Primordial black holes are a hypothetical class of black hole that formed not from dying stars but from extreme density fluctuations in the first fraction of a second after the Big Bang. Unlike the stellar-mass and supermassive black holes astronomers routinely observe, primordial black holes could span an enormous range of masses, from lighter than a grain of sand to thousands of times heavier than the Sun. None have been confirmed to exist, but they remain one of the most actively pursued ideas in cosmology because finding even one would open a window onto conditions in the earliest moments of the universe and could solve the long-standing puzzle of what dark matter actually is.
How They Would Have Formed
The most widely studied formation scenario starts with inflation, the brief period of exponential expansion thought to have occurred within the first tiny fraction of a second. During inflation, quantum fluctuations in the density of matter and energy were stretched to macroscopic scales. Most of those fluctuations were small and simply became the seeds of galaxies. But if some fluctuations were unusually large on small scales, the overdense patches they produced could have collapsed directly into black holes once they re-entered the observable universe during the subsequent radiation era. This collapse-of-large-amplitude-perturbation picture is the most commonly analyzed route to primordial black hole formation in single-field inflation models.1PubMed. Constraining Primordial Black Hole Formation from Single-Field Inflation
Inflation is not the only possibility, though. A completely different class of formation mechanism involves cosmological phase transitions. In the very early universe, the fundamental forces and particle fields went through dramatic shifts as the cosmos cooled, somewhat like water freezing. If any of those transitions were sudden enough, patches of the universe would have changed state at slightly different times. The patches that transitioned last could have developed overdensities large enough to collapse into black holes.2Physical Review D. Primordial black holes from supercooled phase transitions Another variant of the phase-transition idea focuses on particles whose mass jumps sharply during the transition. Those particles get blocked by the advancing bubble walls between old and new vacuum states, piling up in front of the walls and creating dense clumps that can trigger black hole formation.3Physics Letters B. Primordial black holes from first-order cosmological phase transitions The diversity of proposed formation channels matters because each one predicts a different range of masses and abundances, which in turn predicts different observable signatures.
The Dark Matter Connection
The idea that dark matter might simply be a population of primordial black holes has been around for decades, and it remains one of the chief motivations for studying them. Dark matter makes up roughly a quarter of the universe’s total energy content, yet no laboratory experiment has detected a dark-matter particle. Primordial black holes would neatly sidestep that problem: they are made of ordinary matter compressed to extreme density, so no new particle physics is required.
The difficulty is that most mass ranges for primordial black holes have already been constrained by observations. Very light ones would have evaporated by now through Hawking radiation. Very heavy ones would show up in gravitational lensing surveys or would heat up stars in small galaxies. What remains is the so-called “asteroid-mass window,” roughly between about 1017 grams and 1022 grams, a range spanning from roughly the mass of a small asteroid to something approaching the mass of a dwarf planet. Within this window, primordial black holes could account for all of the dark matter without violating any current observational limit.4arXiv. Primordial black holes: the asteroid mass window A detailed reassessment of the constraints found that the window narrows when you account for the expected spread of masses, but it stays open.5SciPost Physics. How open is the asteroid-mass primordial black hole window?
These asteroid-mass black holes would be incredibly tiny in physical size, far smaller than an atom, yet each would weigh as much as a mountain or a small moon. That combination makes them almost invisible to conventional telescopes, which is precisely why the window remains open. Closing it, or finding something in it, will require new observational techniques that are still being developed.
Hawking Radiation and Evaporation
One of the most important theoretical properties of any black hole, but especially a small one, is Hawking radiation. According to Stephen Hawking’s 1974 prediction, quantum effects near a black hole’s event horizon cause it to emit particles and slowly lose mass. For stellar-mass black holes this effect is negligibly slow. But for primordial black holes of very low mass, evaporation can be dramatic. Black holes lighter than about 5 × 1014 grams at the time of their formation would have completely evaporated by today, meaning they cannot contribute to the present-day density of dark matter unless they left behind some kind of stable remnant.6ScienceDirect. Primordial black hole constraints with Hawking radiation—A review
Primordial black holes in a heavier range, between about 1017 and 1023 grams, would still be radiating particles of all types today, though not quickly enough to have disappeared. That radiation is the only practical way to detect or constrain the abundance of black holes in this mass range.7ScienceDirect. Primordial black hole constraints with Hawking radiation—A review Gamma-ray and X-ray telescopes look for the diffuse glow that a population of evaporating primordial black holes would produce, and the absence of such a glow sets upper limits on how many can exist at those masses.
