arxiv: 2605.06267 · v1 · submitted 2026-05-07 · 🌌 astro-ph.CO · physics.space-ph

Recognition: unknown

Revisiting the Constancy of the Speed of Light: Galaxy Cluster Mass Bias Implications

Javier E. Gonzalez, Marcelo Ferreira, R. F. L. Holanda, S. H. Pereira

Pith reviewed 2026-05-08 05:34 UTC · model grok-4.3

classification 🌌 astro-ph.CO physics.space-ph

keywords speed of light constancygalaxy clustersX-ray gas mass fractionsupernovae Iamass biascosmological testsfundamental constants

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The pith

Galaxy cluster and supernova data support constant speed of light except under one mass calibration

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

The paper tests the constancy of the speed of light by comparing X-ray gas mass fractions in galaxy clusters against luminosity distances from type Ia supernovae in the Pantheon+ sample. It uses the SH0ES prior on the Hubble constant and the baryon-to-matter density ratio from galaxy clustering observations to limit dependence on any particular cosmological model. Three different cluster mass bias calibrations are examined: CLASH, CCCP, and Planck-based estimates. No deviation from constant c is found with the first two priors, while the Planck calibration produces only marginal consistency at the 2 sigma level. This outcome demonstrates that the test result is sensitive to the choice of mass bias scheme.

Core claim

Combining X-ray gas mass fraction measurements from galaxy clusters with SNe Ia luminosity distances from Pantheon+, while adopting the SH0ES H0 prior and the Omega_b/Omega_m ratio from galaxy clustering, yields no deviation from a constant speed of light when CLASH or CCCP mass bias priors are used; the Planck-based calibration produces mild tension, with constant c only marginally consistent at the 2 sigma level.

What carries the argument

The comparison of observed X-ray gas mass fractions in clusters to the expected baryon fraction, set against supernova luminosity distances, under fixed H0 and density ratio priors to constrain possible redshift or distance dependence in c.

If this is right

The test minimizes reliance on a specific cosmological model through the chosen priors on H0 and the baryon fraction.
Conclusions about constant c vary with the cluster mass calibration scheme adopted.
The Planck calibration introduces a 2 sigma tension that may point to residual systematics in mass estimates.
The method offers a route to probe fundamental constants using cluster observations with reduced model dependence.

Where Pith is reading between the lines

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

Future surveys with tighter independent mass constraints could resolve whether the Planck tension reflects calibration issues or a real variation in c.
The same cluster-plus-supernova comparison could be applied to test other constants or extensions of general relativity.
Cross-calibration between CLASH, CCCP, and Planck methods would strengthen the reliability of this test for fundamental physics.

Load-bearing premise

The adopted cluster mass bias calibrations from CLASH, CCCP, and Planck correctly capture the true total masses without systematic errors that could mimic or mask variations in the speed of light.

What would settle it

An independent measurement of cluster total masses that aligns with CLASH or CCCP values but produces a clear deviation from constant c in the same gas fraction and distance data would falsify the no-variation conclusion.

Figures

Figures reproduced from arXiv: 2605.06267 by Javier E. Gonzalez, Marcelo Ferreira, R. F. L. Holanda, S. H. Pereira.

**Figure 1.** Figure 1: FIG. 1: Left: Gas mass fraction measurements used in this work. Right: Gaussian Process reconstruction of view at source ↗

