++/pvhub velocity-field reconstruction adequately corrects redshifts for large-scale coherent motions when fitting H0.
- domain assumption Fiducial flat ΛCDM with Ωm = 0.3151 is adequate for converting redshifts to distances at z < 0.1.
- ad hoc to paper The group-selection corrections δFP and δSN remove the bias introduced by the shallower group catalogs.
read the original abstract
The Dark Energy Spectroscopic Instrument (DESI) in its first Data Release (DR1) already provides more than 100,000 galaxies with relative distance measurements. The primary purpose of this paper is to perform the calibration of the zero-point for the DESI Fundamental Plane and Tully-Fisher relations, which allows us to measure the Hubble constant, $H_0$. This sample has a lower statistical uncertainty than any previously used to measure $H_0$, and we investigate the systematic uncertainties in absolute calibration that could limit the accuracy of that measurement. We improve upon the DESI Early Data Release Fundamental Plane $H_0$ measurement by a) using a group catalog to increase the number of calibrator galaxies and b) investigating alternative calibrators in the nearby Universe. Our baseline measurement calibrates to the SH0ES/Pantheon+ type Ia supernovae, and finds $H_0=73.7\pm 0.06\;(\text{stat.})\pm 1.1\;(\text{syst.})$ km s$^{-1}$ Mpc$^{-1}$. Calibrating to surface brightness fluctuation (SBF) distances yields a similar $H_0$. We explore measurements using other calibrators, but these are currently less precise since the overlap with DESI peculiar velocity tracers is much smaller. In future data releases with an even larger peculiar velocity sample, we plan to calibrate directly to Cepheids and the tip of the red giant branch, which will enable the uncertainty to decrease towards a percent-level measurement of $H_0$. This will provide an alternative to supernovae as the Hubble flow sample for $H_0$ measurements.
Figures
Forward citations
Cited by 1 Pith paper
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The DESI DR1 Peculiar Velocity Survey: growth rate measurements from the maximum likelihood fields method
DESI DR1 peculiar velocity data yields fσ8(z_eff=0.07) = 0.450 ± 0.055, consistent with Planck ΛCDM and GR growth index γ = 0.58 ± 0.11.
Reference graph
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