Even primordial black holes that evaporated long ago can leave traces. If they evaporated around the time of Big Bang nucleosynthesis, the burst of high-energy particles they released could have altered the primordial abundances of light elements like helium and deuterium. A recent detailed analysis found that to produce any detectable effect on those element abundances, a primordial black hole would need to have had a mass above about 109 grams, a threshold somewhat higher than some earlier estimates suggested.8Journal of Cosmology and Astroparticle Physics. Primordial black holes evaporating before Big Bang nucleosynthesis
How Astronomers Search for Them
Since no primordial black hole has been directly observed, the field is driven by increasingly creative search strategies. The approaches depend heavily on what mass range you are probing.
For primordial black holes heavier than about a thousandth of the Sun’s mass, gravitational microlensing is the workhorse technique. When a compact object passes between a distant star and an observer, its gravity bends and magnifies the star’s light in a characteristic way. Surveys monitoring millions of stars in the Magellanic Clouds and the Andromeda galaxy have placed strong limits on how many primordial black holes of various masses could populate our galaxy’s dark-matter halo.9Journal of Cosmology and Astroparticle Physics. Primordial black hole stellar microlensing constraints: understanding their dependence on the density and velocity distributions These limits are not ironclad, though. The constraints on heavier primordial black holes depend quite sensitively on assumptions about how fast dark matter moves near us, meaning that uncertainties in the local velocity distribution propagate directly into the limits.10Journal of Cosmology and Astroparticle Physics. Primordial black hole stellar microlensing constraints: understanding their dependence on the density and velocity distributions Observations of the Andromeda galaxy using the Subaru telescope’s wide-field camera have pushed these constraints further by looking for very short lensing events that would signal lighter primordial black holes.11arXiv. Microlensing constraints on Primordial Black Hole abundance with Subaru Hyper Suprime-Cam observations of Andromeda
At the heavy end of the spectrum, primordial black holes with masses well above the Sun’s can be constrained by their gravitational effects on stars in small, dark-matter-dominated galaxies. Ultrafaint dwarf galaxies are particularly useful for this. A population of massive compact objects orbiting in such a galaxy would gradually heat up the stellar population through gravitational interactions, puffing out the galaxy’s visible extent. Observations of the half-light radius of ultrafaint dwarfs set limits on how much of their dark matter could be in the form of heavy compact objects.12Physical Review D. Constraints on dark matter from dynamical heating of stars in ultrafaint dwarfs. I. MACHOs and primordial black holes
Gravitational Waves as a Detection Tool
Gravitational-wave astronomy has opened an entirely different avenue for hunting primordial black holes. The key insight is that standard stellar evolution has a hard time producing black holes lighter than about 1.4 solar masses, because that is roughly the minimum mass for a neutron star. Any black hole confirmed to be below that threshold would be extremely difficult to explain as the endpoint of a star’s life, making it a strong primordial candidate.
The LIGO and Virgo collaborations have conducted dedicated searches for mergers involving at least one subsolar-mass compact object. These searches have looked for black holes with masses as low as a tenth of the Sun’s mass. So far, no detections have been made, and the most stringent upper limits on the merger rate of subsolar-mass binary black holes come from the full third observing run.13Monthly Notices of the Royal Astronomical Society. Search for subsolar-mass black hole binaries in the second part of Advanced LIGO’s and Advanced Virgo’s third observing run The earlier half of that same run had already set strong limits.14PubMed. Search for Gravitational Waves from the Coalescence of Subsolar-Mass Binaries in the First Half of Advanced LIGO and Virgo’s Third Observing Run These null results do not rule out primordial black holes, but they do constrain how many of them could be merging in the nearby universe.
