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What Are Primordial Black Holes?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Origins of the Primordial Black Hole

Stephen Hawking first mentioned primordial black holes (PBHs) in the 1970s as he developed his ideas for cosmology, findings that were a potential consequence of the radiation-dominated universe, a brief period in the early history of the universe. In a random fashion, different parts of the Universe expanded at different rates, and gravity also worked in different ways, depending on the volume and density of the region it was in. For some places, gravity could so greatly exceed the rate of universal expansion and the pressure of a collapsing object that the region filled solely with photons would collapse onto itself, forming a PBH. Assuming the minimum radius of a Planck length, these PBHs would be at minimum 10 micrograms in mass. They would be so small that through Hawking radiation, the PBHs could disappear over the lifetime of the universe, meaning that not many would be left today. But to get a true gauge of how realistic they could be, the inflation model needed some fine-tuning (Hawking).

In 1996, Garica-Bellido, Andre Linde, and David Wands found that inflation could cause “sharp peaks in the spectrum of density flux” when the Universe was young. At that time, quantum effects were rampant in such a small space, and the uncertainty principle allowed for large peaks in energy density. These peaks were further magnified by inflation and led to areas where black holes formed directly from photon groupings. If models hold true, they predict that those black holes could have formed in clusters as PBHs, and then were distributed across the Universe as it expanded and became the dark matter we see (Garcia 40, Crane 39).

Each of these early PBHs would be 1/100 to 1/10,000 a solar mass. Over time, through chance encounters, they could merge together and possibly be the seeds of supermassive black holes. And in a 2015 update to that work, Garcia-Bellido and Clesse found that the wide range of density fluctuations because of the energy levels and spatial properties at that time of the Universe. would result in a wide range and number of PBHs. The density of them out there could be as much as 1 million within a several light-year span, which on a per mass basis would fall in line with dark matter predictions. And because of their origin of photon collapsing, they could be of any size and not limited to Schwarzschild considerations (for photons are radiative in nature while host stars are matter in nature, leading to sizing limits) (Garcia 40-2, Crane 39).



Understanding the drive behind finding PBHs comes from trying to understand if dark matter is made of WIMPs (Weakly Interacting Massive Particles) or MACHOs (Massive Compact Halo Objects), both unproven concepts. But something that already has lots of evidence in its favor is black holes, and they have many characteristics that MACHOs would have. But, and this is key, some more properties would be needed if they were to be MACHO candidates, such as a certain galactic distribution, patterns in the cosmic web, and gravitational lensing effects, all of which we haven’t seen yet. Nothing so far has yielded the expected MACHO response, and so they are no longer a major candidate for dark matter. But don’t confuse that with scientists giving up on them. They have conducted a microgravity lensing observation to try and place some limits on the mass of these objects. After such a search in the Small Magellanic Cloud, no MACHO candidates were spotted, and so scientists knew from that data that the largest MACHO could be 10 solar masses but expect them to be much smaller than that. Naturally, scientists moved on and looked for WIMPs, but that search has gained more attention and yet equally lacking results as its counterpart. Some models predict PBHs could be WIMP factories via Hawking radiation considerations, for size is inversely correlated to temperature. Therefore, a small object like a PBH should be very hot, therefore, radiative. If WIMPs exist, then collisions between them should create a distinctive gamma ray that is yet unseen. So now the spotlight is once again on MACHOs, for there is a type of black hole that would be a perfect MACHO candidate: a PBH. Hard to see, yet offering the gravitational pull needed, they would be a great target (Garcia 40, BEC, Rzetelny, Crane 40).

Hunting for PBHs

We can hunt for PBHs through several methods. One would be gravity waves, but the sensitivity needed to spot a wave from a PBH merger doesn’t exist yet (more on this soon). Another method would come from ultrafaint dwarf galaxies, for they are directly a product of dark matter acclimation. Observations from WFIRST may shed light on these and determine some of the dark matter properties of them, including if they are made of PBHs. A third option would be to look for unexpected changes in the positions of stars in the Milky Way. Gaia is making a 3D map of the star distribution and can spot any unanticipated changes in their positions, leading a smoking gun hopefully to some PBHs. Yet another option would be to examine the 20 cm wavelength hydrogen that exists from the early Universe, looking for ionization that would arise from PBHs hitting the hydrogen with X-rays. Finally, we can hunt for distortions in the cosmic microwave background, for PBHs (if they formed early enough) should have left an impression on it and affected the microwaves we see today (Garcia 42).

One such study was done in the late 2000s to early 2010s when Alexander Kashlinsky and a team at NASA looked at the cosmic X-ray background and the cosmic infrared background to see if any interesting patterns emerged. The infrared data indicated that an excessive glow was seen, indicating that the structures seen at the time are not enough to account for them. The X-ray data also pointed to this, hinting at the only known object which has this wide an output of energy, and that is a black hole as its halo and a correction disc interact with each other and their surroundings. From the time that the backgrounds originated, only PBHs should have been possible, hinting at their possible existence, and based on the data it implies that PBH binaries are possible too. They would have just the right energy and glancing interaction to form, and once enough gravitational and EM radiation had left the system, the PBHs could merge to make more modern-sized black holes, just like theory predicts (Reddy, Kashlinksky, BEC, Rzetelny).

A PBH interacting with a star.

A PBH interacting with a star.

