Constraints on Ultra-Light Axions from the DESI DR1 Full Shape, Planck and ACT

Emanuele Castorina; Emiliano Sefusatti; Ennio Salvioni; Francesco Verdiani; Matteo Viel

REVIEW 3 major objections 2 minor 1 cited by

Joint DESI DR1 full-shape and CMB data set the tightest limits yet on ultra-light axions as a dark-matter subcomponent across 10^{-32}–10^{-24} eV.

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 →

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T0 review · grok-4.5

2026-07-14 02:29 UTC pith:7YQLANPX

load-bearing objection First DESI DR1 full-shape ULA subcomponent bounds that tighten CMB-only limits by >2× at the lightest masses; the usual EFT modeling assumption is load-bearing and unchecked from the abstract alone. the 3 major comments →

arxiv 2607.11876 v1 pith:7YQLANPX submitted 2026-07-13 astro-ph.CO hep-ph

Constraints on Ultra-Light Axions from the DESI DR1 Full Shape, Planck and ACT

Francesco Verdiani , Emanuele Castorina , Ennio Salvioni , Emiliano Sefusatti , Matteo Viel This is my paper

classification astro-ph.CO hep-ph

keywords ultra-light axionsdark matterDESI DR1full-shape analysisCMB constraintsEffective Field Theory of LSSgalaxy power spectrumcosmological parameters

verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

This paper claims that combining the full-shape DESI DR1 galaxy power spectra with Planck and ACT cosmic microwave background data produces the strongest existing upper bounds on the fraction of ultra-light axions that can contribute to the total matter density in the mass window 10^{-32} eV to 10^{-24} eV. The analysis relies on Effective Field Theory modeling of large-scale structure to extract information from the galaxy clustering spectra. At the lightest masses the joint data set improves on CMB-only limits by more than a factor of two; near 10^{-29} eV, for example, ultra-light axions are restricted to less than 0.3 percent of the total matter density. A mild preference for a small ultra-light-axion component around 10^{-26} eV appears in the DESI luminous red galaxy sample alone, but disappears once CMB measurements are included. The result therefore both tightens the allowed parameter space for these particles and shows that current hints of a detection do not survive the joint analysis.

Core claim

When DESI DR1 full-shape galaxy power spectra are analyzed jointly with Planck and ACT CMB data inside the Effective Field Theory of large-scale structure, the allowed fraction of ultra-light axions as a dark-matter subcomponent is reduced by more than a factor of two relative to CMB-only constraints at the lowest masses, reaching f_a ≲ 0.3 percent near m_a ∼ 10^{-29} eV and establishing the most stringent limits to date across the window 10^{-32}–10^{-24} eV.

What carries the argument

The full-shape analysis of the DESI DR1 galaxy power spectra performed with the Effective Field Theory of Large Scale Structure, jointly fitted with Planck and ACT CMB likelihoods; this supplies the additional late-time clustering information that tightens the ultra-light-axion fraction bounds.

Load-bearing premise

The Effective Field Theory modeling of the DESI galaxy spectra is assumed to remain free of residual theoretical bias across the mass window and scale cuts that are used, so that systematics do not artificially tighten or shift the reported bounds.

What would settle it

A re-analysis of the same DESI DR1 spectra that employs a different set of scale cuts or an independent theoretical model of the galaxy power spectrum and finds an ultra-light-axion fraction bound that is weaker by a factor of two or more at m_a ∼ 10^{-29} eV would falsify the claimed improvement.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

Ultra-light axions near 10^{-29} eV can constitute at most a few parts per thousand of the total matter density.
Any mild preference for an ultra-light-axion component around 10^{-26} eV seen in luminous red galaxy samples alone is erased by the addition of CMB data.
Future constraints at masses ≳ 10^{-25} eV will require both smaller-scale galaxy clustering and CMB lensing measurements and simultaneous advances in theoretical modeling.
CMB-only analyses are no longer the limiting factor for the lightest ultra-light axions once DESI full-shape information is included.

Where Pith is reading between the lines

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

The factor-of-two improvement at the lightest masses implies that forthcoming DESI data releases will begin to probe ultra-light-axion fractions below the 0.1 percent level if modeling systematics remain under control.
The disappearance of the BOSS-like preference once CMB data are added suggests that similar late-time clustering hints in other surveys are likely to be prior-volume or modeling artifacts rather than genuine signals.
Extending the same full-shape pipeline to smaller scales will test whether the current mass-window edge at 10^{-24} eV is a true physical boundary or merely a modeling limitation.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Desk Editor's Note

First DESI DR1 full-shape ULA subcomponent bounds that tighten CMB-only limits by >2× at the lightest masses; the usual EFT modeling assumption is load-bearing and unchecked from the abstract alone.

read the letter

The one thing you need to know is that this paper gives the first full-shape DESI DR1 constraints on ultra-light axions as a dark-matter subcomponent, and the joint run with Planck plus ACT improves the CMB-only limits by more than a factor of two for the lightest masses. Near 10^{-29} eV they push the allowed fraction down to 0.3 percent of total matter. That is a clean observational update if the pipeline holds.

