Small primordial black holes (PBHs) are one of the hot topics in astronomy and cosmology today. These hypothetical black holes are believed to have formed soon after the Big Bang, resulting from pockets of subatomic matter so dense that they underwent gravitational collapse. At present, PBHs are considered candidates for dark matter, a possible source of primordial gravitational waves, and a resolution to various problems in physics. However, no definitive PBH candidate has been observed so far, leading to proposals for how we may find these miniature black holes.
Recent research has suggested that main-sequence neutron and dwarf stars might contain small PBHs in their interiors that are slowly consuming their gas supply. In a recent study, a team of physicists extended this idea to include a new avenue for potentially detecting PBHs. Basically, we could search inside objects like planets, asteroids, or employ large plates or slabs of metal to detect PBHs for signs of their passage. By detecting the microchannels these bodies would leave, scientists could finally confirm the existence of PBHs and shed light on some of the greatest mysteries in cosmology today.
The research was conducted by De-Chang Dai, a physicist at National Dong Hwa University in Taiwan and the Center for Education and Research in Cosmology and Astrophysics (CERCA) at Case Western Reserve University, and Dejan Stojkovic, a physicist from the High Energy Physics and Cosmology group at the State University of New York Buffalo. The paper that details their findings recently appeared online and is being reviewed for publication in the journal Physics of the Dark Universe.
Scientists have been fascinated by PBHs for decades since Russian scientists Igor D. Novikov and Yakov Zeldovich predicted their existence in 1966. They were also a source of interest for Stephen Hawking, whose work on PBHs led to his breakthrough discovery in 1974 that black holes can evaporate over time. While larger and intermediate black holes would take longer to evaporate than the current age of the Universe (ca. 13.8 billion years), smaller PBHs may have already or could be in the process of doing so.
However, interest in PBHs has experienced a renaissance in recent years because they serve as dark matter candidates, a source of primordial gravitational waves (GWs), and more. Like Dark Matter, their existence could help resolve some major cosmological mysteries, but no confirmed observations have been made yet. As De-Chang and Stojkovic told Universe Today via email, this is what motivated them to propose novel detection methods:
“If an asteroid, or a moon, or a small planet (planetoid) has a liquid core surrounded by a solid crust, then a small PBH will consume the dense liquid core relatively quickly (within weeks to months). The crust will remain intact if the material is strong enough to support gravitational stress. Thus, we will end up with a hollow structure. If the central black hole is ejected (due to collisions with other objects), the density will be lower than the usual density of a rocky object with a liquid core.”
In addition, De-Chang and Stojkovic calculated the gravitational stress small PBHs would generate. They then compared this to the compressive strength of materials that make up a planet's crust - such as silicate minerals (rock), iron, and other elements. They also considered the strongest manufactured materials, such as multi-wall carbon nanotubes. “We found, for example, that granite can support hollow structures up to the radius of 1/10 of the Earth’s radius,” said Stojkovic. “That is why we should concentrate on planetoids, moons, or asteroids.”
These calculations offer a means to search for evidence of PBHs in space and here on Earth. Possible candidate planetoids, moons, or asteroids could be identified in our Solar System by observing their mass and radius to provide estimates of their density. This would allow astronomers to identify potentially hollow objects for follow-up studies by probes, landers, and other robotic space missions. Alternatively, they recommend that sensors be built to search for PBHs by detecting their passage. Said Stojkovic:
“If a small PBH passes through some solid material, it will leave a straight long tunnel of the radius comparable to the PBH's radius. For example, a 1023 g PBH should leave a tunnel with a radius of 0.1 micron. [The energies] that such PBHs can have are significant, but [the energies] which they deposit into the material are very low. In fact, such a PBH can even pass through a human body, and we would not even notice because human body tissue has a very low tension.”
In this vein, scientists can scan for micro tunnels in commonplace materials we find lying around (like glass or rocks). At the same time, say De-Chang and Stojkovic, large slabs of polished metal could be prepared for this purpose. Similar to neutrino detection, these slabs would need to be isolated so that any sudden change in their properties could be recorded. “The expected flux of these PBHs is very small and we may end up finding nothing, but a possible payoff of finding PBHs will be huge, especially since such experiments will be very cheap,” said Stojkovic.
As De-Chang added, it has been proposed in recent years that some primordial black holes may be hidden in stars. Stephen Hawking once proposed the idea, which became the basis of two studies, one released in 2019 and another this past year. “It is also proposed that primordial black holes may radiate Gamma rays. Strong gamma rays in the Milky Way's dark matter halo can be a good hint for the existence of primordial black holes,” said De-Chang. “Gravitational microlensing can be another way to identify the primordial black holes.”
Further Reading: arXiv
Understanding the Potential Detection of Primordial Black Holes
In this section, we will delve into the categorization of primordial black holes, proposed experimental designs to uncover their existence, and their implications for our understanding of fundamental physics.
Categorizations of Primordial Black Holes
Primordial black holes can be categorized based on their mass and formation conditions:
| Mass Range | Formation Mechanism | Expected Detection Methods |
|---|---|---|
| < 1015 g | Fluctuations post-Big Bang | Micro-tunnel detection, gravitational lensing |
| 1015 g - 107 M☉ | High-density regions | Microlensing events, gamma-ray bursts |
| > 107 M☉ | Collapse of massive primordial clouds | Gravitational wave detection, x-ray observational astronomy |
Experimental Designs for Detection
The detection of primordial black holes hinges on innovative experimental designs and requires collaborative efforts from various fields of astrophysics:
- Utilizing Gravitational Wave Observatories to detect merging events of PBHs.
- Engaging in Gamma Ray Observations to trace potential sputtering from nearby primordial black holes.
- Conducting experiments based on Particle Physics that simulate conditions under which PBHs were created.
Detection methods continue to evolve as technology improves, allowing scientists to develop increasingly innovative approaches to unravel some of the universe's greatest mysteries.
The Role of Simulations
Computational astrophysics plays a vital role in modeling the formation and growth of primordial black holes:
| Simulation Aspect | Purpose | Output |
|---|---|---|
| Formation Scenarios | Understand conditions post-Big Bang | Mass distribution of PBHs |
| Gravitational Dynamics | Track PBH interactions | Insights into microlensing effects |
| Radiative Properties | Assess observable emissions | Theoretical spectra for detection |
Simulations and numerical methods are increasingly valuable in aiding astronomers to visualize and explore phenomena that are otherwise impossible to observe directly.
Conclusion
The search for primordial black holes is at the forefront of contemporary astrophysics and cosmology. Best guesses regarding their properties and formation have sparked a myriad of theoretical models and proposals for detection methods. As technology progresses and our understanding of the universe expands, primordial black holes may yet reveal themselves as vital pieces of the cosmic puzzle.
