Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes.
Yakov Borisovich Zel’dovich and Igor Dmitriyevich Novikov in 1966 first proposed the existence of such black holes. The theory behind their origins was first studied in depth by Stephen Hawking in 1971.
Depending on the model, primordial black holes could have initial masses ranging from 10−8 kg (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.
Primordial black holes are non-baryonic and as such are plausible dark matter candidates.
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.
Primordial black holes belong to the class of massive compact halo objects. 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).
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. 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.
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”.
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.
Problems for which primordial black holes have suggested as a solution include the beside dark matter problem also the cosmological domain wall problem and the cosmological monopole problem. Since primordial black holes do not necessarily have to be small (they can have any size), they may 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.