The Elusive Lightweight Black Hole: Key to Dark Matter? express
The gravitational waves detectors LIGO and Virgo picked up a deep rumble — the aftermath of two invisible objects merging. Their combined mass turned out to be less than the Sun's. Stellar evolution doesn't produce such lightweight black holes: a dying star doesn't collapse into such a tiny finale.
This signal has revived Stephen Hawking's hypothesis of primordial black holes — clumps of matter born in the first moments after the Big Bang, described by the Standard Model of physics. If this detected pair really is such relics, they could be part of dark matter, the invisible glue that holds galaxies together. Calculations allow that at least 4% of dark matter consists of such objects.
Here's the twist: a half-solar-mass black hole is just a couple of kilometers across: you could walk around it in half an hour, yet its pull is stronger than a star cluster's. The signal S251112cm is still being studied, but the lack of a visible flash is typical for black holes. Confirmation would open a direct path from spacetime ripples to solving the universe's composition.
Original abstract
as written by the authors arXiv:2602.21295
The detection of sub-solar mass black holes is a milestone of modern astrophysics as it would open a window either onto new stellar physics or could potentially unveil the nature of Dark Matter as Primordial Black Holes (PBHs). On November 12, 2025, the LIGO-Virgo-KAGRA (LVK) collaboration reported the compact binary merger candidate S251112cm, a system with no obvious electromagnetic counterpart, consistent with binary black hole merger with a chirp mass in the range $$0.1-0.87 \, M_\odot$$. The probability that at least one component has mass $$<$$1 $$M_{\odot}$$ is $$>99\%$$. Inspired by this trigger, we tested if a population of PBHs formed at Quantum Chromodynamics epoch with a broad mass function could account for a signal of this type. Our results, corresponding to a predicted event rate of $$0.8 \,\text{yr}^{-1}$$ as seen by LVK O3b, suggest that the observed merger rate of $$0.23^{+0.86}_{-0.218}\,\text{yr}^{-1}\;(95\%\;\text{C.L.})$$ if the trigger is confirmed as an astrophysical event would be compatible with such a model. Our predicted detection rate is also in agreement with current LVK expectations for stellar-mass binaries, remaining consistent with a scenario in which a non-negligible fraction of the $$3-200 \;M_\odot$$ mergers observed by LVK originate from Primordial Black Holes. If confirmed, this detection would place a lower limit to the PBH abundance $$f_{PBH}>0.04$$ for our adopted model.
A black hole with half the Sun's mass has a radius of about 1.5 kilometers, such an object could fit into a small town center, but would pull in everything around it with monstrous force.