Whether gravitational microlensing surveys of the Large Magellanic Cloud (LMC) can place robust, quantitative limits on the fraction of the Milky Way halo made of stellar-mass primordial black holes (PBHs) is contested. The 20-year OGLE survey detected 13 microlensing events toward the LMC, consistent with lensing by known stellar populations alone, and concluded that compact objects cannot comprise a substantial fraction of the halo dark matter. Hawkins and García-Bellido (2025) contested this conclusion, arguing that the OGLE detection efficiencies are unvalidated in crowded LMC fields, that the expected self-lensing background was not recovered, that luminous stellar lenses were not excluded spectroscopically, that modern Gaia-informed Galactic rotation curves reduce the predicted optical depth, and that only a candidate-by-candidate reappraisal of the older MACHO sample could settle its claimed compact-body population. The OGLE collaboration replied (Mróz et al. 2025) with a first-cut modern re-reduction of the MACHO images, efficiency cross-validation tests, and halo-model arguments, while explicitly deferring a full MACHO reanalysis. The dispute therefore remains unresolved, and the validation questions identified below are still open.

Background

Primordial black holes (PBHs) are hypothetical black holes formed from early-Universe density fluctuations rather than stellar collapse; if sufficiently numerous, they would act as dark matter. A classic probe of compact dark-matter objects is gravitational microlensing: when a foreground compact body crosses the line of sight to a background star, it temporarily magnifies the star's light in a smooth, symmetric, achromatic brightening whose duration (the Einstein-radius crossing time) is set by the lens mass, the geometry, and the relative velocity. The expected event rate is expressed through the microlensing optical depth τ\tau, the instantaneous probability that a given source is being significantly lensed, which is proportional to the mass density of compact objects along the line of sight. Surveys monitor millions of stars in the Magellanic Clouds to probe compact objects in the Milky Way halo. The MACHO project (the acronym doubles as the name for Massive Astrophysical Compact Halo Objects) monitored 11.9 million LMC stars for 5.7 years and found 13-17 candidate events, far more than expected from known stars, implying roughly 20% of a standard halo in compact objects of about 0.50.5-0.60.6 solar masses (Alcock et al. 2000). The EROS survey (Expérience de Recherche d'Objets Sombres, a French-led microlensing survey of the Magellanic Clouds) found no excess, and successive phases of the OGLE survey (Optical Gravitational Lensing Experiment) found only a trickle of events. In 2024 the OGLE collaboration published its 20-year OGLE-III/IV LMC survey of 78.7 million stars (Mróz et al. 2024): 13 events selected by objective criteria, an optical depth of τ=(0.121±0.037)×10−7\tau = (0.121 \pm 0.037) \times 10^{-7} consistent with lensing by stars in the Milky Way disk and the LMC itself ('self-lensing'), and the conclusion, titled 'No massive black holes in the Milky Way halo', that compact objects over a broad mass range cannot make up more than about 1-10% of the halo dark matter. Hawkins and García-Bellido (2025) contest these limits. Their critique turns on long-known systematics of microlensing surveys. The detection efficiency, the probability that a real event of a given duration passes the survey's selection cuts, must be estimated by Monte Carlo methods, traditionally by injecting artificial events into real light curves, and is sensitive to crowding: blending with unresolved neighboring stars, finite-source and limb-darkening corrections when the lens transits the resolved stellar disk, variable seeing, and the observing cadence. They argue the OGLE efficiencies in crowded LMC fields are unvalidated and likely overestimated; that roughly 40 self-lensing events expected from earlier inner-LMC optical-depth estimates went undetected; that spectroscopic or multi-band follow-up of the candidate events should reveal luminous disk or LMC lenses, especially for long-duration events implying massive lenses; that the halo model adopted by OGLE overstates the mass at LMC radii compared with declining rotation curves informed by Gaia data (Gaia being the European Space Agency's all-sky astrometry mission), under which their own reanalysis of the MACHO and EROS data permits an almost entirely PBH halo (García-Bellido and Hawkins 2024); and that the MACHO claims could only be overturned by showing individual candidates are not microlensing, which they assert has never been achieved. The OGLE team's reply (Mróz et al. 2025) and the 2026 reviews by Green and by Carr et al. leave the underlying scientific question, how much of the Galactic halo can be stellar-mass compact objects and whether Magellanic-Cloud microlensing robustly constrains it, actively disputed.

The methodology behind these questions is documented in the literature: the OGLE detection efficiencies rest on Monte Carlo injection of artificial events into real light curves and on cross-checks against survey-overlap expectations and Galactic-bulge optical depths, and archival MACHO photometry has been re-reduced with modern difference-imaging techniques (subtraction of a reference image to isolate time-variable sources).

Problem Statement

Establish, with quantified systematics, whether microlensing surveys toward the Magellanic Clouds, above all the 20-year OGLE survey of the Large Magellanic Cloud, can place robust, quantitative limits on the fraction of the Galactic dark-matter halo composed of stellar-mass primordial black holes or other compact objects, and thereby resolve the MACHO-OGLE discrepancy that this question instantiates. The problem comprises five linked sub-questions: (1) whether the OGLE detection efficiencies in crowded LMC fields are validated independently and specific to that survey, given the finite-source, limb-darkening, blending, variable-seeing and cadence effects that bear on them; (2) whether the survey pipeline recovers the self-lensing and Milky-Way-disk-lens background expected from known stellar populations under the survey's actual footprint; (3) whether luminous stellar lenses of the candidate events can be identified or excluded, particularly for long-duration events where a disk star should be detectable; (4) what optical depth at LMC radii follows from Galactic mass models consistent with modern Gaia and DESI measurements (Gaia: the European Space Agency's all-sky astrometry satellite; DESI: the Dark Energy Spectroscopic Instrument galaxy survey), which currently span roughly 0.20.2-0.4×10120.4 \times 10^{12} solar masses enclosed within 50 kpc, with the model uncertainty propagated into the inferred halo compact-object fraction; and (5) what compact-object constraints follow from a candidate-by-candidate reappraisal of the full MACHO sample. These five sub-questions are independent rather than jointly required. The scientific target is the maximum compact-object fraction of the Milky Way halo that Magellanic-Cloud microlensing can robustly certify, stated together with its quantified systematic uncertainty, or else a demonstration that no robust certification is possible, thereby either substantiating or withdrawing the claim that stellar-mass primordial black holes are excluded as a substantial dark-matter component.

References

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