To date it has not been possible to detect these primordial black holes .
This is a subject of intense study at present, since it is assumed that these ultra-compact objects could be part of the much sought after dark matter of the universe.
One of the most surprising predictions of Einstein’s general theory of relativity is the existence of black holes: regions of space with such a strong gravitational pull that not even light can escape them.
When a massive enough star uses up all its fuel and its core collapses in on itself, the end result is a stellar-born black hole . Its mass ranges from 3 to 100 times that of our Sun.
On the other hand we have supermassive black holes, monsters with masses of millions of suns. It is worth noting the observations of two of these giants: one in the center of the galaxy M87 and the most recent one in the center of our Milky Way ( Sagittarius A* ).
The following animation shows a comparison in size of both supermassive black holes:
However, there is another type of black hole whose origin is not stellar: it is about primordial or primitive black holes .
Formed in the early moments of the universe by the collapse of extremely dense regions, these black holes can have masses much less than that of our Sun.
As an example, a primordial black hole with a mass equivalent to Mount Everest would be about the size of an atom.
These tiny black holes lose mass at a much faster rate than their more massive companions, emitting what is called Hawking radiation until they evaporate completely.
To date it has not been possible to detect these primordial black holes. This is a subject of intense study at present, since it is assumed that these ultra-compact objects could be part of the much sought after dark matter of the universe .
An alternative scenario for the detection of such atomic-sized black holes is proposed in a recent study .
In this investigation, the characteristic signal of the interaction between one of these black holes and one of the densest objects in the universe: a neutron star is studied.
But before going into this new astrophysical model in detail, let’s explain what a neutron star is .
Neutron stars: one of the densest objects in the universe
When a massive star runs out of fuel, its core collapses, giving rise to a stellar black hole. However, the end may be different (depending on the initial mass of our dying star) and give rise to a neutron star.
They are very small and extremely dense stars. To give us an idea, its mass is of the order of 1.5 times that of our Sun, compressed into a sphere of only 20 kilometers in diameter (the size of the island of Manhattan).
Its density is so high that a tablespoon of a neutron star would contain a mass of millions of tons.
The youngest neutron stars belong to a subclass called pulsars . These objects spin at extremely high speeds (even faster than a kitchen blender) emitting radiation in very narrow beams that periodically reach Earth.
Over time, these pulsars cool down and lose their rotation speed, making their detection difficult (only the most energetic pulsars have been observed).
What happens when an atomic-sized black hole and a neutron star meet?
Primordial black holes could be located in galactic regions where the concentration of dark matter is remarkably high. Thus, they could roam the universe (moving at different speeds and directions) and interact with other astronomical objects (such as massive black holes or neutron stars).
In this sense, a primordial black hole of atomic size could meet an old neutron star (whose temperature is very low and has lost practically all its rotation speed). According to recent research , the frequency of these encounters would be of the order of 20 events per year, although most would be difficult to observe (due to their enormous distance and proper orientation with respect to Earth).
Two possible scenarios are considered: first, when the primordial black hole is captured by the neutron star. Second, when the atomic-sized black hole approaches from very far away, it circles the neutron star and moves away again (the so-called scattering scenario).
Depending on the type of event (ie capture or scattering) a characteristic and unique signal would be generated, serving as a means of identifying such interactions (and indirect evidence for the existence of such tiny black holes).
The following animation describes in detail the interaction between these two astronomical objects:
The aforementioned signal consists of a burst of highly energetic radiation of limited duration: the so-called gamma- ray bursts (or GRBs). They are possibly the most energetic events that occur in the universe.
Some very specific gamma ray bursts
These sudden explosions have durations from milliseconds to several hours. They occur at very far distances from Earth and emit very energetic radiation in the form of very narrow beams.
The shorter bursts are possibly due to the merger of neutron stars or black holes, while the longer bursts are caused by the death of massive stars (so-called supernovae ).
In the case at hand, the gamma-ray burst would have a duration of about 35 seconds and would meet a very specific condition: an emission that increases smoothly and progressively, followed by an abrupt and rapid decrease in just a few hundredths of a second.
Will we be able to detect black holes of this type?
It is a difficult question to answer, given the complexity of finding these tiny black holes.
However, if modern gamma-ray observatories (such as the Fermi-LAT space telescope ) are capable of detecting gamma-ray bursts with the characteristics mentioned in said research, we could say that we have observed the interaction between a black hole atomic-sized primordial and a neutron star (occurred in the early universe).
This would imply indirect evidence of these primordial black holes predicted by Stephen Hawking . This is not an easy task (such gamma-ray bursts may never be found), but we cannot completely rule out such a possibility: only time will tell us.