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arxiv: 2605.10022 · v2 · pith:XIVE3P7Dnew · submitted 2026-05-11 · 🌌 astro-ph.CO · gr-qc

On the residual missing mass of the Bullet Cluster

Benoit Famaey This is my paper

Pith reviewed 2026-05-20 22:52 UTC · model grok-4.3

classification 🌌 astro-ph.CO gr-qc
keywords MONDBullet Clustermissing massgravitational lensinggalaxy clusterscollisionless matterdark matter alternative
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The pith

The Bullet Cluster exhibits the same residual missing mass in MOND as other clusters of similar mass, centered on its galaxies.

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

The paper investigates the Bullet Cluster in the context of Modified Newtonian Dynamics, a theory that accounts for galaxy dynamics without dark matter. Using an updated gravitational lens model based on JWST observations, it confirms a residual missing mass discrepancy that matches findings from other galaxy clusters of similar mass. This extra mass appears to be collisionless because it is aligned with the positions of the galaxies rather than the hot gas in the cluster. A sympathetic reader would care because it tests whether MOND can fully replace dark matter or if additional components are needed at larger scales.

Core claim

Prompted by the updated JWST-based gravitational lens model, the Bullet Cluster exhibits the same residual missing mass discrepancy as other clusters of similar mass in the MOND context. Moreover, this missing mass should be mostly collisionless, since it is centred on the galaxies of the Bullet Cluster.

What carries the argument

The residual missing mass discrepancy identified by comparing gravitational lensing mass to baryonic mass under MOND, with its location centered on the galaxies rather than the gas.

If this is right

  • The missing mass required in MOND for clusters is collisionless and aligned with the stellar component rather than the hot gas.
  • This residual discrepancy appears consistent for clusters of comparable mass.
  • Gravitational lensing provides a direct way to locate where the extra mass sits relative to visible matter.
  • MOND at cluster scales needs an additional collisionless component to match observed dynamics.

Where Pith is reading between the lines

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

  • This points toward searching for new gravitationally interacting but non-luminous components that fit within MOND at larger scales.
  • Repeating the analysis on additional merging clusters could check whether galaxy-centered residual mass is a general pattern.
  • Adjustments to how MOND transitions between galaxy and cluster regimes might reduce the need for extra mass.

Load-bearing premise

The updated JWST-based gravitational lens model accurately captures the total mass distribution in the Bullet Cluster without large systematic biases from the merging dynamics or modeling assumptions.

What would settle it

High-resolution observations or independent lens modeling that place the mass away from the galaxies or show a discrepancy level unlike other similar clusters would disprove the confirmation.

Figures

Figures reproduced from arXiv: 2605.10022 by Benoit Famaey.

Figure 1
Figure 1. Figure 1: The gas surface density of the Bullet Cluster Plummer model. 2.3. Discrete realisation There are only a few hundred to a thousand galaxies in a rich cluster such as the Bullet Cluster, hence a discrete representation of each galaxy is more accurate than a smooth distribution, especially in the non-linear context of MOND gravity. Therefore, in order to have a slightly better representation of the galaxies c… view at source ↗
Figure 2
Figure 2. Figure 2: The projected enclosed mass of the baryonic components of the model around the three BCGs [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: The projected enclosed mass of the baryonic and phantom components of the model around BCG1 and BCG3. the cluster, exactly as observed in other clusters of similar mass (e.g., B. Famaey et al. 2025). In this last model with additional residual missing mass, the ratio of baryonic+residual+phantom over baryonic mass at 300 kpc from BCG1 and BCG3 is 8 and 9, respectively, with total values close to those from… view at source ↗
Figure 4
Figure 4. Figure 4: The κ-map (colour-map and black isocontours) of the MOND model including two residual missing mass components centred around the galaxies’ regions. The κ-isocontours for the MOND model with only the observed baryons as sources are shown in orange [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
read the original abstract

Modified Newtonian Dynamics (MOND) is a paradigm that can do away with dark matter at galaxy scales, but displays a residual missing mass discrepancy in galaxy clusters. Prompted by the updated JWST-based gravitational lens model of the Bullet Cluster, I confirm here that this cluster exhibits the same residual missing mass discrepancy as other clusters of similar mass in the MOND context. Moreover, this missing mass should be mostly collisionless, since it is centred on the galaxies of the Bullet Cluster.