What is actually new is the data combination itself: DESI DR1 galaxy power spectra analyzed with current EFT-of-LSS methods, plus ACT and Planck. The mass window and the subcomponent framing are standard, but the quantitative improvement and the concrete numbers are not already in the literature. They also flag the mild DESI-only preference around 10^{-26} eV that mirrors earlier BOSS hints and then vanishes once CMB is added. That is reported without spin, which is useful.

The soft spot is exactly the one the abstract itself names: the EFT modeling of the galaxy spectra has to remain unbiased across the mass window and the scale cuts they chose. Higher masses will need smaller scales and better theory. Because we only have the abstract, we cannot inspect the likelihoods, covariances, or residual systematics, so the claimed tightening rests on that assumption. Nothing looks circular or forced.

This is for people who keep ULA bounds current or who run DESI full-shape pipelines. Anyone updating dark-sector constraints will want the numbers. It is important enough inside the field and uses the standard toolkit, so it deserves a serious referee rather than a desk reject. I would send it out.

Referee Report

3 major / 2 minor

Summary. This manuscript reports updated constraints on ultra-light axions (ULAs) as a subcomponent of dark matter in the mass window 10^{-32}–10^{-24} eV, obtained from a joint full-shape analysis of DESI DR1 galaxy power spectra with Planck and ACT CMB data, using methods based on the Effective Field Theory of Large Scale Structure. The abstract claims that the joint analysis improves CMB-only bounds by more than a factor of two at the lightest masses, with ULAs limited to as little as 0.3% of the total matter density near m_a ∼ 10^{-29} eV, constituting the most stringent limits to date. A mild preference for a ULA subcomponent near m_a ≈ 10^{-26} eV in the DESI LRG sample is reported, mirroring earlier BOSS hints, but is stated to vanish upon combination with CMB data. The abstract notes that extending constraints to m_a ≳ 10^{-25} eV will require both smaller-scale data and concurrent improvements in theoretical modeling.

Significance. If the quantitative bounds and the claimed factor-of-two improvement hold under full scrutiny of the analysis pipeline, the work would deliver the tightest existing limits on ULA dark-matter fractions across a cosmologically important mass window and would illustrate the power of combining DESI full-shape clustering with CMB data for dark-sector constraints. Transparent reporting of a DESI-only preference that disappears with CMB is a methodological strength. The adoption of EFT-of-LSS methods is appropriate in principle for the scales involved, and the results would be of clear interest to both the cosmology and axion communities. Credit is due for framing the higher-mass regime as limited by modeling rather than data alone.

major comments (3)

[Abstract] The central quantitative claims—more than a factor-of-two improvement over CMB-only constraints and f_a ≲ 0.3% of total matter density near m_a ∼ 10^{-29} eV—are load-bearing for the paper’s significance. With only the abstract available, these claims cannot be checked against the likelihood pipeline, scale cuts, covariance treatment, posterior contours, or direct CMB-only versus joint comparisons. The full manuscript must present these results with explicit figures and tables so that the improvement factor and the 0.3% bound can be independently assessed.
[Abstract (final sentence)] The abstract correctly identifies that probing m_a ≳ 10^{-25} eV requires concurrent improvements in theoretical modeling. The claimed unbiased improvement across the full quoted window nevertheless rests on the assumption that EFT-of-LSS counterterms and the adopted scale cuts remain adequate at intermediate masses. Residual theoretical systematics that could artificially tighten or shift the reported f_a(m_a) bounds must be quantified (e.g., via mock-based validation, theory-error budgets, or explicit scale-cut stability tests); without that demonstration the ‘most stringent limits to date’ claim is not fully supported.
[Abstract] The mild DESI LRG preference near m_a ≈ 10^{-26} eV that vanishes with CMB data is reported transparently, but the manuscript must show that this feature is not driven by unmodeled systematics in the DESI-only analysis (sample selection, multipole range, or EFT nuisance marginalization). A clear comparison of DESI-only versus joint posteriors, with nuisance-parameter diagnostics, is required to establish that the preference is statistical rather than systematic.

minor comments (2)

[Abstract] The abstract is clear and appropriately cautious. In the full manuscript the definition of the ULA fraction f_a (relative to total matter versus dark matter only) should be stated unambiguously at first use, and the precise DESI samples, redshift bins, and multipoles employed should be listed early.
Without the full text it is not possible to assess figure clarity, notation consistency, or completeness of the reference list; these presentation items should be checked once the complete manuscript is available.

