Astronomy has long been an arena of human curiosity, pushing the boundaries of our understanding of the universe. One of the most enigmatic phenomena within this vast field is the concept of black holes. Typically associated with the death of massive stars, black holes are regions in space where gravity is so intense that not even light can escape. However, a lesser-known but equally fascinating type of black hole has been theorized to exist: the primordial black hole (PBH). These black holes, which may have formed in the chaotic conditions of the early universe, are vastly different from those born from dying stars. While no primordial black hole has ever been observed, recent theoretical research co-led by the University at Buffalo (UB) offers a fresh perspective on how their existence might be confirmed, providing a tantalizing glimpse into the possible role PBHs play in the universe, particularly as dark matter.
Primordial black holes have long been the subject of theoretical studies. They are thought to have formed in the first moments after the Big Bang when the universe was a hot, dense place. The theory proposes that fluctuations in the density of matter in the early universe could have led to regions where gravity was strong enough to cause collapse, creating black holes even before the formation of stars. Unlike stellar black holes, which are formed from the collapse of large stars, primordial black holes would have a much smaller mass, but they would still possess immense density. A primordial black hole might be as heavy as a mountain, but compressed into a region the size of an atom. Due to their tiny size and immense density, these black holes could play a key role in the mystery of dark matter, a substance that makes up about 85% of the universe’s mass but has yet to be directly detected.
The search for primordial black holes has been an ongoing challenge. Although their properties make them intriguing candidates for dark matter, finding direct evidence of their existence has proven elusive. A recent study, co-led by Dejan Stojkovic, Ph.D., a professor of physics at UB, proposes a novel approach to detect these theoretical objects by thinking both large and small. According to Stojkovic, the signature of a primordial black hole could manifest in two distinct ways. One approach looks at large cosmic objects, such as planets or asteroids, where a primordial black hole might be trapped within the object’s core. The other proposes examining smaller, everyday materials here on Earth, where a primordial black hole could have passed through solid matter, leaving behind microscopic evidence.
The study, published in the December 2024 issue of Physics of the Dark Universe, suggests that a primordial black hole could be detected in objects like planets or moons if the black hole became trapped within them. If the object had a liquid core, the black hole could absorb this liquid, since the density of the liquid is higher than the outer solid layer of the object. Over time, this process could leave behind a hollow structure, essentially turning the object into a “hollow planetoid.” These hollow objects would be detectable through telescopic observations, particularly by studying their orbits. An object with low density for its size would likely indicate that it is hollow, as the density would be too low for a solid object of that size.
However, such hollow objects could not be arbitrarily large. The study calculated that a hollow object with a primordial black hole trapped within it could not exceed about one-tenth of the Earth’s radius. Any larger and the object would collapse under its own tension. This limitation would make it more likely that such objects would be minor planets or asteroids, rather than fully developed planets. The potential discovery of such hollow objects would be groundbreaking. The researchers believe that if primordial black holes are responsible for dark matter, detecting these hollow planetoids could provide crucial evidence for the theory.
In addition to cosmic objects, primordial black holes might also leave behind smaller, more subtle signatures. For instance, a PBH might pass through a solid material, such as a rock, metal, or glass, leaving behind a straight tunnel. The study proposes that such tunnels, although extremely small—on the order of microns—could be detectable in materials that are hundreds, thousands, or even millions of years old. These tunnels would result from the high-speed passage of a PBH through the material, and although the probability of a PBH passing through a given object is extremely low, the idea of searching for such marks in ancient materials offers an intriguing new direction for research.
Stojkovic and his colleagues estimate that the likelihood of a PBH passing through a billion-year-old rock and leaving behind a detectable tunnel is extraordinarily small—around 0.000001. Even so, the researchers argue that the potential reward of detecting a primordial black hole would far outweigh the minimal cost of such searches. The cost of such research would be relatively low, especially compared to the profound implications that the discovery of a PBH would have for our understanding of dark matter and the early universe.
While the chances of encountering a PBH in your lifetime are incredibly small, Stojkovic notes that these black holes would not be dangerous if they did pass through you. The speed at which a primordial black hole would travel through matter is so high that it would not transfer significant energy during its passage. Just as a bullet passing through a window leaves a hole but doesn’t shatter the glass, a PBH moving through human tissue would likely leave no noticeable effects. In fact, its impact would be so minimal that humans would be unlikely to even notice it.
This study is part of a broader effort to develop new theoretical frameworks to tackle some of the biggest unsolved problems in physics, such as the nature of dark matter. The problem of dark matter has stumped scientists for decades, and traditional models based on quantum mechanics and general relativity have yet to provide satisfactory answers. Stojkovic emphasizes that solving the mysteries of the universe may require entirely new approaches—frameworks that go beyond the existing paradigms.
The possibility that primordial black holes could be a form of dark matter is just one of the many exciting avenues of research in contemporary astronomy and cosmology. As the study suggests, even though the odds of directly detecting these black holes are slim, the potential implications are vast. If confirmed, primordial black holes could fundamentally reshape our understanding of the early universe, providing a new piece of the puzzle in the search for dark matter.
As research in this field continues, the search for primordial black holes highlights the importance of thinking beyond conventional boundaries. The universe is full of mysteries waiting to be uncovered, and breakthroughs in theoretical research often come from unexpected directions. Whether through the detection of hollow planetoids in distant galaxies or the discovery of microscopic tunnels in ancient materials, the hunt for primordial black holes represents an exciting frontier in modern astronomy—one that may one day reveal the elusive nature of dark matter and offer insights into the fundamental workings of the cosmos.
Source: University at Buffalo