section_path: Formation and evolution/Gravitational collapse/Primordial black holes and the Big Bang
section_name: Primordial black holes and the Big Bang
paragraph_text: Primordial black holes and the Big Bang
  
  Gravitational collapse requires great density. In the current epoch of the universe these high densities are found only in stars, but in the early universe shortly after the Big Bang densities were much greater, possibly allowing for the creation of black holes. High density alone is not enough to allow black hole formation since a uniform mass distribution will not allow the mass to bunch up. In order for primordial black holes to have formed in such a dense medium, there must have been initial density perturbations that could then grow under their own gravity. Different models for the early universe vary widely in their predictions of the scale of these fluctuations. Various models predict the creation of primordial black holes ranging in size from a Planck mass (m P = ℏ c / G {\displaystyle m_{P}={\sqrt {\hbar c/G}}} ≈ 1.2 × 10 GeV/ c ≈ 2.2 × 10 kg) to hundreds of thousands of solar masses.
  
  Despite the early universe being extremely dense, it did not re-collapse into a black hole during the Big Bang, since the expansion rate was greater than the attraction. Following inflation theory there was a net repulsive gravitation in the beginning until the end of inflation. Since then the Hubble flow was slowed by the energy density of the universe.
  
  Models for the gravitational collapse of objects of relatively constant size, such as stars, do not necessarily apply in the same way to rapidly expanding space such as the Big Bang.