Another interesting study was conducted by Miguel Zumalacarregui and Uros Seljak (both at Berkeley Center for Cosmological Physics) in 2017 when they noted that the lower bounds for PBHs that fit with cold dark matter models (from 10 to 100 solar masses) were also a range that LIGO was looking for in its mergers. While methods for determining how PBHs are scattered on a small scale make testing this difficult, the authors also noted that gravitational lensing of Type Ia supernova, the standard candle of astronomy, could be an effective test for PBHs, especially because of their range (of depth) and their reliability of luminosity. It could also look at the mass range that LIGO was capable of as well. After looking for the dimming effect that a PBH-caused microlensing event would generate and removing errors from instrument uncertainty and high red shifted events (because of those inherent unknowns), no lensing events for PBHs with masses greater than 1/100 a solar mass were seen for 740 bright supernovas, narrowing the possible ranges of PBH masses as well as their contribution to dark matter models. It would seem that if PBHs contribute then it is at most 40% (Zumalacarregui, Rzetelny, Gohd).

What would happen if a PBH encountered a star? It turns out, not much – if it's a large enough star. The PBH would go through the star and not consume it nor destroy it. However, it would cause vibrations to resonate on the surface of the star as the surface becomes unpinned from the gravity interaction with the PBH. Simulations have shown what astronomers should expect to see if viewing a star that has undergone this interaction, so it’s just a matter of observing and being lucky at this point (Princeton).

However, this picture is not true if it were a neutron star, a very dense object indeed. The PBH would intake matter, decreasing the radius of the star and therefore increasing its spin rate as conservation of angular momentum dictates. If enough is consumed, the star will spin fast enough to send neutrons flying away and at enough speeds to collide and make heavy elements. Calculations show that this mechanism would account for all the heavy elements we see in the known Universe. And as an added bonus, this process could lead to fast radio bursts and also generate matter-antimatter signals we see at the center of the galaxy (Lucy).

All of this refers back to that picture of PBHs as our dark matter candidate. But what if PBHs are not the only form of dark matter, but a fraction of it? Misao Sasaki and several other Japanese astrophysicists examined GW150914, a LIGO event from 2015, and ran models to see if any PBH scenario could explain the merger. They found that if PBHs were mainly half a solar mass and considered to be in sufficient quantities for them to be dark matter was considered, then the merger was not possible, but instead, if PBHs were in smaller quantities, then their potential lower merger rate would match the upper rate established by GW150914 (Sasaki).

Other Possibilities for PBHs

Okay, so what other mysteries could PBHs help us out with? For starts, they could explain why we seem to have a lack of dwarf galaxies around as satellites to other galaxies. The current theory as to their missing presence is that baryonic matter mechanics at that scale is not fully understood. PBHs could perhaps be stealing material and preventing dwarf galaxies. They could also provide a gravitational kick to remove stars from even clustering in the first place (Garcia 43).

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PBHs would also solve why we don’t have more intermediate galaxies out there. Why? Those missing dwarf galaxies are not there to combine to make smaller galaxies than we are used to, breaking the chain of growth. Another mystery PBHs would fix would be the formation of supermassive black holes. How did they start? Most models show it would take billions of years for the necessary gravitational collapses necessary for them, yet we know it happened much faster than that. But the accumulation and gravitational collapse of PBHs would speed up the process and maybe even be responsible for intermediate black holes as well (Garcia 43, Crane 41).

What they wouldn’t solve would be dark energy, the mysterious force that is accelerating the expansion of the Universe. Some postulated that the PBH’s Hawking radiation (a huge consequence and success of quantum mechanics with black holes) could lead to such an effect as the black hole slowly evaporated away, decreasing their gravitational mass and therefore increasing the Universe’s expansion. However, the time it would take for this evaporation to happen is much longer than the current age of the Universe and anything that did radiate out would be so highly redshifted that it would be inconsequential on a Universal scale. Sorry, but black holes don’t solve everything (Clesse, Crane 40).

Works Cited

BEC Crew. “A NASA physicist just gave us the most promising new candidate for dark matter.” Science Alert, 30 May 2016. Web. 24 Sept. 2018.

Clesse, Sebastien and Juan Garcia-Bellides. “Black Holes, Dark Matter.” Scientific American Novemeber 2017. Print. 8.

Crane, Leah. “Dark Secrets.” New Scientist. New Scientist Ltd., 12 Oct. 2019. Print. 39-41.

Garcia-Bellides, Juan and Sebasten Clesse. “Black Holes from the Beginning of Time.” Scientific American Jul. 2017. Print. 40-3.

Gohd, Chelsea. "Black holes can't explain dark matter." Kalmbach Publishing Co., 11 Oct. 2018. Web.

Hawking, Stephen. “Gravitationally Collapsed Objects of Very Low Mass.” Monthly Notices of the Royal Astronomical Society. Vol. 152, 09 Nov. 1971. Print. 75-8.

Kashlinksky, A. “LIGO Gravitational Wave detection, Primordial Black Holes, and the Near-IR Cosmic Infrared Background Anisotropies.” The Astrophysical Journal Letters. 01 June 2016. Print. 1-6.

Lucy, Michael. "Primordial black holes may create gold and other heavy elements." Cosmos. Web. 29 Aug. 2019.

Princeton University. “Black hole, star collisions may illuminate universe’s dark side.” Kalmbach Publishing Co., 22 Sept. 2011. Web. 24 Sept. 2018.

Reddy, Francis. “NASA Scientists Suggests Possible Link Between Primordial Black Holes and Dark Matter.” NASA, 06 Aug. 2017. Web. 27 May 2018.

Rzetelny, Xaq. “MACHOs make a return with gravitational wave discovery.” Conte Nast., 26 May 2016. Web. 24 Sept. 2018.

Sasaki, Misao et al. “Primordial Black Hole Scenario for the Gravitational-Wave Event GW150914.” arXiv: 1603.08338v2.

Zumalacarregui, Miguel and Uros Seljak. “No LIGO MACHO: Primordial Black Holes, Dark Matter and Gravitational Lensing of Type 1a Supernovae.” arXiv: 1712.02240v1.

© 2019 Leonard Kelley

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