Formation of the universe without (above) and with (below) primordial black holes

In cosmology, primordial black holes (PBHs) are hypothetical black holes that formed soon after the Big Bang. In the inflationary era and early radiation-dominated universe, extremely dense pockets of subatomic matter may have been tightly packed to the point of gravitational collapse, creating primordial black holes without the supernova compression needed to make black holes today. Because the creation of primordial black holes would pre-date the first stars, they are not limited to the narrow mass range of stellar black holes.

In 1966, Yakov Zeldovich and Igor Novikov first proposed the existence of such black holes,[1] while the first in-depth study was conducted by Stephen Hawking in 1971.[2] However, their existence has not been proven and remains hypothetical. In September 2022, primordial black holes were proposed by some researchers to explain the unexpected very large early galaxies discovered by the James Webb Space Telescope (JWST).[3][4]

PBHs have long been considered possibly important if not nearly exclusive components of dark matter,[5][6][7][8] the latter perspective having been strengthened by both LIGO/Virgo interferometer gravitational wave and JWST observations.[9][10] Early constraints on PBHs as dark matter usually assumed most black holes would have similar or identical ("monochromatic") mass, which was disproven by LIGO/Virgo results,[11][12][13] and further suggestions that the actual black hole mass distribution is broadly platykurtic were evident from JWST observations of early large galaxies.[9][10]


Depending on the model, primordial black holes could have initial masses ranging from 10−8 kg[14] (the so-called Planck relics) to more than thousands of solar masses. However, primordial black holes originally having mass lower than 1011 kg would not have survived to the present due to Hawking radiation, which causes complete evaporation in a time much shorter than the age of the Universe.[15] Primordial black holes are non-baryonic,[16] and as such are plausible dark matter candidates.[10][5][11][12][8][13][9] Primordial black holes are also good candidates for being the seeds of the supermassive black holes at the center of massive galaxies, as well as of intermediate-mass black holes.[17][3][4]

Primordial black holes belong to the class of massive compact halo objects (MACHOs). They are naturally a good dark matter candidate: they are (nearly) collision-less and stable (if sufficiently massive), they have non-relativistic velocities, and they form very early in the history of the Universe (typically less than one second after the Big Bang).[18] Nevertheless, critics maintain that tight limits on their abundance have been set up from various astrophysical and cosmological observations, which would exclude that they contribute significantly to dark matter over most of the plausible mass range.[19] However, new research has provided for the possibility again, whereby these black holes would sit in clusters with a 30-solar-mass primordial black hole at the center.[20][21]

Simulation of two black holes colliding

In March 2016, one month after the announcement of the detection by Advanced LIGO/VIRGO of gravitational waves emitted by the merging of two 30 solar mass black holes (about 6×1031 kg), three groups of researchers proposed independently that the detected black holes had a primordial origin.[22][23][24][25] Two of the groups found that the merging rates inferred by LIGO are consistent with a scenario in which all the dark matter is made of primordial black holes, if a non-negligible fraction of them are somehow clustered within halos such as faint dwarf galaxies or globular clusters, as expected by the standard theory of cosmic structure formation. The third group claimed that these merging rates are incompatible with an all-dark-matter scenario and that primordial black holes could only contribute to less than one percent of the total dark matter. The unexpected large mass of the black holes detected by LIGO has strongly revived interest in primordial black holes with masses in the range of 1 to 100 solar masses. It is still debated whether this range is excluded or not by other observations, such as the absence of micro-lensing of stars,[26] the cosmic microwave background anisotropies, the size of faint dwarf galaxies, and the absence of correlation between X-ray and radio sources toward the galactic center.

