REVIEW 1 major objections 1 minor 2 references
SHARP on the ELT can deliver precise near-infrared time delays for tens of AGN, enabling calibration-free H0 measurements via the SARM method.
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 →
T0 review · grok-4.3
2026-07-02 17:47 UTC pith:CPQYN6ZR
load-bearing objection This is a science case for SHARP on the ELT to scale up SARM for more AGN and H0, but it contains no calculations or data to support the feasibility of the claimed precision. the 1 major comments →
Precision near-IR spectroscopy for understanding AGN physics and shed light on the H0 tension -- SHARP Science Book
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
SHARP's sensitivity and multi-object spectroscopy will enable efficient long-term monitoring of existing GRAVITY targets with minimal time investment, and systematic RM campaigns for the fainter AGN that will be observed by GRAVITY+. These advances will give precise infrared lags for tens of AGN, enabling geometric distance measurements and a robust, calibration-free determination of H0. Beyond cosmology, SHARP will allow detailed studies of BLR structure and kinematics in the infrared, advancing understanding of AGN physics and the measurements of SMBH masses.
What carries the argument
The SARM method, which derives geometric distances by combining near-infrared reverberation-mapping time delays of the broad-line region with spectroastrometric angular-size measurements.
Load-bearing premise
The SARM method remains valid and accurate when extended to fainter AGN using the specific near-IR emission lines accessible to GRAVITY+ and that the future SHARP instrument will achieve the sensitivity and operational performance required for efficient long-term monitoring.
What would settle it
A set of infrared lags measured with SHARP that, when paired with GRAVITY+ angular sizes for the same AGN, produces an H0 value inconsistent with both early-universe and late-universe determinations, or a failure to recover expected time delays across the sample.
If this is right
- Precise infrared lags become available for tens of AGN rather than a handful.
- Geometric distances to those AGN can be derived without external calibration.
- A calibration-free value of H0 follows directly from the combined SARM data.
- Infrared studies of BLR structure and kinematics become routine for a larger population.
- SMBH mass estimates improve through better constraints on BLR geometry and dynamics.
Where Pith is reading between the lines
- If the resulting H0 matches one side of the current tension, it would isolate which class of existing measurements carries the dominant systematic.
- The same monitoring strategy could be adapted to other future near-IR spectrographs on large telescopes to increase the sample size further.
- Kinematic maps from the new data might distinguish between competing models of how the broad-line region is illuminated and responds to continuum changes.
- A successful campaign would provide an independent anchor for the distance ladder that bypasses both Cepheid and supernova steps.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript is a science case proposing that the SHARP near-IR spectrograph on the ELT, with its sensitivity and multi-object mode, will enable efficient long-term RM monitoring of GRAVITY targets and systematic campaigns on fainter GRAVITY+ AGN. This would yield precise near-IR lags for tens of AGN, allowing SARM to deliver geometric distances and a calibration-free H0 measurement while also advancing BLR and SMBH mass studies.
Significance. If the unquantified projections hold, the work would supply an independent geometric route to H0 that bypasses distance-ladder systematics, potentially clarifying the current tension, and would expand the sample of IR BLR kinematics. The manuscript itself contains no new data, calculations or validation, so its significance is prospective and contingent on instrument performance and SARM extrapolation.
major comments (1)
- [Abstract] Abstract: the assertion that SHARP will deliver 'precise infrared lags for tens of AGN' enabling geometric H0 rests on the untested assumption that lag uncertainties remain sub-10% for the fainter GRAVITY+ targets; no exposure-time calculations, S/N estimates or lag-recovery simulations are supplied to support this central projection.
minor comments (1)
- [Abstract] Abstract, first sentence: 'remains on of the most' is a typographical error and should read 'remains one of the most'.
