How much can shape degeneracies affect the Hubble constant measurement?
Abstract
Cluster lens models are constrained almost entirely by observed lensed image positions, leaving a large space of mass perturbations with arbitrary amplitudes such that they reproduce every image exactly. Such degeneracies are known as shape degeneracies (ShaDes). We show that these ShaDes shift the observed time delays, by the difference of the perturbing potential between the two images, without depending on the number of source/lens planes. Bounding their amplitude by preserving the positivity of the convergence, image multiplicity, and parity, in MACS J0138.0−2155, they can contribute at least ≃7 days to the delay between the detected images of SN-Encore (i.e., 17.5% of the measured value of $39.8$ days) and ≃112 days (i.e., $2.8−−3.7\%$ of predicted value of $3,000-4000$ days) to the reappearance intervals predicted for SN-Encore and SN-Requiem. These error bars transfer one-to-one to H0, and remain invisible in comparisons of independent models.
Related
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- The SRG/eROSITA All-Sky Survey. Early dark energy and Hubble constant from the cluster mass function
BibTeX
@misc{meena2026much,
title = {{How much can shape degeneracies affect the Hubble constant measurement?}},
author = {Ashish K. Meena and Prasenjit Saha and Liliya L. R. Williams and Derek Perera},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.15999},
eprint = {2610.15999},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.15999},
}
RIS
TY - UNPB
TI - How much can shape degeneracies affect the Hubble constant measurement?
AU - Ashish K. Meena
AU - Prasenjit Saha
AU - Liliya L. R. Williams
AU - Derek Perera
PY - 2026
DA - 2026/10/14
PB - arXiv
DO - 10.48550/arxiv.2610.15999
UR - https://arxiv.org/abs/2610.15999
AB - Cluster lens models are constrained almost entirely by observed lensed image positions, leaving a large space of mass perturbations with arbitrary amplitudes such that they reproduce every image exactly. Such degeneracies are known as shape degeneracies (ShaDes). We show that these ShaDes shift the observed time delays, by the difference of the perturbing potential between the two images, without depending on the number of source/lens planes. Bounding their amplitude by preserving the positivity of the convergence, image multiplicity, and parity, in MACS J0138.0−2155, they can contribute at least ≃7 days to the delay between the detected images of SN-Encore (i.e., 17.5% of the measured value of $39.8$ days) and ≃112 days (i.e., $2.8−−3.7\%$ of predicted value of $3,000-4000$ days) to the reappearance intervals predicted for SN-Encore and SN-Requiem. These error bars transfer one-to-one to H_0, and remain invisible in comparisons of independent models.
ER -