Some clever ways to discover primordial black holes

Primordial black holes (PBHs) have recently attracted much attention in the physics community. One of the primary reasons is the possible link to dark matter. In fact, if PBHs can be proven to exist, there is a very good chance that they are the same dark matter, the invisible stuff that makes up 85% of the mass of the universe. If proven, it would certainly be a Nobel-level discovery in astrophysics.

But to prove it, someone has to find them first. So far, PBHs exist only in theory. But we're getting closer to proving that they exist, and a new paper from Marcos Flores of the Sorbonne and Alexander Kusenko of UCLA explores some ideas about how we might eventually find PBHs and thus dark matter. Can prove or disprove their relation to matter. ,

Drs. Flores and Kusenko focus on understanding PBH formation principles and then predicting how those structures can be detected even with modern instruments. The typical black hole that we know of exists when supermassive stars collapse under their own weight.

Fraser discusses PBH.

PBHs formed before any stars of such size collapsed, so they must have formed using a different mechanism. The paper details a theoretical PBH formation process that includes a detailed mathematical look at particle asymmetry and how it fits with other models of particle physics. But how can astronomers see those structures?

One way is to look at the loss of angular momentum. Astronomers can observe “halos” of particles from as early as the universe. In many cases, they are moving faster. However, if their spin slowed down dramatically, it could indicate that a PBH was forming in the surrounding region, sucking some of the energy from that angular momentum by pulling the particles towards itself.

Another way is to look at a new favorite mechanism of astronomers everywhere – gravitational waves. It is not entirely clear whether the formation of PBHs can generate gravitational waves. Nevertheless, the paper discusses some outlines that could potentially lead to a theory as to whether they do or not.

Fraser discusses with Dr. Celeste Keith how hard it is to find PBH.

Supersymmetry provides one of those frameworks. In some cases, the early universe operating under the principles of supersymmetry may form a PBH that will create a gravitational wave that the next generation of gravitational wave detectors could potentially detect. Specifically, this would include what the paper calls “poltergeist mechanisms” resulting in space-time disturbances in some theories.

The final way to detect these PBHs is to monitor gravitational lenses. Gravitational microlensing has been looked at in some experiments, such as the Optical Gravitational Lensing Experiment (OGLE) and the Subaru Telescope's HyperSupreme-Cam (HSC), where there are no known massive objects capable of causing such lensing. PBHs, which would be effectively invisible to those telescopes, may provide an explanation, although other explanations must first be ruled out.

Other theories offer other opportunities to detect PBHs, including observing the interactions of “Q-balls” or theoretical large “blobs” of matter. If enough of these are collected together, they can potentially form a PBH.

Ultimately, there are more questions than answers surrounding these mysterious objects. If they exist, they could answer many of them. However, more data is needed to prove this beyond any reasonable doubt. Experimenters are already moving as fast as possible to develop new and better detectors that can help in the search for PBHs. If they exist, it's just a matter of time before we find them.

learn more:
Flores and Kusenko – New views on the formation and astrophysical detection of primordial black holes.
UT – The universe may be full of ultralight black holes that can't die
UT – If we could find them, primordial black holes would explain a lot about the universe
UT – Neutron stars may capture primordial black holes

Main Image:
Illustration of colliding black holes.
Credit – Caltech/R. Hurt (IPAC)

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