The production of black holes in the early universe could explain the nature of dark matter

An international team of cosmologists led by the Department of Theoretical Physics of the Autonomous University of Madrid (UAM) and the Institute of Theoretical Physics UAM/CSIC has proposed, in an article published in the Journal of Cosmology and Astroparticle Physics , that holes Primordial blacks could have been produced by an increase in the temperature of the universe during its first moments of life.

According to the authors, that same increase in temperature would have caused the formation of gravitational waves (small deformations of space-time that propagate in a similar way to waves on the surface of a lake) that we could detect today with a system of detector satellites with the proper sensitivity.

“Since primordial black holes and gravitational waves have their origin in the same physical process, the detection of the latter would constitute a valuable source of information. For example, the frequency of gravitational waves would make it possible to determine the mass of primordial black holes”, the authors highlight.

In this way, the work jointly offers a new hypothesis to explain the nature of dark matter.

Dark matter

One of the main open problems in cosmology is the nature of ‘dark matter’, which is the most abundant type of matter in the universe (approximately 85% of the total matter). Its existence is inferred from astrophysical and cosmological observations, such as the rotational motion of spiral galaxies.

Unlike ordinary matter (the stuff we are made of and experience in our everyday lives), dark matter is not directly observed. The word “dark” refers to this physical property. And logically, this makes the study of its composition and origin extremely difficult.

The most popular hypothesis assumes that dark matter consists of particles that interact very weakly (particularly with electromagnetic radiation). However, the persistent lack of detections of such particles (despite enormous experimental efforts devoted to it) leads physicists to consider other possibilities as well.

Primordial black holes

One possibility is primordial black holes. Like any black hole, primordial black holes are extremely dense regions of spacetime, to the point that not even light can escape their gravitational pull.

The name “primordial” refers to the fact that, unlike usual black holes, its origin is not the collapse of stars in the late universe, but rather high concentrations of matter and energy from the early universe.

These areas of high density could have formed from quantum fluctuations generated in a phase of the universe known as “primordial inflation.” Cosmologists believe that during this phase the universe expanded exponentially rapidly, an idea that was proposed in the 1980s to explain, among other things, the great similarity between very distant regions of the universe.

In addition, primordial inflation is also capable of accounting for the seeds of density that gave rise to the structures present in the universe, such as galaxies. Similarly, primordial seeds of sufficient density could explain the formation of black holes.

Transient rise in temperature

Most cosmological inflation models assume that the temperature of the universe during primordial inflation was extremely low. The work carried out at the UAM studies the effects of a transient increase in temperature during this phase.

“This increase in temperature would be related to the formation of a population of black holes. As a consequence of the sudden increase in temperature during inflation, the number of primordial black holes increases considerably, being able to account for the amount of dark matter present in the universe”, the authors detail.

“To account for the dark matter in the universe,” they add, “the mass of each primordial black hole would have to be about a million times less than that of Earth. Due to their high density, these black holes would have a size comparable to that of an atom, which corresponds to gravitational waves of about 0.01 Hz in frequency.”

“This type of gravitational waves —they continue— is beyond the precision of currently existing experiments, but they could be observed with future detectors in the next decade. In particular, the international LISA collaboration, already underway, has the right characteristics to achieve this.”

“The detection of these types of gravitational waves could represent not only a sensational milestone in the physics of gravitation, but also a fundamental advance in solving the dark matter problem. In addition, it would mean a great progress, perhaps radical, in our way of understanding the physics of the early universe”, the authors conclude.

The Autonomous University of Madrid, commonly known as la Autónoma, is a Spanish public university located in MadridSpain. The university was founded in 1968 by royal decree. UAM is widely respected as one of the most prestigious universities in Europe. According to the QS World University Rankings 2022, UAM is ranked as the top university in Spain and has consistently ranked as #1 in Spain in the El Pais University rankings, published annually.

Among its notable alumni, which include every president that the Supreme Court of Spain and Constitutional Court of Spain has had, is the current King of Spain, Felipe VI, who studied the Licenciatura en Derecho (Law) and is the president of UAM’s alumni society.

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