Are ultrashort microlensing events caused by planets or primordial black holes?
In plain words
Surveys watching millions of stars see a few brightenings lasting less than a day, caused by passing objects about as heavy as the Earth. They are usually attributed to planets drifting free of any star, but primordial black holes would produce identical events.
Precise statement
OGLE bulge data contain 6 ultrashort events that Niikura et al. (2019, arXiv:1901.07120) reproduce with Earth-mass PBHs forming about a percent of dark matter, while MOA (Sumi et al. 2023, arXiv:2303.08280) attributes such events to about 21 free-floating planets per star. Determine $f_{\mathrm{PBH}} = \Omega_{\mathrm{PBH}} / \Omega_{c}$ for $M = 1e-7\ \text{to}\ 1e-5\ M_{\mathrm{sun}}$ ($2e26\ \text{to}\ 2e28\ \mathrm{g}$) by separating lens populations through event-rate dependence on direction (bulge, disk, halo, lines of sight to the LMC and M31) and through lens-distance information.
What would settle it
Nancy Grace Roman Space Telescope microlensing statistics with parallax for short events, compared between the bulge and halo-dominated lines of sight.
Status in the literature
Unverified note
Mroz et al. (2024, Nature 632, 749) bound $f_{\mathrm{PBH}} < 0.1$ for $1.3e-5 \text{ to } 860\,M_{\mathrm{sun}}$ toward the LMC, leaving the Earth-mass range less constrained.