arxiv: 2604.06888 · v1 · submitted 2026-04-08 · 🌌 astro-ph.CO

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Low-redshift-agnostic BAO Constraints on Binned Dark-energy Density Evolution from DESI DR1 and DR2

Qian-Mo Liu , Gong-Bo Zhao

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Pith reviewed 2026-05-10 18:21 UTC · model grok-4.3

classification 🌌 astro-ph.CO

keywords BAOdark energy densityDESIredshift binsdata compressionexpansion historycosmological constraintsanisotropic distances

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The pith

A linear compression of BAO distances removes the low-redshift transverse mode to produce nearly uncorrelated constraints on binned dark-energy density evolution above the first node.

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

The paper develops a compression method for anisotropic BAO data that replaces absolute transverse distances with their adjacent increments while keeping radial distances. This isolates information on the expansion history above the lowest measured redshift and avoids mixing in unknown contributions from below that redshift. When applied to DESI DR1 and DR2 measurements, the resulting constraints on piecewise-constant dark-energy density parameters in separate redshift bins are almost uncorrelated with one another. All bins remain consistent with a constant dark-energy density within the uncertainties. The approach is presented as a conservative baseline that marginalizes over local matter density and distance-scale parameters using broad priors.

Core claim

The authors replace the standard set of transverse BAO distances D_M/r_d with adjacent increments ΔD_M/r_d while retaining the radial distances D_H/r_d. Because the mapping is linear, the covariance matrix transforms exactly. This compression deliberately removes one absolute transverse-distance mode below the first BAO node and preserves the remaining information needed to reconstruct the expansion history above that node. Applied to the DESI DR1 and DR2 anisotropic BAO data, the procedure yields almost uncorrelated constraints on the piecewise-constant interval parameters X_j for the normalized dark-energy density. All bins are consistent with X=1 within current uncertainties, providing a低

What carries the argument

The linear replacement of absolute transverse comoving distances D_M/r_d by adjacent increments ΔD_M/r_d, while retaining radial distances D_H/r_d, to remove the low-redshift transverse mode.

If this is right

Each X_j interval is constrained mainly by BAO information from its own redshift range.
The compressed likelihood acts as a conservative band-power-like estimate of dark-energy evolution.
One nonlocal transverse mode and stronger global assumptions are deliberately projected out or marginalized over.
The resulting X_j constraints serve as a low-redshift-agnostic BAO baseline rather than a fully prior-free reconstruction.
All current bins remain consistent with constant dark-energy density (X=1).

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same differencing approach could be applied to other distance indicators such as supernovae to isolate high-redshift signals in a comparable way.
If future surveys detect a deviation from X=1 in a specific bin using this method, it would point to evolving dark energy without contamination from low-redshift assumptions.
The technique underscores how careful data compression can reduce reliance on global priors when testing for time-varying dark energy.

Load-bearing premise

The linear compression removes only the unwanted low-redshift transverse mode while preserving all information relevant to reconstructing the expansion history above the first node, and broad external priors on bin-local matter-density and distance-scale parameters introduce no bias into the X_j constraints.

What would settle it

A future data set in which the compressed X_j constraints become strongly correlated or one bin deviates significantly from unity while a standard full-analysis reconstruction of the same data remains consistent with X=1 would indicate that the compression has either lost relevant information or introduced bias.

Figures

Figures reproduced from arXiv: 2604.06888 by Gong-Bo Zhao, Qian-Mo Liu.

**Figure 2.** Figure 2: Correlation matrices among the effective interval-wise parameters [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗

read the original abstract

We present a low-redshift-agnostic compression of anisotropic baryon acoustic oscillation (BAO) distances to constrain the normalized dark-energy density evolution, $X(z)\equiv \rho_{\rm DE}(z)/\rho_{\rm DE}(0)$, above the lowest BAO redshift node $z_1$. Standard BAO summaries include the transverse comoving distance $D_{\rm M}/r_{\rm d}$, which depends on the integral of $H^{-1}(z)$ from $z=0$ to $z$ and therefore mixes the expansion history at $z<z_1$ with the higher-redshift signal. We instead replace the set $D_{\rm M}(z_i)/r_{\rm d}$ by adjacent increments $\Delta D_{\rm M}(z_i,z_{i+1})/r_{\rm d}$ while retaining the radial distances $D_{\rm H}(z_i)/r_{\rm d}$. The mapping is linear, so the covariance propagates exactly. This compression intentionally removes one absolute transverse-distance mode, namely the additive contribution to $D_{\rm M}/r_{\rm d}$ below the first BAO node, and preserves the remaining information relevant to reconstructing the expansion history above $z_1$. Applied to DESI DR1 and DR2 anisotropic BAO measurements, the method yields almost uncorrelated constraints on piecewise-constant interval parameters $X_j$. In this sense, the compressed likelihood provides a conservative band-power-like estimate of dark-energy evolution: each interval is constrained mainly by BAO information from its own redshift range, while one nonlocal transverse mode and stronger global assumptions are deliberately projected out or marginalized over. Because our baseline analysis also marginalizes over bin-local matter-density and distance-scale parameters with broad external priors, the resulting $X_j$ constraints should be interpreted as a low-redshift-agnostic BAO baseline rather than as a fully prior-free reconstruction. All bins are consistent with $X=1$ within current uncertainties.

