Other meanings of black hole
Astrophysics
A primordial black hole is a hypothetical type of black hole that formed not from the collapse of a star, but from the gravitational collapse of overdense regions in the extremely early universe, within the first second after the Big Bang.1 Unlike stellar black holes, which have masses at least a few times that of the Sun, primordial black holes could have masses ranging from as small as a fraction of a gram (if they formed at very early times) to many thousands of solar masses.2 They are a leading candidate for dark matter and have profound implications for cosmology, gravitational wave astronomy, and particle physics.
Primordial black holes are thought to have formed from the collapse of density fluctuations in the early universe, which were generated during cosmic inflation.1 The probability of collapse depends on the amplitude and spectral index of the fluctuations; a critical threshold must be exceeded for a region to collapse into a black hole.3 Other mechanisms include the collapse of cosmic strings, domain walls, or other topological defects, as well as the direct collapse of scalar field perturbations during phase transitions.4
The mass of a primordial black hole at formation is roughly equal to the mass within the Hubble horizon at that time, which scales with the cosmic time: a black hole forming at time t after the Big Bang has a mass of about the Planck mass times the square root of the time in Planck units.2 This means that black holes forming at 10-43 seconds would have masses around 10-5 grams, while those forming at 1 second could be as massive as 105 solar masses.2
In 1974, Stephen Hawking showed that black holes emit thermal radiation due to quantum effects near the event horizon, causing them to lose mass and eventually evaporate.5 The evaporation time is proportional to the cube of the mass: a black hole of mass 1015 grams would evaporate in about the age of the universe (13.8 billion years).5 Thus, primordial black holes with masses below about 1015 grams would have already evaporated, potentially leaving observable signatures such as gamma-ray bursts or relics.6
Observations of the extragalactic gamma-ray background place strong constraints on the abundance of primordial black holes with masses around 1015 grams, as their evaporation would produce a characteristic spectrum of high-energy photons.6 The non-detection of such radiation implies that these black holes cannot constitute a significant fraction of dark matter.6
Primordial black holes are a compelling dark matter candidate because they are non-baryonic, stable (if massive enough), and could have formed with the right abundance to explain the observed dark matter density.7 The mass range for dark matter primordial black holes is tightly constrained by various observations: microlensing surveys (such as MACHO and EROS) rule out masses from 10-7 to 10 solar masses, while dynamical constraints from wide binaries and the cosmic microwave background limit other ranges.7 The remaining allowed window includes asteroid-mass black holes (1017 to 1023 grams) and possibly some higher-mass windows.7
Recent studies suggest that primordial black holes could explain the unexpectedly high merger rates observed by LIGO/Virgo, as they could form binaries in the early universe and merge today.8 However, the exact fraction of dark matter they constitute remains uncertain, with some analyses allowing up to 100% in certain mass windows.7
Primordial black holes have not yet been directly detected, but numerous observational probes constrain their existence. These include gravitational lensing (both microlensing and strong lensing), gravitational waves from mergers, the cosmic microwave background (via accretion effects), and the absence of Hawking radiation.67
The Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo have detected gravitational waves from merging black holes with masses of tens of solar masses, which could be primordial in origin.8 Future observatories like the Laser Interferometer Space Antenna (LISA) will be sensitive to lighter black holes, potentially detecting primordial black holes in the mass range 10-1 to 103 solar masses.8
Another promising avenue is the observation of neutron stars being captured by primordial black holes, which would cause the neutron star to be accreted and eventually collapse, producing a gamma-ray burst.7 The absence of such events places limits on the abundance of primordial black holes in certain mass ranges.7
One lesser-known aspect is the role of primordial black holes in the early universe's reionization: if they exist, they could have emitted X-rays that ionized the intergalactic medium, affecting the cosmic microwave background anisotropies.9
Another intriguing possibility is that primordial black holes could be the seeds for supermassive black holes at the centers of galaxies, which are observed to exist just a few hundred million years after the Big Bang.10 These seeds would need to be massive enough to grow rapidly, and primordial black holes of ~105 solar masses could serve this role.
Additionally, primordial black holes could have formed with a wide range of masses, including Planck-mass relics that might be stable and constitute a component of dark matter, though this is speculative.2
Finally, the concept of primordial black holes has been used to probe physics beyond the Standard Model, such as the existence of extra dimensions or phase transitions in the early universe, which could enhance their formation.4
Primordial black holes remain hypothetical, but they are a rich area of theoretical and observational research, with implications for fundamental physics and cosmology.
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