Primordial holes formed under the exotic conditions of the Big Bang could have become their own source of matter and radiation. The standard history of the early universe goes like this. When our universe was incredibly young, it went through a period of incredibly rapid expansion known as inflation. Then inflation rose and filled the universe with particles and radiation in the hot Big Bang. The universe then expanded and cooled, while the density of matter and radiation fell. Matter eventually united conscious stars, galaxies, and clusters.

But new research published on the preprint server arXiv, show that this simple story may be missing an important ingredient: primordial black holes. Currently, we know of only one guaranteed way to create black holes. This is due to the death of massive stars. When they collapse on themselves at the end of their lives, they reach a density high enough to overcome any other force and trigger the formation of a black hole.

But the early universe may have been so exotic that it created black holes in its own way. When the swelling ceased and the universe began to cool, the process was not smooth and gentle. Instead, it was incredibly violent, with huge changes in energy and mass from place to place. It’s possible that pockets of the universe would spontaneously reach densities high enough to form black holes directly on their own, without having to go through star formation first. These are the so-called primitive black holes.

Cosmological observations have already placed severe constraints on the number of primordial black holes that may have populated the early universe. But there is still a place for their presence. And a research team has compiled a paper exploring an unexpected consequence of the formation of these primordial black holes.

We know from the work of Stephen Hawking that black holes are not completely black. They actually glow faintly due to an exotic quantum process known as Hawking radiation. For normal-sized black holes, this is a very inefficient process. An ordinary black hole emits only one radiation particle each year. But smaller black holes emit much more radiation.

If primordial black holes were small enough, they would have completely evaporated, leaving no trace of their existence, while the universe was still in its infancy. But the researchers found that this leads to an intriguing situation. As these primordial black holes evaporated, they released their own streams of radiation and matter.

Despite the expansion of the universe, if enough primordial black holes evaporate, the density of matter and radiation can remain constant. This would lead to an expanded black hole-based big bang scenario.

Eventually all primordial black holes will perish and the rest of cosmological history will continue without them. But they would leave their mark. Changes in matter and radiation density could potentially have long-term effects that we have been able to detect to date. And the evaporation of primordial black holes causes the formation of gravitational waves that can persist even today. We may not find direct evidence of the existence of primordial black holes, but researchers have discovered that we can find thin fingerprints all over the universe.