Simulated Author's Rebuttal
We thank the referee for their constructive review of this science case proposal. The major comment correctly identifies a limitation in the current manuscript: the central projections lack supporting quantitative calculations. We respond to this point below and commit to revisions that strengthen the presentation without altering the prospective nature of the document.
read point-by-point responses
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Referee: [Abstract] Abstract: the assertion that SHARP will deliver 'precise infrared lags for tens of AGN' enabling geometric H0 rests on the untested assumption that lag uncertainties remain sub-10% for the fainter GRAVITY+ targets; no exposure-time calculations, S/N estimates or lag-recovery simulations are supplied to support this central projection.
Authors: We agree that the manuscript does not include exposure-time calculations, S/N estimates or lag-recovery simulations. The projections in the abstract and main text are based on scaling relations from existing GRAVITY RM results, published 8 m-class near-IR RM campaigns, and preliminary sensitivity estimates provided by the SHARP instrument team; they are not derived from new calculations performed for this paper. In the revised version we will insert a short new subsection (and update the abstract wording) that supplies order-of-magnitude exposure-time estimates for representative GRAVITY+ targets using the ELT exposure-time calculator, together with a transparent discussion of the assumptions required to reach sub-10 % lag precision. Full end-to-end lag-recovery simulations remain outside the scope of a science-case document, but we will cite the relevant literature on RM error budgets to make the extrapolation more explicit. revision: yes
Circularity Check
No circularity: prospective proposal with no derivations or fitted quantities
full rationale
The manuscript is a forward-looking science proposal describing the anticipated capabilities of the SHARP spectrograph for AGN monitoring and SARM-based H0 measurements. It contains no equations, no fitted parameters, no derivation chains, and no load-bearing self-citations that reduce a result to its own inputs. All claims are qualitative statements about future instrument performance and method extension; none are presented as derived predictions that could be circular by construction. This matches the reader's assessment of zero circularity.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption SARM method yields accurate geometric distances independent of calibration when applied to near-IR lines
read the original abstract
The persistent tension between early- and late-Universe measurements of the Hubble constant (H0) remains on of the most significant challenges in modern cosmology. The Spectroastrometry and Reverberation Mapping (SARM) method offers a promising, calibration-independent approach to address this issue by combining time-delay measurements of the Broad-Line Region (BLR) with interferometric angular size determinations. Current implementations of SARM, however, are limited by the difficulty of performing near-infrared reverberation mapping (RM) on the same emission lines observed by GRAVITY, restricting applications to only a few bright AGN. We propose using the capabilities of SHARP, the next-generation near-infrared spectrograph for the Extremely Large Telescope (ELT), to overcome these limitations. SHARP's sensitivity and multi-object spectroscopy will enable (1) efficient long-term monitoring of existing GRAVITY targets with minimal time investment, and (2) systematic RM campaigns for the fainter AGN that will be observed by GRAVITY+. These advances will give us precise infrared lags for tens of AGN, enabling geometric distance measurements and a robust, calibration-free determination of H0. Beyond cosmology, SHARP will allow detailed studies of BLR structure and kinematics in the infrared, advancing our understanding of AGN physics and with repercussion on the measurements of Supermassive Black Holes (SMBH) masses.
Reference graph
Works this paper leans on
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[1]
Spatially resolved broad-line region in a quasar at z=4: Dynamical black hole mass and prominent outflow. Astron. Astrophys. 706, A99. http://dx.doi.org/ 10.1051/0004-6361/202557285, arXiv:2509.13911. GRAVITY Collaboration, Abuter, R., Accardo, M., Amorim, A., Anugu, N., Ávila, G., Azouaoui, N., Benisty, M., Berger, J.P., Blind, N., Bonnet, H., Bourget, P...
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[2]
The demography of massive dark objects in galaxy centers. Astron. J. 115, 2285–2305. http://dx.doi.org/10.1086/300353, arXiv:astro-ph/9708072. Onken, C.A., et al., 2004. Supermassive black holes in active galactic nuclei. Astrophys. J. 615, 645. Osterbrock, D.E., Ferland, G.J., 2006. Astrophysics of Gaseous Nebulae and Active Galactic Nuclei. University S...
work page internal anchor Pith review Pith/arXiv arXiv doi:10.1086/300353 2004
discussion (0)
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