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arxiv: 2606.23314 · v1 · pith:A5T7RSG2new · submitted 2026-06-22 · 🌌 astro-ph.CO

Wedge-avoidance Fisher Forecasts for Primordial Non-Gaussianity from Dark-Ages 21-cm Power Spectrum and Bispectrum

Chen Zhang , Furen Deng , Yidong Xu , Bin Yue , Yuewei Wen , Xuelei Chen This is my paper

Pith reviewed 2026-06-26 07:28 UTC · model grok-4.3

classification 🌌 astro-ph.CO
keywords 21-cm cosmologyprimordial non-Gaussianityforeground wedgeFisher forecastsdark agesinflationary featurespower spectrumbispectrum
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The pith

Foreground wedge avoidance in 21-cm observations significantly weakens constraints on primordial non-Gaussianity.

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

The paper introduces a Fisher forecast framework that directly incorporates the loss of Fourier modes caused by avoiding the foreground wedge when measuring the 21-cm power spectrum and bispectrum from the dark ages. This mode loss is propagated into the variances used to forecast constraints on primordial non-Gaussianity amplitudes in two inflation models that produce oscillatory features. The central finding is that the wedge effect removes many modes in both observables, producing substantially weaker bounds than forecasts that omit it. A sympathetic reader would care because it supplies a more realistic limit on what lunar arrays can extract about early-universe physics. The framework is presented as usable for any redshift where the wedge is severe.

Core claim

By building a wedge-aware Fisher framework and applying it to resonant and step inflation models, the authors demonstrate that wedge avoidance reduces available Fourier modes substantially in both the cylindrical 21-cm power spectrum and reduced bispectrum, resulting in much weaker constraints on PNG amplitudes for a lunar far-side array that includes thermal noise.

What carries the argument

The wedge-aware Fisher framework that propagates the removed modes from foreground wedge avoidance directly into the variances of the cylindrical 21-cm power spectrum and reduced bispectrum.

If this is right

  • Constraints on PNG amplitudes become significantly weaker in both resonant and step models once wedge avoidance is included.
  • The same mode-loss effect appears in forecasts for both the power spectrum and the bispectrum.
  • The method applies to 21-cm forecasts at any redshift where the foreground wedge is important, including the epoch of reionization.
  • Thermal noise from a lunar array is folded into the forecasts alongside the wedge effect.

Where Pith is reading between the lines

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

  • Array designs for high-redshift 21-cm observations may need to prioritize baselines that stay outside the wedge to restore some lost constraining power.
  • The similar degradation in both observables suggests that adding the bispectrum does not automatically compensate for wedge losses.
  • Accurate foreground modeling becomes a limiting factor for any PNG search that relies on high-redshift 21-cm data.

Load-bearing premise

Foreground wedge avoidance can be accurately modeled by directly propagating the removed modes into the variances of the power spectrum and bispectrum without additional unaccounted systematics or modeling errors.

What would settle it

A measurement or simulation that shows the actual variance in the retained cylindrical modes differs substantially from the prediction obtained by simple removal of wedge modes would falsify the framework.

read the original abstract

The Dark Ages offer a uniquely clean window on primordial physics, making the redshifted 21-cm signal a promising probe of primordial non-Gaussianity (PNG). Forecasts for interferometric 21-cm measurements must account for foreground wedge avoidance, which removes a substantial fraction of Fourier modes, and this effect is especially severe at high redshifts. We develop a wedge-aware Fisher framework in which this mode loss is propagated directly into the variances of the cylindrical 21-cm power spectrum and reduced bispectrum. As a case study, we apply the method to forecast PNG constraints in two inflation models with different oscillatory features, a resonant model and a step model, for a lunar far-side array including thermal noise. Since these features affect both the power spectrum and the bispectrum, we apply wedge avoidance to both forecasts and compare their constraining power on PNG amplitudes in these oscillatory feature models. We find that wedge avoidance reduces available Fourier modes significantly in both observables, leading to much weaker constraints on PNG. This framework is broadly applicable to 21-cm power spectrum and bispectrum forecasts across redshifts and is particularly useful in regimes where foreground wedge effect is severe, such as the epoch of reionization and the higher-redshift Dark Ages.