In May 2016, Alexander Kashlinsky suggested that the observed spatial correlations in the unresolved gamma-ray and X-ray background radiations could be due to primordial black holes with similar masses, if their abundance is comparable to that of dark matter.[27]

In August 2019, a study was published opening up the possibility of making up all dark matter with asteroid-mass primordial black holes (3.5 × 10−17 – 4 × 10−12 solar masses, or 7 × 1013 – 8 × 1018 kg).[28]

In September 2019, a report by James Unwin and Jakub Scholtz proposed the possibility of a primordial black hole (PBH) with mass 5–15 ME (Earth masses), about the diameter of a tennis ball, existing in the extended Kuiper Belt to explain the orbital anomalies that are theorized to be the result of a 9th planet in the solar system.[29][30]

In October 2019, Derek Inman and Yacine Ali-Haïmoud published an article in which they discovered that the nonlinear velocities that arise from the structure formation are too small to significantly affect the constraints that arise from CMB anisotropies[31]

In September 2021, the NANOGrav collaboration announced that they had found a low-frequency signal that could be attributed to gravitational waves and potentially could be associated with PBHs.[32]

In September 2022, primordial black holes were used to explain the unexpected very large early (high redshift) galaxies discovered by the James Webb Space Telescope.[3][4]

On 26 November 2023, evidence, for the first time, of an overmassive black hole galaxy (O.B.G.), the result of "heavy black hole seed formation from direct collapse", an alternative way of producing a black hole other than the collapse of a dead star, was reported. This discovery was found in studies of UHZ1, a very early galaxy containing a quasar, by the Chandra X-ray Observatory and James Webb Space Telescope.[33][34]


Primordial black holes were possibly formed by the collapse of overdense regions in the inflationary or early radiation-dominated universe.[35]

Primordial black holes could have formed in the very early Universe (less than one second after the Big Bang) during the inflationary era, or in the very early radiation-dominated era. The essential ingredient for the formation of a primordial black hole is a fluctuation in the density of the Universe, inducing its gravitational collapse. One typically requires density contrasts (where is the density of the Universe) to form a black hole.[36]

Production mechanisms

There are several mechanisms able to produce such inhomogeneities in the context of cosmic inflation (in hybrid inflation models.) Some examples include:

Axion inflation

Axion inflation is a theoretical model in which the axion acts as an inflaton field. Because of the time period it is created at, the field is oscillating at its minimal potential energy. These oscillations are responsible for the energy density fluctuations in the early universe.[37]


Reheating is the transitory process between the inflationary and the hot, dense, radiation-dominated period. During this time the inflaton field decays into other particles. These particles begin to interact in order to reach thermal equilibrium. However, if this process is incomplete it creates density fluctuations, and if these are big enough they could be responsible for the formation of PBH.[38]

Cosmological phase transitions

Cosmological phase transitions may cause inhomogeneities in different ways depending on the specific details of each transition. For example, one mechanism is concerned with the collapse of overdense regions that arise from these phase transitions, while another mechanism involves highly energetic particles that are produced in these phase transitions and then go through gravitational collapse forming PBHs.[39]


Dark matter problem

The dark matter problem, proposed in 1933 by Swiss-American astronomer Fritz Zwicky, refers to the fact that scientists still don't know what form dark matter takes. PBH can solve that in a few ways. First, if PBHs accounted for all or a significant amount of the dark matter in the universe, this could explain the gravitational effects seen in galaxies and galactic clusters. Secondly, PBHs have different proposed production mechanisms. Unlike WIMPs, they can emit gravitational waves that interact with regular matter. Finally, the discovery of PBHs could explain some of the observed gravitational lensing effects that couldn't arise from ordinary matter. While evidence that primordial black holes may constitute dark matter is inconclusive as of 2023, researchers such as Bernard Carr and others are strong proponents.[9][10][40][11][12][8][13][5][41]

Galaxy formation

Since primordial black holes do not necessarily have to be small (they can have any size), they may have contributed to formation of galaxies, such as those earlier than expected.[3][4]

