Did LIGO Just Detect Primordial Black Holes?
Barely on the media radar is the fact that since LIGO/VIRGO began their O3 observing run in April, 5 gravitational wave candidates have been observed. The first three appeared to be BH-BH mergers like those familiar from the O1 and O2 runs.
The 4th and 5th events occurred on 4/25 and 4/26. At least 3 of the masses in these events are in the < 3 solar mass range, which is below the observed range of conventional core-collapse black holes.
The conventional thinking is that the 4/25 event involved a binary neutron star merger, and the 4/26 event is possibly a BH-NS merger. Immediately many observatories around the world, and in space, turned their telescopes to the inferred locations of these two events searching for transient fireworks reminiscent of the 2017 NS-NS merger event. So far there appears to be no transient event that can be firmly linked to the observed GW events.
One possibility that has not been mentioned publicly is that the low-mass objects could be primordial black holes. Whereas conventional core-collapse BHs have been observed with masses > 5 solar mass, BHs with masses < 3 solar mass are not expected.
On the other hand, primordial black holes are not subject to these mass limitations. Various authors have suggested that there are observational reasons to think that the dark matter is composed of astrophysical PBHs with masses as low as 0.2 solar mass, and a range between 0.2 and about 35 solar mass.
If the 4/25 and 4/26 events are fully confirmed and no obvious transients are observed to accompany them, then the low-mass objects could be the first confirmed observations of primordial black holes.
With LIGO/VIRGO and other GW observatories estimated to be observing about 4 GW events per month, things are guaranteed to get very interesting in the near future.