A separate gravitational-wave signal, this one at much lower frequencies, has attracted attention from a different angle. Pulsar timing arrays like NANOGrav have detected a background hum of gravitational waves permeating the cosmos. One proposed explanation is that these waves were generated by the same large density fluctuations that would have produced primordial black holes. However, a careful analysis showed that if you assume those fluctuations follow the simplest statistical distribution, they would overproduce primordial black holes, creating far more than observational limits allow. Even accounting for more realistic models of inflation that introduce statistical asymmetry, single-field inflationary scenarios are disfavored as the source of the signal.15PubMed. Recent Gravitational Wave Observation by Pulsar Timing Arrays and Primordial Black Holes: The Importance of Non-Gaussianities
Looking ahead, next-generation detectors should be far more capable. The space-based LISA observatory and ground-based instruments like the Einstein Telescope and Cosmic Explorer could detect extreme mass-ratio inspirals, where a tiny black hole spirals into a much larger one. These events would allow mass measurements precise enough to confirm a subsolar-mass companion down to roughly one-hundredth of a solar mass, at distances of hundreds of megaparsecs for LISA and several gigaparsecs for the Einstein Telescope.16PubMed. Detecting Subsolar-Mass Primordial Black Holes in Extreme Mass-Ratio Inspirals with LISA and Einstein Telescope A confirmed subsolar-mass black hole would be a landmark discovery, as it would represent a type of source that cannot form through any known stellar process.
Seeds for Supermassive Black Holes
One of the more puzzling observations in modern astronomy is the existence of billion-solar-mass black holes at the centers of galaxies when the universe was less than a billion years old. Getting that heavy that fast is surprisingly hard to explain with conventional growth scenarios, which typically start with the remnant of a massive star and build up through gas accretion. Primordial black holes offer an alternative starting point.
A recent model proposes that primordial black holes born from a non-Gaussian distribution of density fluctuations would have been naturally clustered inside the densest regions of the early universe, the same overdensities that later became dark-matter halos. Inside a young halo, these primordial black holes would accrete gas, lose orbital energy through friction with the surrounding gas, sink toward the center, form binary pairs, and undergo runaway mergers. The result is a massive seed of roughly 10,000 to 100,000 solar masses forming by the time the universe was only a few hundred million years old, in rare halos that collapsed between redshifts of about 20 and 40.17arXiv. Primordial black holes as supermassive black holes seeds Seeds that heavy can grow into the observed supermassive black holes through ordinary accretion in the remaining time, a much more comfortable path than starting from a stellar-mass remnant.
What If Planet Nine Is a Primordial Black Hole
Orbital anomalies among distant objects beyond Neptune have led some astronomers to propose that an unseen planet, roughly five to ten times Earth’s mass, lurks at a distance of around 500 astronomical units. Most discussions assume this object, nicknamed Planet Nine, is a conventional icy world. But a pair of researchers pointed out that a primordial black hole of the same mass would fit the gravitational evidence equally well and could also explain an excess of short-duration microlensing events seen in the OGLE survey.18PubMed. What If Planet 9 Is a Primordial Black Hole? A primordial black hole with five to ten Earth masses would be roughly the size of a grapefruit, making it far too small to reflect sunlight or emit detectable thermal radiation.
This idea is testable. A black hole of that mass sitting in the outer solar system would occasionally swallow small icy bodies from the Oort Cloud. Each impact would produce a brief flare of radiation as the infalling material heated up. The Vera C. Rubin Observatory’s Legacy Survey of Space and Time, which is beginning operations, should be capable of either detecting these flares or ruling out the black-hole hypothesis within roughly a year of observations.19The Astrophysical Journal Letters. Searching for Black Holes in the Outer Solar System with LSST The hypothesis is deliberately speculative, but it illustrates how primordial black holes could lurk in unexpected places.
Primordial Black Holes Eating Neutron Stars
If primordial black holes are drifting through the galaxy as part of the dark-matter halo, they would occasionally pass through dense stellar objects. A neutron star, with its extreme density, makes an especially interesting target. A subsolar-mass primordial black hole passing through a neutron star can lose enough energy through interactions with the ultra-dense stellar material to become gravitationally trapped. Once captured, it would spiral inward to the core and consume the neutron star from the inside out.20Journal of Cosmology and Astroparticle Physics. Revisiting Primordial Black Hole Capture by Neutron Stars The observable consequence would be the sudden disappearance of a pulsar or the emission of a burst of gravitational waves as the neutron star collapsed. Watching for unexplained neutron-star disappearances in regions where dark matter is dense, like the galactic center, is another potential way to constrain or detect primordial black holes.
