Primordial black hole

In 1971, Stephen Hawking introduced the idea that black holes may exist that are smaller than stellar mass, possibly formed during the high-density phase of the big bang. A primordial black hole is a hypothetical type of black hole that is formed in the early universe due to the gravitational collapse of important density fluctuations. Several mechanisms have been proposed to produce the inhomogeneities at the origin of primordial black hole formation, like in the context of cosmic inflation, reheating, or a phase transition.

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 with a mass lower than 10¹¹ kg would have evaporated (due to Hawking radiation) in a time much shorter than the age of the Universe, and so cannot have survived in the present Universe. A noticeable exception is the case of Planck relics that could eventually be stable. The abundance of primordial black holes could be as important as the one of dark matter, to which they are a plausible candidate. 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 (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). Nevertheless, tight limits on their abundances have been set up from various astrophysical and cosmological observations, so that it is now excluded that they contribute importantly to the dark matter over most of the plausible mass range.

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. Two of them 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 the interest for primordial black holes with masses in the range of 1 to 100 solar masses. It is however still unclear and debated whether this range is excluded or not by other observations, such as the absence of micro-lensing of stars, the cosmic microwave background anisotropies, the size of faint dwarf galaxies, and the absence of correlation between X-ray and radio sources towards the galactic center.

...
Wikipedia