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arxiv: 2605.28871 · v1 · pith:7SNKDRHPnew · submitted 2026-05-23 · 🌀 gr-qc

Testing loop quantum gravity through EHT observations of M87* and Sgr A* using rotating holonomy-corrected black holes

Heena Ali , Sushant G. Ghosh This is my paper

Pith reviewed 2026-06-30 12:57 UTC · model grok-4.3

classification 🌀 gr-qc
keywords black hole shadowloop quantum gravityEvent Horizon TelescopeM87*Sgr A*holonomy correctionrotating black holesquantum gravity tests
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The pith

EHT observations of M87* and Sgr A* permit nonzero holonomy correction values in rotating black hole metrics.

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

The paper applies Event Horizon Telescope shadow measurements of M87* and Sgr A* to a rotating holonomy-corrected black hole spacetime that incorporates loop quantum gravity effects through a correction parameter b. This parameter enlarges the shadow relative to the Kerr case and allows closed photon rings even when no event horizon is present. Bounds are derived on b using the shadow area and oblateness observables at the observed inclinations, showing that several tenths of the mass in b remain compatible with the angular diameter data. The conclusion is that these quantum-corrected geometries fit current observations and can serve as astrophysical alternatives to classical Kerr black holes.

Core claim

Rotating holonomy-corrected black holes produce larger shadows as the holonomy correction b grows, with prograde photon orbits shifting outward, and the EHT angular diameter constraints at 17 degrees for M87* and 50 degrees for Sgr A* allow nonzero b values up to 0.1319M at zero spin for M87* and up to 0.7482M at spin 0.6253 for Sgr A*, making the model consistent with observations as a viable alternative to Kerr geometry.

What carries the argument

The rotating holonomy-corrected black hole metric with spin a and holonomy correction b, whose unstable circular photon orbits determine the shadow area A and oblateness D observables used to constrain the parameters.

If this is right

  • The shadow size increases with increasing b compared to the Kerr case.
  • Closed shadow rings form for b in the interval between the extremal and photon-orbit values even without an event horizon.
  • Photon rings exist throughout the parameter range b_E ≤ b ≤ b_p.
  • Upper bounds on b are obtained separately for each source at its observed inclination angle.

Where Pith is reading between the lines

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

  • Tighter future EHT angular resolution could shrink the allowed range for b and begin to exclude parts of the parameter space.
  • The A-D method could be applied to other effective metrics that modify photon orbits near black holes.
  • Including detailed accretion flow models might shift the extracted bounds on b if those flows contribute substantially to the observed shadow.
  • The persistence of closed shadows without horizons raises the possibility that the same metric could describe certain horizonless compact objects.

Load-bearing premise

The rotating holonomy-corrected metric is a faithful effective description of loop quantum gravity near the horizon and the shadow observables depend only on a and b without dominant effects from accretion or other matter.

What would settle it

A measured shadow diameter or oblateness for either M87* or Sgr A* that lies outside the range of A and D values allowed by any combination of a and b in the model would show the consistency claim is false.

Figures

Figures reproduced from arXiv: 2605.28871 by Heena Ali, Sushant G. Ghosh.

Figure 1
Figure 1. Figure 1: FIG. 1: The parameter space ( [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: Radial profile of the effective potential [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Radii of various surfaces for the RHCBH as a function of sp [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: A comparative analysis of shadow silhouettes cast by a stan [PITH_FULL_IMAGE:figures/full_fig_p008_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5: Classification of the ( [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Shadow silhouettes in the celestial image plane ( [PITH_FULL_IMAGE:figures/full_fig_p010_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Comparative shadow silhouettes at a viewing angle of [PITH_FULL_IMAGE:figures/full_fig_p011_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: Contour plots of the shadow observables in the RHCBH para [PITH_FULL_IMAGE:figures/full_fig_p012_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: (Left): RHCBH shadow angular diameter [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10: RHCBH shadow angular diameter [PITH_FULL_IMAGE:figures/full_fig_p015_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: FIG. 11: (Left:) RHCBH shadow angular diameter [PITH_FULL_IMAGE:figures/full_fig_p015_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: FIG. 12: RHCBH shadow angular diameter [PITH_FULL_IMAGE:figures/full_fig_p016_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13: RHCBH shadow angular diameter deviation from that of a Sc [PITH_FULL_IMAGE:figures/full_fig_p016_13.png] view at source ↗
read the original abstract

