Discovering Substellar Dark Matter Halos with Astrometric Weak Lensing of Multiply Imaged Quasars
Abstract
We propose time-domain astrometric weak lensing of multiply imaged quasars as a probe of substellar dark matter (DM) halos. In ΛCDM, each macro-image's light traverses many microhalos, producing a stochastic centroid motion with a calculable red power spectrum. Halos with crossing times longer than the survey impart a relative angular acceleration between image pairs. The response peaks at subhalo scale radii 0.005-2 pc and masses 3×10−6-102 M⊙, 12-14 orders of magnitude below the smallest detected DM structures, down to the damping cutoffs of a 100 GeV thermal relic. We forecast the sensitivity of ten-year campaigns with 300 epochs for two benchmark systems: the galaxy-lensed quadruple B1422+231 at a 0.1 μas per-epoch precision forecast for extended-path intensity correlation, and the cluster-lensed triple SDSS J1029+2623 at 1 μas. The angular-acceleration channel reaches the standard ΛCDM microhalo population with a variance signal-to-noise ratio of order ten for the galaxy lens (order unity after stellar microlensing subtraction) and 0.35 for the cluster under the optimistic assumption that 10% of the projected mass density at the image positions resides in surviving microhalos. A positive detection would: raise universal lower bounds on the DM particle mass to ≳400 keV for fermions and ≳10−12 eV for bosons; constrain the DM kinetic decoupling temperature to ≳ 10 MeV; and be sensitive to the running of the spectral tilt at the 0.01 level over 15-20 e-folds of the primordial curvature power spectrum. A robust null result across well-characterized lenses would constrain microhalo survival and/or imply violations of the above inequalities. These signatures motivate differential astrometric observations with extreme 0.1-1 μas light-centroiding precision on multiply imaged quasars. [Abridged]
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BibTeX
@misc{tilburg2026discovering,
title = {{Discovering Substellar Dark Matter Halos with Astrometric Weak Lensing of Multiply Imaged Quasars}},
author = {Ken Van Tilburg and David E. Kaplan},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2608.27557},
eprint = {2608.27557},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.27557},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Discovering Substellar Dark Matter Halos with Astrometric Weak Lensing of Multiply Imaged Quasars
AU - Ken Van Tilburg
AU - David E. Kaplan
PY - 2026
DA - 2026/10/02
PB - arXiv
DO - 10.48550/arxiv.2608.27557
UR - https://arxiv.org/abs/2608.27557
AB - We propose time-domain astrometric weak lensing of multiply imaged quasars as a probe of substellar dark matter (DM) halos. In ΛCDM, each macro-image's light traverses many microhalos, producing a stochastic centroid motion with a calculable red power spectrum. Halos with crossing times longer than the survey impart a relative angular acceleration between image pairs. The response peaks at subhalo scale radii 0.005-2 pc and masses 3×10^−6-10^2 M_⊙, 12-14 orders of magnitude below the smallest detected DM structures, down to the damping cutoffs of a 100 GeV thermal relic. We forecast the sensitivity of ten-year campaigns with 300 epochs for two benchmark systems: the galaxy-lensed quadruple B1422+231 at a 0.1 μas per-epoch precision forecast for extended-path intensity correlation, and the cluster-lensed triple SDSS J1029+2623 at 1 μas. The angular-acceleration channel reaches the standard ΛCDM microhalo population with a variance signal-to-noise ratio of order ten for the galaxy lens (order unity after stellar microlensing subtraction) and 0.35 for the cluster under the optimistic assumption that 10% of the projected mass density at the image positions resides in surviving microhalos. A positive detection would: raise universal lower bounds on the DM particle mass to ≳400 keV for fermions and ≳10^−12 eV for bosons; constrain the DM kinetic decoupling temperature to ≳ 10 MeV; and be sensitive to the running of the spectral tilt at the 0.01 level over 15-20 e-folds of the primordial curvature power spectrum. A robust null result across well-characterized lenses would constrain microhalo survival and/or imply violations of the above inequalities. These signatures motivate differential astrometric observations with extreme 0.1-1 μas light-centroiding precision on multiply imaged quasars. [Abridged]
ER -