Cosmological domain wall problem

The cosmological domain wall problem, proposed in 1974 by Soviet physicist Yakov Zeldovich, discussed the formation of domain walls during phase transitions of the early universe and what could arise from their large energy densities. PBHs could serve as a solution to this problem in various ways. One explanation could be that PBHs can prevent the formation of domain walls due to them exerting gravitational forces on the surrounding matter making it clump and theoretically preventing the formation of said walls. Another explanation could be that PBHs could decay domain walls; if these were formed in the early universe before PBHs then due to gravitational interactions these could eventually collapse into PBHs. Finally, a third explanation could be that PBHs do not violate the observational constraints; if PBHs in the 1015-1016 gram mass range were to be detected then these would have the right density to make up all dark matter in the universe without violating constraints, thus the domain wall problem wouldn't arise.[42]

Cosmological monopole problem

The Cosmological monopole problem, also proposed by Yakov Zeldovich in the late 1970s, consisted of the absence of magnetic monopoles nowadays. PBHs can also serve as a solution to this problem. To start, if magnetic monopoles did exist in the early universe these could have gravitationally interacted with PBHs and been absorbed thus explaining their absence. Another explanation due to PBHs could be that PBHs would have exerted gravitational forces on matter causing it to clump and dilute the density of magnetic monopoles.[43]

String theory

Main article: String theory

General relativity predicts the smallest primordial black holes would have evaporated by now, but if there were a fourth spatial dimension – as predicted by string theory – it would affect how gravity acts on small scales and "slow down the evaporation quite substantially".[44] In essence, the energy stored in the fourth spatial dimension as a stationary wave would bestow a significant rest mass to the object when regarded in the conventional four-dimensional space-time. This could mean there are several thousand primordial black holes in our galaxy. To test this theory, scientists will use the Fermi Gamma-ray Space Telescope, which was put into orbit by NASA on June 11, 2008. If they observe specific small interference patterns within gamma-ray bursts, it could be the first indirect evidence for primordial black holes and string theory.[needs update]

Observational limits and detection strategies

A variety of observations have been interpreted to place limits on the abundance and mass of primordial black holes:

Lifetime, Hawking radiation and gamma-rays: One way to detect primordial black holes, or to constrain their mass and abundance, is by their Hawking radiation. Stephen Hawking theorized in 1974 that large numbers of such smaller primordial black holes might exist in the Milky Way in our galaxy's halo region. All black holes are theorized to emit Hawking radiation at a rate inversely proportional to their mass. Since this emission further decreases their mass, black holes with very small mass would experience runaway evaporation, creating a burst of radiation at the final phase, equivalent to a hydrogen bomb yielding millions of megatons of explosive force.[45] A regular black hole (of about 3 solar masses) cannot lose all of its mass within the current age of the universe (they would take about 1069 years to do so, even without any matter falling in). However, since primordial black holes are not formed by stellar core collapse, they may be of any size. A black hole with a mass of about 1011 kg would have a lifetime about equal to the age of the universe. If such low-mass black holes were created in sufficient number in the Big Bang, we should be able to observe explosions by some of those that are relatively nearby in our own Milky Way galaxy. NASA's Fermi Gamma-ray Space Telescope satellite, launched in June 2008, was designed in part to search for such evaporating primordial black holes. Fermi data set up the limit that less than one percent of dark matter could be made of primordial black holes with masses up to 1013 kg. Evaporating primordial black holes would have also had an impact on the Big Bang nucleosynthesis and change the abundances of light elements in the Universe. However, if theoretical Hawking radiation does not actually exist, such primordial black holes would be extremely difficult, if not impossible, to detect in space due to their small size and lack of large gravitational influence.

Temperature anisotropies in the cosmic microwave background: Accretion of matter onto primordial black holes in the early Universe should lead to energy injection in the medium that affects the recombination history of the Universe. This effect induces signatures in the statistical distribution of the cosmic microwave background (CMB) anisotropies. The Planck observations of the CMB exclude that primordial black holes with masses in the range 100–104 solar masses contribute importantly to the dark matter,[46] at least in the simplest conservative model. It is still debated whether the constraints are stronger or weaker in more realistic or complex scenarios.

