The Giants before Giants

 Primordial Black Holes

Hello friends, it’s been a long while since I wrote a blog. My apologies for such a long break. But don’t worry, today’s topic of discussion is the most asked about. Let’s talk about primordial black holes (PBHs). But before I begin, I’d suggest you read a few of my previous articles and brush yourself up. Do revise particle physics, gravitational lensing, the big bang theory, dark matter, and Hawking radiation. I’d provide the links as the words come by. Without further adieu, let’s start.

Primordial black holes are said to be the black holes that formed immediately after the big bang. The time of their birth and their nature could give us a pretty good idea about the nature of the early universe. If these theoretical bodies exist, they would be the first celestial body formed. Although they have not been detected and exist in some theories only, they are an excellent candidate for dark matter.

The Big Bang and the creation of Primordial Black Holes

For the formation of PBHs, there are three requirements and these requirements are the hindrances too to their existence. The formation of black holes requires extremely high density which is not a problem at the beginning of the universe. All mass that is known and unknown, was in a space lesser than the diameter of an atom. Well, the mass part supports our theory but we need a density differential too. Without direction, there would be no point for the mass to face or the direction in which to get attracted to. This lack of a density differential combined with cosmic inflation would prohibit its growth. Now, the homogeneous mass distribution does not have a direction to move to and its speed of expansion is much more than the speed it can conjure to get attracted to a nearby mass. All of these effects, seem to make the existence of a primordial black hole almost impossible. BUT, there is one thing we have yet not considered. The effects that come into place only when the sizes are this small, a.k.a, the quantum effects.

The universe in question is of the size of an atom or smaller than it. Quantum fluctuations rule the universe, creating a kind of static fuzz. This fuzz when subjected to cosmic inflation would have turned the littlest of distances to distances in light-years. Some of these fluctuations were intense enough to resist the local expansion of the universe and form a black hole.

Current observation of the oldest light, or the first light that appeared when the universe became suitable for its travel, the cosmic microwave background radiation shows tiny differences in density of matter from one point in space to the next. Working backwards, these spaces would have been much more nuanced.

Cosmic Microwave Background Radiation

PBHs formed in a radiation dominated era. Most of its mass must have come from non-baryonic mass energy. A thing worth noting is when we talk about baryonic mass, normally, we do not mean the baryonic particles we studied in particle physics. A justified cause for particle physicists' angst, but we use baryonic mass to talk about particles that formed in the Big Bang nucleosynthesis. This mass encompasses the masses of light elements that formed, like hydrogen, helium, and small amounts of lithium and beryllium. By observing the current ratios of these elements we can infer the total density of the baryonic matter, and we get 4% or so of the total density of the universe corresponding to regular matter.

The regular black holes have evolved from this baryonic matter as the universe grew, but this is just the case of matter redistribution and has no effect on its overall density. So, we call these regular black holes baryonic black holes. Since the PBHs evolved before Big Bang nucleosynthesis and after inflation, they are called non-baryonic black holes. This would account for 27% of the unaccounted mass in the Universe, making it an excellent candidate for dark matter. But, in the more real sense we know these black holes were formed from electrons, protons and neutrons and protons and neutrons are baryons. Thus, these black holes are not non-baryonic, they are just called that. And, not to forget they might have existed even before, the baryons were formed at all.

The current candidacy for black holes is not new. It has been around for a while and the mystery is close to being solved. Primordial black holes belong to the class of massive compact halo objects(MACHOs). They are nearly collision-less, stable if sufficiently massive, have non-relativistic velocities, and are formed very early in the history of the Universe.

Let’s explore PBHs candidacy for dark matter a bit more.

Various cosmological events allow for a very small range of masses for the existence of dark matter. PBHs that originally had masses in the range of 10²¹ Kg would have vanished till now in what we call Hawking Radiation. Hawking Radiation is a phenomenon we have extensively discussed before. In short, it is a process by which black holes lose mass and sooner or later cease to exist.

Hawking Radiation

A heavy massed black hole mass when passes in front of a large luminous body we observe what we call gravitational lensing. Depending upon the masses it could even be twinkling or micro-lensing. This phenomenon rules out a few more mass ranges.

Gravitational Lensing

Very heavy PBHs would affect the structure of star clusters, it would disrupt orbits of binary star systems or might fall into a neutron star causing it to explode. The lack of these events in sufficient numbers also rules out a lot of matter ranges.

After a lot of calculations, we have two possibilities. Lots of primordial black holes with masses similar to large asteroids or a small number of really big PBHs. Again, one more problem that comes up with big PBHs is feeding. With masses approximating twenty to a hundred times that of solar masses, it would have left its mark on the cosmic microwave background radiation. What do you think?

All right guys, I would conclude this article here. There are lots and lots of things I would have loved to discuss with you but the word limit doesn’t allow me to do so. Mention your questions in the comments and let’s keep discussing.

Auf Wiedersehen!

Image sources: Google Images