Earlier this year, astrophysics history was made with the announcement of a conclusive detection of gravitational waves at the Laser Interferometer Gravitational-Wave Observatory (LIGO). The source of these barely-there ripples along the surface of space-time was the collision of binary black holes, each one around 30 times as massive as our own Sun, located some 1.3 billion light-years from Earth.
The gravitational wave observation also represents the first detection of a black hole merger, one of the few cosmic events extreme enough to kick off detectable gravitational waves. Of the energy released, some 5.3 × 1047 joules was radiated away as gravitational waves—enough to warp a laser beam on Earth by about one-thousandth the width of a single proton.
In a paper published Wednesday in the Physical Review Letters, researchers from Johns Hopkins University wonder if those two black holes might offer something more: dark matter. This is a possibility offered by some fringe-y theories—rather than consisting of esoteric ghostly particles, DM consists of normal matter in esoteric configurations, such as primordial black holes (PBHs).
Stephen Hawking offered up the possibility of PBHs for the first time in 1971 and they remain very much so a hypothetical construct. While black holes as we usually understand them are the results of collapsing stars, PBHs would have been formed as dense areas of cosmic raw materials in the very early universe collapsed in on themselves. So, the first stars would have been born alongside the first black holes. Crucially, PBHs should be small enough to be widely distributed around a galaxy's halo region—where dark matter is thought to live.
That all sounds well and good, but the PBH theory of dark matter has taken some serious if not fatal hits in recent years, including recent studies that have completely excluded most possible mass ranges for the objects. (This is based on expected interactions between PBHs at varying masses and neutron stars.)
Still, there are believers, or at least those interested enough in the possibility to pursue unlikelihoods. In any case, the Hopkins paper doesn't just come out and say that the LIGO black hole merger involves PBHs, just that the anticipated rate for merging PBHs within a galactic halo is not excluded from the merger rate implied by the gravitational wave (GW) observations.
"Distinguishing whether any individual GW event, or even some population of events, are from PBH [dark matter] or more traditional astrophysical sources will be daunting," the Hopkins team notes. "Still, there are some prospects. Most apparently, PBH mergers will be distributed more like small-scale [dark matter] halos and are thus less likely to be found in or near luminous galaxies than [black hole] mergers from more traditional astrophysical sources."
Further work, the astrophysicists conclude, should focus first on looking for unexpected masses within galactic halos that can't be linked to more traditional astrophysical sources. Clues may also be revealed by probing the geometry of gravitational waves registered on Earth and by looking at the cosmic microwave background for lower frequency signals than those expected from normal black hole activity.
Don't expect a sudden groundswell of interest in PBH dark matter theories, but as searches for more mainstream dark matter candidates continue to come up empty, fringe theories can only become less fringe.
An open-access version of the Hopkins paper can be accessed at the arXiv pre-print server.