**Figure 2.** Figure 2: FIG. 2: Left: Posterior probability distribution of view at source ↗

**Figure 3.** Figure 3: FIG. 3: ∆ view at source ↗

**Figure 4.** Figure 4: FIG. 4: ∆ view at source ↗

read the original abstract

In recent years, improvements in galaxy cluster observations have enabled a variety of tests of fundamental physics using these systems. In this work, we test the constancy of the speed of light, $c$, by combining X-ray gas mass fraction measurements from galaxy clusters with SNe Ia luminosity distance measurements from Pantheon+. We adopt the SH0ES prior on $H_0$ and the $\Omega_b/\Omega_m$ ratio from galaxy clustering observations, thereby minimizing the dependence of our analysis on any specific cosmological model. We explore different assumptions for the cluster mass calibration (mass bias), including \textsc{CLASH}, \textsc{CCCP}, and Planck-based estimates. We find no deviation from a constant $c$ when adopting \textsc{CLASH} or \textsc{CCCP} priors, while Planck-based calibration yields a mild tension, with the hypothesis of constant $c$ being only marginally consistent at the $2\sigma$ level, indicating a non-negligible sensitivity of the results to the adopted calibration scheme.

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 test for constant c from cluster gas fractions shifts from null to 2σ tension depending on which mass bias prior is chosen.

read the letter

The main result is that whether this analysis finds a deviation from constant c depends on the cluster mass calibration. CLASH and CCCP priors produce no tension, while the Planck-based one yields a marginal 2σ inconsistency. The paper updates the test by using the Pantheon+ supernova sample and systematically varying those three mass bias assumptions. It also brings in the SH0ES H0 prior and the baryon fraction from galaxy clustering to limit dependence on a full cosmological model. This keeps the focus on the c test itself and makes the sensitivity to mass bias explicit, which is a useful practical point given how uncertain cluster masses remain. The fitting and data handling appear standard and the citations track the usual sources for these measurements. The soft spot is exactly what the stress test flags: the conclusion tracks the prior choice, and there is no joint marginalization or expanded error budget that would make the outcome robust across calibrations. Any unaccounted difference in hydrostatic bias, selection, or lensing-X-ray mismatch between the calibrations could create or remove the apparent tension without involving c. The abstract states the sensitivity but does not resolve it. This work is for people who already follow cluster cosmology and tests of fundamental constants. It is an incremental refinement rather than a new method, but the clear prior variation gives it some value for anyone using similar data sets. I would send it to peer review because the analysis is transparent, the data are public, and the central caveat is stated directly, so referees can weigh the priors themselves.

Referee Report

2 major / 2 minor

Summary. The manuscript tests the constancy of the speed of light c by combining X-ray gas mass fraction measurements from galaxy clusters with Pantheon+ Type Ia supernova luminosity distances. External priors on H0 (from SH0ES) and Ωb/Ωm (from galaxy clustering) are adopted to reduce cosmological model dependence. Three cluster mass-bias calibrations (CLASH, CCCP, Planck) are explored; the analysis reports no deviation from constant c under CLASH or CCCP priors but a marginal 2σ tension under the Planck calibration, and concludes that results are sensitive to the adopted mass-bias scheme.

Significance. If the central result were robust to mass-calibration choices, the work would provide an interesting, relatively model-independent probe of fundamental physics using cluster observables. The use of external priors on H0 and Ωb/Ωm is a clear strength that limits dependence on a specific cosmology. However, the reported sensitivity to mass-bias priors (changing the conclusion from consistency to marginal tension) indicates that the test is not yet independent of cluster-mass systematics, limiting its immediate impact on constraints on c.

major comments (2)

[Abstract and §4] Abstract and §4 (results): The central claim that constant c is 'only marginally consistent at the 2σ level' under Planck calibration (while consistent under CLASH/CCCP) is load-bearing, yet the manuscript presents no joint marginalization or combined error budget over the three mass-bias priors. Because the outcome flips with the choice of prior, the sensitivity statement does not yet establish robustness of the constant-c hypothesis.
[§3] §3 (methodology): The analysis imports the three mass-bias factors as fixed priors without propagating their mutual inconsistencies or possible redshift-dependent systematics (e.g., hydrostatic bias differences between X-ray and lensing calibrations). This directly controls the reported tension level and must be addressed for the conclusion to be independent of calibration choice.

minor comments (2)

