Small primordial black holes (PBHs) are one of the hottest topics
Scussi 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.
Small primordial black holes: candidates for dark matter?
Currently, small primordial black holes are considered a candidate for dark matter, a possible source of primordial gravitational waves, and a solution to various problems in physics. However, no definitive small primordial black hole candidate has been observed so far, which has led to proposals for how we might find these miniature black holes.
Recent research has suggested that main-sequence dwarf and neutron stars may contain small PBHs within them that are slowly depleting their gas supply.
In a recent study, a team of physicists extended this idea to include a new avenue for potentially detecting small ones black holes primordial. In practice, we could search inside objects such as planets and asteroids or use large metal plates or plates to detect PBHs for signs of their passage.
By detecting the microchannels left behind by these bodies, scientists could finally confirm the existence of PBHs and shed light on some of the biggest 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 by Dejan Stojkovic, a physicist in the Physics Group of high energy and cosmology at the State University of New York Buffalo.
The paper detailing 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, ever since Russian scientists Igor D. Novikov and Yakov Zeldovich predicted their existence in 1966. They have also been a source of interest to Stephen Hawking, whose work on small primordial black holes led to his groundbreaking discovery in 1974 that black holes can evaporate over time.
While larger and intermediate black holes would take longer than the current age of the Universe (about 13.8 billion years) to evaporate, smaller PBHs may have already done so or may be in the process of doing so.
Interest in small primordial black holes has experienced a resurgence in recent years because they serve as candidates for dark matter, a source of primordial gravitational waves (GWs), and more. Like dark matter, their existence could help solve some important cosmological mysteries, but no confirmed observations have yet been made.
As De-Chang and Stojkovic explained to Universe Today via email, this is what drove them to propose new 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 or months). The crust will remain intact if the material is strong enough to support the gravitational stress.”
“So, 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.”
Furthermore, De-Chang and Stojkovic calculated the gravitational stress that small primordial black holes would generate. They then compared this with the compressive strength of the materials that make up a planet’s crust, such as silicate (rock) minerals, iron and other elements. They also considered stronger handcrafted materials, such as multi-walled carbon nanotubes.
“We found, for example, that granite can support hollow structures up to 1/10 of the Earth’s radius,” Stojkovic said. “That’s why we should focus on planetoids, moons or asteroids.”
These calculations offer a means to search for evidence of small, primordial black holes 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 building sensors to search for small primordial black holes by detecting their passage.
Stojkovic said: “If a small PBH passes through a solid material, it will leave a long straight tunnel of radius comparable to the radius of the PBH. For example, a 1023 g PBH should leave a tunnel with a radius of 0.1 micron. [Le energie] that such PBHs may have are significant, but [le energie] that deposit in the material are very low. In fact, such a PBH can even pass through a human body, and we wouldn’t even notice it because the tissue of the human body has a very low voltage.”
In this vein, scientists can scan micro tunnels in common materials we find around (like glass or rocks). At the same time, De-Chang and Stojkovic say, large sheets of polished metal could be prepared for this purpose. Similar to neutrino detection, these plates would need to be isolated so that any sudden changes in their properties could be recorded.
“The expected stream of these small primordial black holes is very small and we might end up finding nothing, but the possible gain in finding a PBH would be huge, especially since such experiments would be very cheap,” Stojkovic said.
As De-Chang added, in recent years it has been proposed that some primordial black holes may be hidden in stars. Stephen Hawking once proposed the idea, which became the basis of two studies, one published in 2019 and another last year.
“It is also proposed that primordial black holes can radiate gamma rays. Strong gamma rays in the Milky Way’s dark matter halo may be a good clue to the existence of primordial black holes,” De-Chang said.
“Gravitational microlensing may be another way to identify primordial black holes.”
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