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

1 major / 2 minor

Summary. The paper develops a wedge-aware Fisher matrix framework for forecasting constraints on primordial non-Gaussianity (PNG) amplitudes in resonant and step inflation models from the cylindrical 21-cm power spectrum and reduced bispectrum during the Dark Ages. Mode loss from foreground wedge avoidance is propagated directly into the variances of both observables for a lunar far-side array that includes thermal noise; the central result is that this mode removal substantially weakens PNG constraints relative to forecasts that ignore the wedge.

Significance. If the direct-propagation approximation holds, the work supplies a practical, broadly applicable tool for including realistic foreground effects in high-redshift 21-cm forecasts. It quantifies how wedge avoidance degrades both power-spectrum and bispectrum constraints on oscillatory PNG features, which is relevant for array design and survey strategy at z ≳ 30.

major comments (1)
  1. [Abstract, §3] Abstract and §3 (Fisher framework): the central claim that wedge avoidance produces 'much weaker constraints' rests on the assumption that excised modes can be propagated directly into the variances of the cylindrical PS and reduced BS without residual systematics, mode-coupling, or bispectrum triangle reconfiguration. No explicit validation or error budget for this approximation is described; if residuals are non-negligible the reported degradation factor is not robust.
minor comments (2)
  1. [§2] Notation for the reduced bispectrum and the precise definition of the cylindrical power spectrum should be stated explicitly in the methods section to allow reproduction of the Fisher matrix.
  2. The abstract states that the framework is 'broadly applicable' but does not indicate whether the code or Fisher matrices will be made public; adding a reproducibility statement would strengthen the paper.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their careful review and constructive feedback. We address the major comment below.

read point-by-point responses

  1. Referee: [Abstract, §3] Abstract and §3 (Fisher framework): the central claim that wedge avoidance produces 'much weaker constraints' rests on the assumption that excised modes can be propagated directly into the variances of the cylindrical PS and reduced BS without residual systematics, mode-coupling, or bispectrum triangle reconfiguration. No explicit validation or error budget for this approximation is described; if residuals are non-negligible the reported degradation factor is not robust.

    Authors: We thank the referee for highlighting this key aspect of our methodology. The manuscript explicitly adopts the approximation that excised modes are propagated directly into the variances of the cylindrical power spectrum and reduced bispectrum (as stated in the abstract and §3) in order to isolate the leading-order impact of wedge avoidance on the Fisher information for PNG amplitudes. This is presented as a practical forecasting tool rather than a full end-to-end simulation. We agree that the current text does not provide an explicit validation, quantitative error budget, or discussion of potential residuals, mode-coupling, or bispectrum triangle reconfiguration. In the revised manuscript we will expand §3 with a new subsection discussing the assumptions and limitations of the direct-propagation approximation, including references to analogous treatments in the 21-cm literature and a qualitative assessment of regimes where residuals could become non-negligible. This will make clear that the reported degradation factors are conditional on the stated approximation. revision: yes

Circularity Check

0 steps flagged

No significant circularity; forward-model Fisher forecast is self-contained

full rationale

The paper presents a modeling framework that directly excises wedge-affected modes from the cylindrical power spectrum and reduced bispectrum variances before computing the Fisher matrix for PNG constraints. This is an explicit modeling choice rather than a derivation that reduces to its own inputs by construction. No self-citations, fitted parameters renamed as predictions, or ansatzes smuggled via prior work are described in the abstract or reader's summary. The central result (weaker constraints after mode removal) follows from the chosen propagation of mode loss and is externally falsifiable against actual 21-cm data or more complete simulations. This qualifies as an honest non-finding under the guidelines.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Review performed on abstract only; full details of parameters and assumptions unavailable. The central claim rests on standard cosmological assumptions about 21-cm signal and foregrounds plus the specific array model.

axioms (1)
  • domain assumption Foreground wedge avoidance removes a substantial fraction of Fourier modes at high redshifts.
    Explicitly stated as the key effect to be propagated into the Fisher matrix.

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discussion (0)

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Reference graph

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