Gamma-ray signatures from annihilating dark matter: If the dark matter in the Universe is in the form of weakly interacting massive particles or WIMPs, primordial black holes would accrete a halo of WIMPs around them in the early universe.[47] The annihilation of WIMPs in the halo leads to a signal in the gamma-ray spectrum which is potentially detectable by dedicated instruments such as the Fermi Gamma-ray Space Telescope.[48]

In the future, new limits will be set up by various observations:

Facilities able to provide PBH measurement

This section is in list format but may read better as prose. You can help by converting this section, if appropriate. Editing help is available. (April 2023)

None of these facilities are focused on the direct detection of PBH due to them being a theoretical phenomenon, but the information collected in each respective experiment provides secondary data which can help provide insight and constraints on the nature of PBHs.s[57]


Space telescopes

Sky Surveys

Very Large Arrays

Fast Radio Bursts observatories

MeV Gamma-Ray Telescopes

GeV and TeV Gamma-Ray Observatories

Difference from direct collapse black holes

A direct collapse black hole is the result of the collapse of unusually dense and large regions of gas, after the radiation-dominated era, while primordial black holes result from the direct collapse of energy, ionized matter, or both, during the inflationary or radiation-dominated eras.[58]

See also


  • S.W. Hawking, Commun.Math. Phys. 43 (1975) 199 : Original article proposing existence of radiation
  • D. Page, Phys. Rev. D13 (1976) 198 : First detailed studies of the evaporation mechanism
  • B.J. Carr & S.W. Hawking, Mon. Not. Roy. Astron. Soc 168 (1974) 399 : Describes links between primordial black holes and the early universe
  • A. Barrau et al., Astron. Astrophys. 388 (2002) 676, Astron. Astrophys. 398 (2003) 403, Astrophys. J. 630 (2005) 1015 : Experimental searches for primordial black holes due to the emitted antimatter
  • A. Barrau & G. Boudoul, Review talk given at the International Conference on Theoretical Physics TH2002 : Cosmology with primordial black holes
  • A. Barrau & J. Grain, Phys. Lett. B 584 (2004) 114 : Searches for new physics (quantum gravity) with primordial black holes
  • P. Kanti, Int. J. Mod. Phys. A19 (2004) 4899 : Evaporating black holes and extra-dimensions
  • Bird, Simeon; Albert, Andrea; Dawson, Will; Ali-Haimoud, Ali-Haimoud, Yacine; Coogan, Adam; Drlica-Wagner, Alex; Feng, Qi; Inman, Derek; Inomata, Keisuke; Kovetz, Ely; Kusenko, Alexander; Lehmann, Benjamin V.; Muñoz, Julian B.; Singh, Rajeev; Takhistov, Volodymyr; Tsai, Yu-Dai. 2022. Snowmass2021 Cosmic Frontier White Paper: Primordial Black Hole Dark Matter [57]
  • Inman, Derek; Ali-Haïmoud, Yacine. 2019. Early structure formation in primordial black hole cosmologies. [31]
  • Lincoln, Don. 2022. Is dark matter real? Astronomy’s multi-decade mystery [59]
  • Vaskonen, Ville; Veermäe, Hardi. 2021. Did NANOGrav See a Signal from Primordial Black Hole Formation? [32]
  • Caputo, Andrea. 2019. Radiative axion inflation. [37]
  • Allahverdi, Rouzbeh; Brandenberger, Robert; Cyr-Racine, Francis-Yan; Mazumdar, Anupam. 2010. Reheating in Inflationary Cosmology: Theory and Applications. [38]
  • Mazumdar, Anupam; White Graham. 2019. Review of cosmic phase transitions: their significance and experimental signatures. [39]
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