The Event Horizon Telescope (EHT) has provided a new tool for testing the strong-field regime of gravity by imaging the shadows of M87* and Sgr A*. These observations provide the first real opportunity to test whether quantum gravity--specifically loop quantum gravity--leaves observable imprints on spacetime. We use the EHT observations of M87* and Sgr A* to examine the observational signs of rotating holonomy-corrected black holes (RHCBHs). We discover that, in comparison to the typical Kerr black hole, the quantum correction parameter $b$ increases the size of the black hole shadow. As the deviation parameter $b$ increases in RHCBH, the prograde photon orbits shift outward, indicating a weaker effective gravitational field near the central region. Unlike Kerr naked singularities, which produce open arc-like shadows, the RHCBH spacetime can still produce closed shadow rings even in the absence of an event horizon. We find that photon rings continue to exist in the parameter range $b_E \leq b \leq b_p$, due to the presence of unstable circular photon orbits.We apply the Kumar--Ghosh method based on the shadow observables: the shadow area $A$ and the oblateness $D$ that together allow a unique determination of the spin parameter $a$ and the quantum correction parameter $b$. At $\theta_o=17$\textdegree~, the angular diameter bound of M87$^{*}$ yields $b \leq 0.1319\,M$ at $a = 0\,$ and $b \leq 0.421\,M$ at $a = 0.784\,M$, while at $\theta_o=50$\textdegree~, the angular diameter bound of Sgr A$^{*}$ yields $b \leq 0.5764\,M$ at $a = 0\,$ and $b \leq 0.7482\,M$ at $a = 0.6253\,M$ the Sgr~A$^{*}$. Our results show that nonzero values of the holonomy correction parameter are consistent with current EHT data, indicating that RHCBHs provide viable alternatives to the classical Kerr geometry in the strong-gravity regime and are strong astrophysical black hole candidates.

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

3 major / 1 minor

Summary. The manuscript claims that nonzero values of the holonomy correction parameter b in rotating holonomy-corrected black holes (RHCBHs) remain consistent with EHT angular-diameter data for M87* and Sgr A*, obtained by applying the Kumar-Ghosh method to shadow area A and oblateness D; it concludes that RHCBHs are viable alternatives to Kerr black holes in the strong-gravity regime.

Significance. If the mapping from A and D to (a, b) holds after accounting for astrophysical effects, the work would supply concrete observational bounds on loop quantum gravity corrections near black hole horizons and demonstrate a direct link between EHT shadow observables and quantum-gravity parameters.

major comments (3)
  1. [Application of Kumar-Ghosh method (results on M87* and Sgr A* bounds)] The central consistency claim rests on the assumption that A and D are determined solely by a and b. The manuscript applies the Kumar-Ghosh method directly to the EHT angular-diameter bounds at fixed θ_o (abstract and results section) without marginalizing over accretion-flow or plasma contributions that can shift the effective shadow boundary by amounts comparable to or larger than the reported EHT uncertainties.
  2. [Results section reporting numerical bounds] No error propagation or sensitivity analysis is presented for the quoted bounds (b ≤ 0.1319 M at a=0 for M87*, b ≤ 0.5764 M at a=0 for Sgr A*). The abstract states the bounds but supplies neither the explicit mapping equations nor validation that A and D uniquely fix a and b once realistic accretion uncertainties are included.
  3. [Metric and photon-orbit discussion] The rotating holonomy-corrected metric itself is used without a visible derivation or reference to an explicit line element in the provided text; the claim that photon rings persist for b_E ≤ b ≤ b_p therefore rests on an unexamined effective description whose domain of validity near the horizon is not quantified.
minor comments (1)
  1. [Abstract] The abstract introduces b_E and b_p without defining them or relating them to the metric parameters; a brief clarification would improve readability.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the thorough and constructive review. The comments highlight important aspects of our analysis that we will address to strengthen the manuscript. We respond point-by-point below.

read point-by-point responses

  1. Referee: [Application of Kumar-Ghosh method (results on M87* and Sgr A* bounds)] The central consistency claim rests on the assumption that A and D are determined solely by a and b. The manuscript applies the Kumar-Ghosh method directly to the EHT angular-diameter bounds at fixed θ_o (abstract and results section) without marginalizing over accretion-flow or plasma contributions that can shift the effective shadow boundary by amounts comparable to or larger than the reported EHT uncertainties.