Circularity Check

0 steps flagged

No significant circularity: observational upper limits on free ULA fraction from external survey+CMB data are not forced by construction.

full rationale

The abstract reports updated upper bounds on the ultra-light axion dark-matter fraction f_a(m_a) obtained by joint full-shape analysis of DESI DR1 galaxy power spectra with Planck and ACT CMB data, using the Effective Field Theory of Large Scale Structure. These are standard Bayesian/frequentist constraints on a free parameter against external observational likelihoods; nothing in the abstract indicates that the reported limits (e.g., f_a ≲ 0.3% near 10^{-29} eV) are obtained by re-fitting a quantity already fixed by construction, by renaming a prior fit, or by a self-citation uniqueness theorem. The mild DESI-only preference that disappears when CMB data are added is an ordinary data-combination result, not a circular prediction. Residual concerns about EFT modeling fidelity across the mass window are correctness/systematics issues, not circularity of the derivation chain. With only the abstract available, no load-bearing step reduces to its own inputs; score 0 is the appropriate honest finding.

Axiom & Free-Parameter Ledger

2 free parameters · 3 axioms · 0 invented entities

Abstract-only review. The central claim rests on standard cosmological assumptions (ΛCDM background, ULA as a subcomponent of dark matter with the usual Jeans-scale suppression, validity of EFT-of-LSS for DESI scales) plus whatever free EFT nuisance parameters and ULA fraction/mass grid are used in the full analysis. No new particles or forces beyond the ULA itself are introduced; the ULA is a standard hypothesized entity being constrained, not invented here.

free parameters (2)

ULA matter fraction f_a(m_a)
The quantity being constrained; for each fixed mass the amplitude is a free parameter fitted to the data. Abstract quotes an upper limit ~0.3% near 10^{-29} eV.
EFT-of-LSS nuisance / counterterm parameters
Standard in full-shape analyses; number and priors not stated in the abstract but required to absorb small-scale galaxy bias and loop corrections.

axioms (3)

domain assumption Ultra-light axions behave as a dark-matter subcomponent with mass-dependent Jeans suppression of structure growth.
Standard ULA cosmology assumption underlying the entire mass-window analysis.
domain assumption Effective Field Theory of Large Scale Structure provides an unbiased model of the DESI galaxy power spectrum on the scales used.
Explicitly invoked as the analysis framework; abstract notes modeling improvements will be needed for higher masses.
domain assumption Planck and ACT CMB likelihoods and DESI DR1 full-shape likelihoods are correctly calibrated and combinable.
Required for the joint constraints that drive the claimed factor-of-two improvement.

pith-pipeline@v1.1.0-grok45 · 6190 in / 2528 out tokens · 22921 ms · 2026-07-14T02:29:12.344593+00:00 · methodology

0 comments

read the original abstract

We present updated bounds on ultra-light axions (ULAs) as a subcomponent of dark matter, derived from the full-shape analysis of the DESI Data Release 1 galaxy power spectra combined with Cosmic Microwave Background (CMB) data from ACT and Planck. We focus on the mass window $10^{-32}\,\mathrm{eV}\leq m_a \leq 10^{-24}\,\mathrm{eV}$, employing state-of-the-art analysis methods rooted in the Effective Field Theory of Large Scale Structure. For the smallest masses, our joint analysis with DESI improves over CMB-only constraints by more than a factor of 2, establishing the most stringent limits to date. For instance, for $m_a \sim 10^{-29}\,\mathrm{eV}$ ULAs are constrained to be a fraction as small as $0.3\%$ of the total matter energy density. The DESI Luminous Red Galaxy sample shows a mild preference for an ULA subcomponent with $m_a \approx 10^{-26}\,\mathrm{eV}$, mirroring previous hints from BOSS, but this preference vanishes upon combination with CMB data. Probing the largest masses, $m_a\gtrsim10^{-25}\,\mathrm{eV}$, in future studies will benefit from extending the data analysis to smaller scales, both for galaxy clustering and CMB lensing, but will also require concurrent improvements in the theoretical modeling.

discussion (0)

Forward citations

Cited by 1 Pith paper

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astro-ph.CO 2026-07 conditional novelty 6.0

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