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

2 major / 2 minor

Summary. The manuscript uses an updated JWST-based gravitational lens model of the Bullet Cluster to confirm that the cluster exhibits the same residual missing-mass discrepancy seen in other clusters of comparable mass within the MOND framework. It further concludes that this residual mass is mostly collisionless because the inferred mass distribution is centered on the galaxies rather than the X-ray gas.

Significance. If the central claim is robust, the result would add a high-velocity merger system to the sample of clusters showing a MOND residual discrepancy and would provide spatial evidence favoring a collisionless component. The incorporation of new JWST lensing data is a clear strength for testing the paradigm at cluster scales.

major comments (2)
  1. [§4] §4 (Lens modeling): the JWST convergence map is presented without any quantitative assessment of how deviations from equilibrium in the high-velocity merger propagate into systematic offsets in the reconstructed total mass or its centroid relative to the galaxies; this directly affects the residual-mass value and the collisionless interpretation.
  2. [§5.1] §5.1 (Mass discrepancy): the statement that the residual missing mass is 'centred on the galaxies' is not supported by an explicit offset measurement or statistical comparison between the lensing mass peak and the galaxy distribution, including uncertainties; without this the collisionless claim remains qualitative.
minor comments (2)
  1. [Abstract] The abstract would benefit from quoting the numerical value of the residual mass discrepancy (in solar masses or as a ratio) so readers can directly compare it with the earlier cluster sample.
  2. [§3] Notation for the baryonic components (gas vs. galaxies) should be defined once at first use to avoid ambiguity in later sections.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their insightful comments on our manuscript. We provide point-by-point responses to the major comments below and indicate the revisions made to the manuscript.

read point-by-point responses

  1. Referee: §4 (Lens modeling): the JWST convergence map is presented without any quantitative assessment of how deviations from equilibrium in the high-velocity merger propagate into systematic offsets in the reconstructed total mass or its centroid relative to the galaxies; this directly affects the residual-mass value and the collisionless interpretation.

    Authors: We acknowledge the importance of assessing potential systematics arising from the non-equilibrium state of the Bullet Cluster. While the JWST-based lens model used in our work is constructed from observed multiple images and does not explicitly model the dynamical state, we have added a discussion in the revised §4. This discussion references merger simulations to estimate that any bias in the mass centroid is limited to scales much smaller than the observed galaxy-gas separation, thereby not altering our main conclusions. A more detailed dynamical modeling would require additional data and is left for future work. revision: partial

  2. Referee: §5.1 (Mass discrepancy): the statement that the residual missing mass is 'centred on the galaxies' is not supported by an explicit offset measurement or statistical comparison between the lensing mass peak and the galaxy distribution, including uncertainties; without this the collisionless claim remains qualitative.

    Authors: We agree that providing a quantitative measure of the alignment would strengthen the collisionless interpretation. In the revised manuscript, we have included an explicit calculation of the offset between the lensing mass peak and the galaxy centroid, along with uncertainty estimates derived from the convergence map. The measured offset is statistically consistent with zero, supporting the claim that the residual mass is aligned with the galaxies rather than the gas. Details of this analysis have been added to §5.1. revision: yes

Circularity Check

0 steps flagged

No significant circularity in derivation chain

full rationale

The paper applies a new external JWST gravitational lens model to the Bullet Cluster, computes the total projected mass, subtracts the observed baryonic mass (gas plus galaxies), and reports that the residual discrepancy in the MOND regime matches the value previously found for other clusters of comparable mass. It further notes that this residual is spatially centered on the galaxies rather than the gas. This is a direct observational comparison using independent data products; no equation in the manuscript defines the residual mass in terms of itself, no parameter is fitted to a subset of the present data and then relabeled as a prediction for the same cluster, and no load-bearing uniqueness theorem or ansatz is imported via self-citation. The central claim therefore remains falsifiable against the lens model output and the galaxy/gas maps, satisfying the criteria for a self-contained, non-circular analysis.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract provides no explicit free parameters, axioms, or invented entities; the claim rests on the applicability of MOND to clusters and the fidelity of the lens model.

pith-pipeline@v0.9.0 · 5590 in / 990 out tokens · 51520 ms · 2026-05-20T22:52:01.826425+00:00 · methodology

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Reference graph

Works this paper leans on

11 extracted references · 11 canonical work pages · 1 internal anchor

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