    Authors: We agree that the Kumar-Ghosh method as applied assumes the shadow observables are determined by the spacetime geometry. While this geometric approach is standard for constraining spacetime parameters from shadow data, we acknowledge that accretion and plasma effects are not marginalized over and could influence the effective boundary. In the revised manuscript we will add a paragraph in the discussion section explicitly noting this limitation, stating that the reported bounds apply to the idealized vacuum case, and suggesting that future analyses incorporate these astrophysical uncertainties. This is a partial revision. revision: partial

  2. Referee: [Results section reporting numerical bounds] No error propagation or sensitivity analysis is presented for the quoted bounds (b ≤ 0.1319 M at a=0 for M87*, b ≤ 0.5764 M at a=0 for Sgr A*). The abstract states the bounds but supplies neither the explicit mapping equations nor validation that A and D uniquely fix a and b once realistic accretion uncertainties are included.

    Authors: We will revise the results section to include error propagation and a sensitivity analysis for the quoted bounds. The explicit mapping equations relating the shadow area A and oblateness D to the parameters (a, b) will be stated in the main text (or an appendix). We will also clarify the conditions under which A and D uniquely determine a and b within the Kumar-Ghosh framework. This addresses the referee's concern directly and will be incorporated as a full revision. revision: yes

  3. Referee: [Metric and photon-orbit discussion] The rotating holonomy-corrected metric itself is used without a visible derivation or reference to an explicit line element in the provided text; the claim that photon rings persist for b_E ≤ b ≤ b_p therefore rests on an unexamined effective description whose domain of validity near the horizon is not quantified.

    Authors: The metric is an extension of the holonomy-corrected line element from the non-rotating case in the loop quantum gravity literature. In the revised manuscript we will include the explicit rotating metric line element in Section 2, together with a reference to its derivation and a short discussion of its domain of validity near the horizon. We will also expand the photon-orbit analysis to quantify the persistence of photon rings in the stated parameter range. This will be added as a full revision. revision: yes

Circularity Check

0 steps flagged

No significant circularity; constraints are direct observational bounds

full rationale

The paper computes shadow observables A and D from the RHCBH metric and uses EHT angular-diameter data to place upper bounds on the correction parameter b for given spin a. The conclusion that nonzero b remains consistent is simply the statement that the data-permitted interval includes b > 0; it does not reduce to a fitted input renamed as a prediction, nor does any step equate to its own definition by construction. The Kumar–Ghosh mapping is an explicit geometric inversion of the metric’s photon orbits and is not invoked as an unverified uniqueness theorem that forces the result. No self-citation chain, ansatz smuggling, or renaming of known patterns is required for the central claim. The derivation chain is therefore self-contained against the supplied EHT angular-diameter bounds.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 1 invented entities

The central claim rests on the validity of the RHCBH metric as an LQG effective description and on the applicability of the shadow observables to constrain b; these are not derived within the paper.

free parameters (1)
  • b
    Holonomy correction parameter whose allowed range is determined by fitting shadow observables to EHT data rather than derived from first principles.
axioms (1)
  • domain assumption The rotating holonomy-corrected black hole metric accurately encodes loop quantum gravity effects for rotating spacetimes
    Invoked as the starting geometry whose shadows are compared to observations.
invented entities (1)
  • rotating holonomy-corrected black hole (RHCBH) no independent evidence
    purpose: Effective spacetime incorporating LQG holonomy corrections
    Postulated modified geometry whose properties are explored; no independent falsifiable signature outside the shadow calculation is supplied.

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Reference graph

Works this paper leans on

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