Quantum Gravity Remnants
What happens at the very end of Hawking evaporation is one of the biggest open questions in theoretical physics. The standard semiclassical calculation says a black hole evaporates completely, but that calculation breaks down as the black hole approaches the Planck mass, the scale where quantum-gravity effects become dominant. Some frameworks, including loop quantum gravity, predict that evaporation halts at or near the Planck mass, leaving behind a stable remnant.21Classical and Quantum Gravity. Gravitational wave hints black hole remnants as dark matter
If remnants exist, the implications are dramatic. Every primordial black hole that formed light enough to have evaporated by now would have left one behind. The universe could be filled with a vast population of Planck-mass relics, each far too tiny and inert to interact with normal matter except through gravity, yet collectively making up some or all of the dark matter. One particularly elegant scenario has tiny primordial black holes evaporating early enough to reheat the universe after inflation while their remnants go on to become the dark matter. This scenario predicts a distinctive gravitational-wave signal near 100 Hz that a next-generation detector like the Einstein Telescope could potentially pick up.22Classical and Quantum Gravity. Gravitational wave hints black hole remnants as dark matter A broader investigation of this remnant-dark-matter idea using loop quantum gravity is also underway.23arXiv. Signatures of loop quantum gravity in primordial black hole cosmologies The appeal of this line of research is that it ties together three otherwise separate problems: the nature of dark matter, the endpoint of black hole evaporation, and the correct theory of quantum gravity.
Tunnels, Hollow Moons, and Other Exotic Signatures
Some of the more imaginative proposals for detecting primordial black holes focus not on the sky but on the ground. If primordial black holes of modest mass pass through solid objects, they would punch straight through, leaving a narrow tunnel in their wake. In principle, researchers could look for unexplained micro-tunnels in very old rocks or even in ancient glass structures that have stood for centuries. On a larger scale, a primordial black hole captured inside a planet, moon, or asteroid could hollow it out from within. Ordinary rocky and icy materials can support a hollow shell up to a certain size, so finding a body whose density or seismic signature indicated a hollow interior would be anomalous and worth investigating.24Physics of the Dark Universe. Searching for small primordial black holes in planets, asteroids and here on Earth These are admittedly long-shot ideas, but they highlight how broadly researchers are casting the net.
At the other end of the speculation spectrum, some theorists have explored what an advanced civilization might do with a primordial black hole. The accretion disk around even a modest-mass primordial black hole would be a formidable energy source, and one group has proposed searching for Dyson-sphere-like structures, shells built to capture radiated energy, around accreting primordial black holes as a potential technosignature.25The Astrophysical Journal. In Search of Extraterrestrial Artificial Intelligence Through Dyson Sphere–like Structures around Primordial Black Holes Follow-up work has even modeled the microlensing signatures such a structure would produce, arguing that upcoming surveys could distinguish a naked primordial black hole from one enclosed in an artificial shell.26The Astrophysical Journal. Microlensing Signatures of Dyson Sphere–like Structures around Primordial Black Holes as Technosignatures of Extraterrestrial Advanced Civilizations This sits firmly in the category of speculative but testable, which describes a surprising amount of primordial black hole research.
Laboratory Tests of Hawking Radiation
Since no actual primordial black hole has been found to study up close, physicists have turned to laboratory analogues to test the underlying theory. Hawking radiation is, at heart, a prediction about what happens to quantum fields near an event horizon, and you can mimic an event horizon in a controlled setting. One approach uses intense light pulses traveling through optical fibers. The pulse creates a moving perturbation in the fiber’s refractive index via the Kerr effect, and probe light traveling through the fiber perceives this perturbation as an event horizon when its group velocity matches the pulse speed. Researchers have observed stimulated Hawking radiation in this setup, confirming that the frequency-mixing process Hawking predicted does occur at an analogue horizon.27PubMed. Observation of Stimulated Hawking Radiation in an Optical Analogue Analogue experiments like this do not prove that astrophysical black holes radiate, but they do demonstrate that the mathematical framework underlying the prediction holds up in physical systems where you can actually control and measure the variables.