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.

Desk Editor's Note private letter to a colleague

The paper's main contribution is a linear compression of DESI BAO data that drops the absolute low-redshift transverse mode while keeping higher-z information, producing nearly uncorrelated binned X(z) constraints consistent with constant dark energy.

read the letter

The useful part is the data compression itself. By swapping absolute D_M/r_d for adjacent increments Delta D_M between redshift nodes and keeping the radial D_H/r_d, the method removes exactly one nonlocal mode tied to the integral below z1. The transformation is linear, so the covariance matrix updates algebraically without approximation. When applied to the DESI DR1 and DR2 anisotropic measurements, this yields X_j constraints for piecewise-constant dark-energy density bins that are almost uncorrelated and all consistent with X=1 after marginalizing broad priors on bin-local matter density and distance-scale parameters. The authors are explicit that this is a conservative baseline rather than a prior-free result, which matches the goal of low-redshift agnosticism. That framing is honest and helps the reader know what the numbers actually mean. The approach is new in the context of DESI summaries and could be adopted for other surveys where one wants to isolate expansion history above a given redshift without mixing in lower-z assumptions. The main limitation is that the current error bars are still large, so the consistency with X=1 is not yet a tight test. The marginalization uses external broad priors, which is reasonable but means the constraints are not fully independent of those choices. Bin definitions and the precise impact on the covariance would need checking in the full text, but nothing in the description suggests a flaw in the linear mapping or the central claim. This is the kind of practical methodological note that observational cosmologists working on BAO or dark-energy reconstruction would find worth reading. It deserves peer review because the technique is cleanly executed and directly applicable to existing and upcoming data sets.

Referee Report

0 major / 3 minor

Summary. The manuscript introduces a low-redshift-agnostic compression technique for anisotropic BAO data from DESI DR1 and DR2. By replacing the transverse comoving distances D_M(z_i)/r_d with their adjacent increments ΔD_M(z_i, z_{i+1})/r_d while retaining the radial distances D_H(z_i)/r_d, the method removes the integral contribution from z=0 to z1, allowing constraints on the binned dark-energy density evolution X(z) = ρ_DE(z)/ρ_DE(0) above z1 without assumptions on the low-redshift expansion history. The linear nature of the mapping ensures exact covariance propagation. The resulting constraints on the piecewise-constant X_j parameters are nearly uncorrelated, and all bins are found to be consistent with X=1 within uncertainties after marginalizing over bin-local matter-density and distance-scale parameters using broad priors. The approach is framed as providing a conservative baseline estimate rather than a fully prior-free reconstruction.

Significance. If the central claims hold, the work provides a useful conservative tool for obtaining nearly uncorrelated, band-power-like constraints on dark-energy evolution directly from BAO data. The exact linear transformation guarantees algebraic covariance propagation without approximation, and the explicit framing as a low-redshift-agnostic baseline (with broad marginalization) makes the results interpretable and reproducible. This could serve as a standard reference method for future DESI and similar surveys when testing deviations from X=1 in a manner that deliberately projects out nonlocal low-z modes.

minor comments (3)

The abstract states that the method 'yields almost uncorrelated constraints' on X_j, but the manuscript should include the numerical correlation matrix (or its off-diagonal elements) for the DESI DR1+DR2 application to allow readers to assess the degree of uncorrelatedness quantitatively.
The specific redshift bin boundaries for the piecewise-constant X_j intervals and the exact BAO redshift nodes z_i from DESI DR1/DR2 used in the analysis should be listed explicitly (e.g., in a table) to ensure full reproducibility of the compressed likelihood.
A brief quantitative comparison between the compressed X_j constraints and those obtained from the standard (uncompressed) BAO likelihood would help illustrate the information loss from removing the single transverse mode and strengthen the 'conservative baseline' interpretation.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive and accurate summary of our manuscript, which correctly captures the low-redshift-agnostic BAO compression technique, the linear mapping, and the conservative framing of the resulting X(z) constraints. We appreciate the recognition of the method's utility as a reference approach for future surveys. No major comments were provided in the report.

