Written by: Sonic Smrit Shrestha - 25061, Grade XII
Posted on: 11 July, 2024
Introduction
A black hole is where nothing is certain and where our understanding of physics starts to break down. A black hole is formed when a star implodes in a supernova explosion which is caused due to the influence of gravity. This is the definition of a normal black hole. But primordial black holes are quite different. A primordial black hole is formed with the Big Bang itself. A primordial black hole is smaller than other black holes and they are not formed by the same method from which a normal black hole is formed.
The primordial black hole could have a mass of as small as 10 to the power minus five grams. Ten to the power minus five grams is the planck mass. The mass of a primordial black hole is similar to the mass of a neuron which is 10^-6. But primordial black holes can have a range of masses, from a golf ball to the mass of a million times of our sun.
Understanding a primordial black hole
Primordial black holes were formed long before any planets or stars were formed in the universe. The primordial black hole existed since the start of the universe and it was first created after the Big Bang. Theorists say that primordial black holes popped into existence because, after the creation of the universe, space was not completely the same at every point. Some areas became denser and hotter in comparison to other areas and had the chance to get collapsed into black holes.
There is only a small frame of time after the Big Bang that primordial black holes could be created. Primordial black holes could have masses as low as 10 power minus seven ounces, or 100,000 times less than a paperclip, up to about 100,000 times greater than the mass of the Sun. The idea of such tiny black holes intrigued Stephen Hawking, who explored their Stephen Hawking got curious about the tiny black holes which led him explore their quantum mechanical properties. This work led to his discovery of Hawking radiation in 1974.
Formation of a primordial black hole
A primary black hole is a black hole that formed very early in the universe. A primordial black hole is not generally formed like the formation of a normal black hole. It is important to note that a star is not necessary for the formation of a black hole. Instead, what you need is a region with a high density of material and a large fluctuation that occurs over a region of space. This fluctuation must increase the amount of matter in the region to the point where it falls within the Schwarzschild radius, allowing for the formation of an event horizon and the formation of a black hole. The fluctuation may have originated from a collapsing star or theoretically the Big Bang.
The primordial black holes could have formed right after the black hole which is known as the planck era. And it began to take shape after that and was connected to various time periods. Scientists say that the very first primordial black hole was formed in a fraction of a second after the formation of the Big Bang. A primordial black hole could have formed with the quartz and other subatomic particles after the formation of the universe. But the primordial black holes don’t last long because primordial black holes are small in size and according to Hawking radiation, the mass of the black hole is inversely proportional to the evaporation of the black hole. So a smaller black hole would last for a shorter period of time than a larger black hole. A primordial black hole is smaller than a normal black hole so it would last for a shorter period of time. Theorists believe that primordial black holes have all evaporated since their formation.
Existence of primordial black holes
Even after 13.8 billion years, Scientists haven’t found definite proof of primordial black holes in our universe. According to Hawking radiation, black holes must emit gamma rays as they evaporate. The primordial black holes should also evaporate and produce gamma rays. These rays can be observed by us through the use of Gamma-ray telescopes. The fact that we don’t see any of these gamma rays places concerns on the number of primordial black holes in existence or if there were any primordial black holes in the first place in our universe. And that puts constraints on its contribution to the overall mass of the universe.
But primordial black holes neatly fill some major gaps in the current understanding of the universe. These black holes could explain the existence and nature of dark matter, the invisible stuff that, according to estimates, makes up some 85% of all matter in the universe. Due to these reasons, the primordial black hole lies in the gray area between physics and fiction. The idea of primordial black holes can explain different gaps in our understanding of the universe but due to their existence not being detected, they cannot be proved as reality, and physics works based on theoretical proof. So black holes cannot be proven to be real only on the basis of theoretical proofs.
Properties of primordial black holes
The characteristics of a primordial black hole are both characteristic and enigmatic. Primordial black holes can have masses ranging from microscopic to supermassive, in contrast to their stellar counterparts, which have a minimum mass requirement for formation. Due to this, primordial black holes have unique properties compared to normal black holes. The presence of primordial black holes is not linked to any visible astronomical object. Due to this reason, primordial black holes are ecliptic and challenging to understand and detect because of their broad mass spectra.
Primordial black holes are unique in part because they may be dark matter candidates. One of the biggest mysteries in contemporary astrophysics is dark matter, an elusive and invisible substance that makes up a larger portion of the universe than what is visible. Primordial black holes present a convincing theoretical candidate that fits with observational constraints, even though the nature of dark matter is still unknown.
Primordial black holes as dark matter candidates
Dark matter is a hypothetical form of matter that is invisible and doesn't interact with ordinary matter. It's made up of particles that don't absorb, reflect, or emit light. Due to this, they cannot be detected through the use of electromagnetic radiation. So the theory is that dark matter is made up of primordial black holes. Those black holes that have a mass bigger than 10 to the power of 15 grams can act as dark matter candidates. Black holes that range from 10 to 20 solar masses can also be dark matter candidates according to our understanding. This is because they are very massive objects stuck in the center of galaxies and they evaporate according to Hawking's theory.
But we don’t know how much a black hole evaporates or how much black holes can evaporate. So the theory is that black holes evaporate till a molecular level and then they stop evaporating at a certain point and these black holes become dark matter. The primordial black holes evaporate faster than normal black holes because of their mass so they would turn into dark matter quickly after the formation of the universe.