[Figure 1] Figure 1 (or equivalent): The caption should explicitly state the numerical values of the three mass-bias priors adopted and the resulting best-fit c/c0 values with uncertainties for each case.
[§2] §2: The notation for the gas-mass fraction f_gas and its relation to the baryon fraction should be clarified with an explicit equation linking it to the assumed constant c.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comments. We address the two major comments point by point below, indicating where revisions will be made to improve clarity while preserving the original intent of exploring calibration dependence.

read point-by-point responses

Referee: [Abstract and §4] Abstract and §4 (results): The central claim that constant c is 'only marginally consistent at the 2σ level' under Planck calibration (while consistent under CLASH/CCCP) is load-bearing, yet the manuscript presents no joint marginalization or combined error budget over the three mass-bias priors. Because the outcome flips with the choice of prior, the sensitivity statement does not yet establish robustness of the constant-c hypothesis.

Authors: We appreciate the referee highlighting this aspect. The manuscript deliberately reports results for each mass-bias calibration separately precisely to illustrate that the level of consistency with constant c depends on the adopted scheme; a joint marginalization is not performed because the three calibrations represent alternative systematic choices rather than statistically combinable measurements. We will revise the abstract and §4 to more explicitly state that the test of constant c is conditional on the mass calibration and to quantify the variation in outcomes across the three cases as an indicator of systematic sensitivity. This will strengthen the presentation without claiming robustness beyond what the separate analyses support. revision: yes
Referee: [§3] §3 (methodology): The analysis imports the three mass-bias factors as fixed priors without propagating their mutual inconsistencies or possible redshift-dependent systematics (e.g., hydrostatic bias differences between X-ray and lensing calibrations). This directly controls the reported tension level and must be addressed for the conclusion to be independent of calibration choice.

Authors: We agree that the mass-bias factors are treated as fixed values drawn from the literature and that a full propagation of their mutual inconsistencies and possible redshift-dependent effects would provide a more comprehensive error budget. The current methodology isolates the impact of each published calibration to demonstrate sensitivity. In the revised manuscript we will expand the discussion in §3 to explicitly note the potential differences (such as between weak-lensing and X-ray hydrostatic estimates) and to state that the reported tension levels are conditional on the chosen calibration. A limited sensitivity test varying the bias factors within published uncertainties will be added if it can be done with existing information; a complete joint propagation of all inter-calibration systematics lies beyond the scope of the present work. revision: partial

Circularity Check

0 steps flagged

No significant circularity; derivation combines independent datasets and external priors without reducing predictions to inputs by construction.

full rationale

The paper tests constancy of c via X-ray gas mass fraction f_gas combined with Pantheon+ SNe Ia distances, adopting independent SH0ES H0 prior and Omega_b/Omega_m from galaxy clustering surveys. Mass bias calibrations (CLASH, CCCP, Planck) are treated as alternative external assumptions drawn from literature, with explicit exploration of sensitivity rather than any claim that a derived quantity equals the input by definition. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations appear in the chain; the result is presented as conditional on the chosen calibration scheme. This structure is self-contained against external benchmarks and does not invoke uniqueness theorems or ansatze from the authors' prior work to force the outcome.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The test rests on the assumption that X-ray gas mass fractions can be combined with luminosity distances to isolate variations in c once mass bias is fixed by external priors; no new entities are introduced.

free parameters (1)

cluster mass bias factor
Different numerical priors for mass bias are adopted from CLASH, CCCP, and Planck; these directly scale the inferred total mass and control whether tension appears.

axioms (2)

domain assumption X-ray gas mass fraction measurements trace the true baryonic content in a manner independent of possible c variation once mass bias is calibrated.
Invoked when combining f_gas data with supernova distances to test constancy of c.
domain assumption The SH0ES H0 prior and galaxy-clustering Ωb/Ωm ratio are independent of the c-constancy hypothesis.
Used explicitly to minimize cosmological model dependence.

pith-pipeline@v0.9.0 · 5489 in / 1397 out tokens · 69829 ms · 2026-05-08T05:34:39.667569+00:00 · methodology

discussion (0)

Reference graph

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