Primordial black holes

Primordial black holes

A primordial black hole is a hypothetical type of black hole that is formed not by the gravitational collapse of a large star but by the extreme density of matter present during the universe's early expansion.

According to the Big Bang Model, during the first few moments after the Big Bang, pressure and temperature were extremely high. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although most regions of high density would be quickly dispersed by the expansion of the universe, a primordial black hole would be stable, persisting to the present.

It has been proposed that primordial black holes, specifically those forming in the mass range of 1014 kg to 1023 kg,[1] could be a candidate for dark matter. This is due to the possibility that at this low mass they would behave as expected of other particle candidates for dark matter. Being within the typical mass range of asteroids, this excludes those black holes too small to persist until our era and those too large to explain gravitational lensing observations.

Possible detection

One way to detect primordial black holes 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 massive burst of radiation at the final phase, equivalent to a hydrogen bomb yielding millions of megatons of explosive force.[2] 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 some of those that are relatively nearby in our own Milky Way galaxy exploding today. NASA's Fermi Gamma-ray Space Telescope satellite, launched in June 2008, is designed in part to search for such evaporating primordial black holes. 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. It has been suggested[3][4] that a small black hole passing through the Earth would produce a detectable acoustic signal. Because of its tiny diameter, large mass compared to a nucleon, and relatively high speed, such primordial black holes would simply transit Earth virtually unimpeded with only a few impacts on nucleons, exiting the planet with no ill effects.

Another way to detect primordial black holes could be by watching for ripples on the surfaces of stars. If the black hole passed through a star, its density would cause observable vibrations.[5]


The evaporation of primordial black holes has been suggested as one possible explanation for gamma-ray bursts. This explanation is, however, considered unlikely. Other problems for which primordial black holes have been suggested as a solution include the dark matter problem, the cosmological domain wall problem[6] and the cosmological monopole problem.[7] Since a primordial black hole does not necessarily have to be small (they can have any size), primordial black holes may also have contributed to the later formation of galaxies.

Even if they do not solve these problems, the low number of primordial black holes (as of 2010, only two intermediate mass black holes were confirmed) aids cosmologists by putting constraints on the spectrum of density fluctuations in the early universe.

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".[8] This could mean there are several thousand black holes in our galaxy. To test this theory scientists will use the Fermi Gamma-ray Space Telescope which was put in 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.


  • 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., 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