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Traditional black holes observed by astronomers fall into one of three categories: stellar-mass black holes, intermediate-mass black holes, and supermassive black holes. Each type of black hole is more massive than our own Sun and forms at least hundreds of thousands of years after the big bang, typically following the death of massive stars. However, there is a type of black hole that astronomers have not yet seen but theorise could exist. These are known as primordial black holes (PBHs).

PBHs are theorised to have been born very early in the life of the universe, just a fraction of a second after the big bang itself. During this brief moment, space was not completely homogenous (identical at all points). Some areas were denser hotter than others, and these denser regions could have collapsed into PBHs. The window for PBH formation was extremely short—roughly one second following the big bang—but in the context of the universe's first moments, a lot could happen in that brief time.

The mass of a PBH depends heavily on when exactly it formed, with estimates ranging from as small as 10^-8kg to more than thousands of solar masses. Stephen Hawking, fascinated by the concept of PBHs, calculated that any PBH with a mass greater than 10^12kg could still exist today, while the smaller ones would have evaporated by now via Hawking radiation. Detecting PBHs is incredibly challenging because they do not necessarily interact with ordinary matter in the same way as normal black holes do. However, they may be detectible through gravitational lensing, their Hawking radiation (for very small PBHs), or their potential contribution to dark matter.

Although it constitutes ~30% of our universe, astrophysicists still do not fully understand the nature of dark matter. PBHs could offer some answers to this mystery. It is theorised that PBHs could be a type of dark matter called MACHOs—massive compact halo objects. These PBHs are thought to reside in the halos (the outer regions) of galaxies. However, as previously mentioned, they are really hard to detect. In addition, recent(ish) studies have determined that even if PBHs of this type exist, they are unlikely to account for all the observed effects attributed to dark matter.

In this model, the universe would be full of black holes. Stars could form around these dense clumps of matter, eventually leading to the creation of solar systems and galaxies over billions of years. If the first stars formed around PBHs, these stars might have appeared earlier in the universe's history than predicted by the standard model. PBHs could thus serve as the seeds from which all black holes form, including the supermassive black hole at the centre of our own Milky Way galaxy.

published: 24/01/25 by kaan evcimen