Circularity Check

0 steps flagged

No significant circularity in the derivation chain

full rationale

The paper defines a fixed linear compression that replaces the set of transverse distances D_M(z_i)/r_d with adjacent increments ΔD_M(z_i, z_{i+1})/r_d while retaining the radial distances D_H(z_i)/r_d. This mapping is algebraic and independent of the target piecewise-constant dark-energy parameters X_j; the covariance is propagated exactly by matrix multiplication. Constraints on the X_j are then obtained by fitting the compressed DESI DR1/DR2 data while marginalizing over bin-local matter-density and distance-scale nuisance parameters using explicit broad external priors. No load-bearing step reduces the claimed X_j constraints to the inputs by construction, no self-citation is invoked to justify uniqueness or an ansatz, and the result is presented as a conservative baseline rather than a tautological prediction. The derivation is therefore self-contained against external data.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The approach rests on standard cosmological assumptions for BAO as a standard ruler and on the choice of piecewise-constant bins; it marginalizes over two classes of nuisance parameters with broad priors rather than introducing new fitted constants or entities.

free parameters (2)

bin-local matter-density parameters
Marginalized with broad external priors as stated in the abstract
distance-scale parameters
Marginalized with broad external priors as stated in the abstract

axioms (2)

domain assumption BAO distances serve as a standard ruler whose radial and transverse components can be separated
Invoked throughout the BAO analysis description
domain assumption Dark-energy density evolution can be represented as piecewise constant in chosen redshift intervals
Basis for the binned X_j parameters

pith-pipeline@v0.9.0 · 5676 in / 1499 out tokens · 39085 ms · 2026-05-10T18:21:55.260679+00:00 · methodology

discussion (0)

Forward citations

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Reference graph

Works this paper leans on

19 extracted references · 4 canonical work pages · cited by 1 Pith paper · 2 internal anchors

[1]

, " * write output.state after.block = add.period write newline

ENTRY address archivePrefix author booktitle chapter doi edition editor eprint howpublished institution journal key month number organization pages publisher school series title misctitle type volume year version url label extra.label sort.label short.list INTEGERS output.state before.all mid.sentence after.sentence after.block FUNCTION init.state.consts ...
[2]

write newline

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[3]

NzXeT Ky 9w^

thebibliography [1] 20pt to REFERENCES 6pt =0pt \@twocolumntrue 12pt -12pt 10pt plus 3pt =0pt =0pt =1pt plus 1pt =0pt =0pt -12pt =13pt plus 1pt =20pt =13pt plus 1pt \@M =10000 =-1.0em =0pt =0pt 0pt =0pt =1.0em @enumiv\@empty 10000 10000 `\.\@m \@noitemerr \@latex@warning Empty `thebibliography' environment \@ifnextchar \@reference \@latexerr Missing key o...

2017
[4]

Data Release 1 of the Dark Energy Spectroscopic Instrument

Abdul Karim, M., et al. 2025 a , arXiv:2503.14745

work page internal anchor Pith review arXiv 2025
[5]

DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman Alpha Forest

Abdul Karim, M., et al. 2025 b , arXiv:2503.14739

work page arXiv 2025
[6]

DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints

Abdul Karim, M., et al. 2025 c , arXiv:2503.14738

work page Pith review arXiv 2025
[7]

G., et al

Adame, A. G., et al. 2025 a , JCAP, 04, 012

2025
[8]

G., et al

Adame, A. G., et al. 2025 b , JCAP, 01, 124

2025
[9]

2020, Astron

Aghanim, N., et al. 2020, Astron. Astrophys., 641, A6, [Erratum: Astron. Astrophys. 652, C4 (2021)]

2020
[10]

The DESI Experiment Part I: Science,Targeting, and Survey Design

DESI Collaboration . 2016, arXiv:1611.00036

work page internal anchor Pith review arXiv 2016
[11]

2022, Astron

DESI Collaboration . 2022, Astron. J., 164, 207

2022
[12]

J., & Hu, W

Eisenstein, D. J., & Hu, W. 1998, Astrophys. J., 496, 605

1998
[13]

J., Zehavi, I., Hogg, D

Eisenstein, D. J., Zehavi, I., Hogg, D. W., et al. 2005, Astrophys. J., 633, 560

2005
[14]

2025, Nature Astron., 9, 1879, [Erratum: Nature Astron

Gu, G., et al. 2025, Nature Astron., 9, 1879, [Erratum: Nature Astron. 9, 1898 (2025)]

2025
[15]

2005, Phys

Huterer, D., & Cooray, A. 2005, Phys. Rev. D, 71, 023506

2005
[16]

2003, Phys

Huterer, D., & Starkman, G. 2003, Phys. Rev. Lett., 90, 031301

2003
[17]

2025, Phys

Lodha, K., et al. 2025, Phys. Rev. D, 112, 083511

2025
[18]

2004, Phys

Wang, Y., & Tegmark, M. 2004, Phys. Rev. Lett., 92, 241302

2004
[19]

write newline

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