In recent years, we have been hearing a lot about massive black holes, even been observing them; supermassive black holes, stellar-mass black holes, or intermediate-mass black holes. However, although no other type has ever been observed, they are thought not the only type of black holes to populate our universe. In effect, a specific “species,” known as primordial black holes, which have been around since the birth of the universe, are theoretically said to exist.

Still, they would be invisible to conventional observation methods. Indeed, primordial black holes (or PBHs), as the term “primordial” suggests, would exist since the big bang and evolve in dimensions that are comparable to the size of an atom but can also range in sizes much greater than that of the Sun. The existence of such black holes was first speculated by Yakov Zel’dovich and Dmitriyevich Novikov in 1966. Their theory was later extended upon and extensively studied by Stephen Hawking in 1971.

The formation of primordial black holes

Discovering Quantum Black Holes Will Solve Several Mysteries of the Universe. But How To Detect Them? 1
Image: NASA/WMAP Science Team

About a second after the Big Bang occurred, the density of space evolved in a very heterogeneous manner, and this sole property of the universe could be the reason why primordial black holes ever popped into existence. The high density of some areas, which were much hotter than others, caused by quantum fluctuations which are the creation and annihilation of paired particles altering the density of space, may have caused the space in these regions to collapse on themselves, forming black holes.

In that period, i.e., the second after the Big Bang occurred, several primordial black holes would have formed. Although most of them are speculated to have evaporated shortly after their creation, if we could figure out how to detect the remaining ones, their proven existence would solve multiple problems in astronomy, notably that of the dark matter covered in the following sections.

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How could we detect a PBH?

Given the heavy and awe-inspiring implications of the potential existence of primordial black holes, scientists have attempted to develop several techniques which could allow us to detect primordial black holes. 

A first suggested detection technique would be the observation of the Cosmic Microwave Background (or CMB), a fingerprint of the early stages of our universe, which would indicate the zones of high X-ray emission caused by primordial black holes having fed off the higher concentration of matter figuring in those regions.

A different experimental technique to detect primordial black holes would be detecting gravitational waves in laboratories like LIGO (the Laser Interferometer Gravitational-Wave Observatory). Indeed, these gravitational waves indicate the disturbance of space-time which then travels to Earth. If detected within the determined threshold and with the calculated parameters, these disturbances may indicate the collapse of space in the early universe to form a primordial black hole. 

Discovering Quantum Black Holes Will Solve Several Mysteries of the Universe. But How To Detect Them? 2
An illustration of gravitational lensing | Image: Nasky/Shutterstock.com

Finally, gravitational lensing would be an indicator for primordial black holes. Gravitational lensing is a concept whereby a star or astronomical object is visible, although situated behind a cluster. However, due to that cluster’s large mass and density, the light of the obscured object distorts around the cluster to reach the observer. In a case where gravitational lensing is observed for an unknown reason or in amounts inexplicable by the density of regular matter, primordial black holes and their high density may explain such a phenomenon: an indicator that they figure in the said cluster. 

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Candidates for dark matter

A central problem in astrophysics is the detection of dark matter. Although it makes up 30% of our universe, we do not know what it is nor exactly where to look. Had it to be confirmed, we would explain multiple behaviors and phenomena we observe, and primordial black holes might be a direction in which to look! Indeed, primordial black holes may be a form of dark matter called MACHOs (Massive compact halo objects) which would be found in the halos of galaxies.

Discovering Quantum Black Holes Will Solve Several Mysteries of the Universe. But How To Detect Them? 3
MACHOs acting as a gravitational lens | Image: Nature.com

A way of spotting these MACHOs would be with the previously mentioned technique of gravitational lensing, whereby the object acting as the lens would here be the MACHO. Secondly, another method to detect these MACHOs would be in a primordial black hole merger which would be invisible but generate gravitational waves potentially to be detected by laboratories such as LIGO. Finally, although the existence of primordial black holes remains speculative, their place in the universe is relatively well defined, and should they, one day, be proven to exist, their make-up would help explain some of the most intricate problems of our universe.

Learn Astrophysics at Home:

Did you always want to learn how the universe works? Then, read our 30-article Basics of Astrophysics series absolutely free of cost. From the popular topics such as stars, galaxies, and black holes to the detailed concepts of the subject like the concept of magnitude, the Hertzsprung Russell diagram, redshift, etc., there is something for everyone in this series. All the articles are given here. Happy reading!

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