4. Cosmology and Supermassive Black Holes

In the previous article, I briefly discussed different types of black holes, particularly how the mere existence of one of them is so confounding that we still haven't found an answer to explain it. Several phenomena in astrophysics can be synonymous with one word, and supermassive black holes are no exception: they are one of space's ultimate mysteries.

Cosmology is essentially the study of the universe as a whole, including its history, the cosmic microwave background radiation, the development of current-day phenomena, and the unraveling of the universe's future. Humanity's entire existence compared to the lifespan of the universe is analogous to 7 minutes and 36 seconds compared to an entire year, and therefore, we observe only the remnants of what was once pure insanity and chaos. In the past 50 years, we have tediously unraveled these remnants like a frog trying to understand a car engine.

The Primordial Universe

The universe we see today was not the same at the time of the Big Bang. Matter was distributed differently, conditions were ripe for absurd phenomena, and the temperature was close to Planck temperature, or about 10^32 Kelvin. Electrons, quarks, and neutrinos were the first fundamental particles to come into being, followed closely by protons and neutrons, and they reigned for about 380,000 years. It took that long for cosmic temperatures to cool down so protons, neutrons, and electrons formed hydrogen and deuterium nuclei. Once the strong nuclear force pulled protons and neutrons together, electrons began orbiting them, and thus the first atoms formed.

Such developments continued as time passed: at year 100 million (referring to 100 million years after the Big Bang), the first stars formed, and by the year 1 billion, galaxy formation was in full swing. Somewhere in between those two timeframes, the first supermassive black holes formed. The oldest black hole we have discovered formed around the year 400 million. Nearly every non-irregular galaxy we've located has a supermassive black hole at its center. Do you see the conundrum here?

Among the first galaxies that formed, there must have been at least one non-irregular galaxy, probably with a supermassive black hole at its center. How is it that in just 600 million years, stellar-mass black holes jumped from roaming around with 30 solar masses to being at the center of a galaxy with a couple million solar masses? Yes, 600 million years is a long time, but keep in mind that's only 4.35% of the universe's current lifespan.

Theories of Supermassive Black Hole (SMBH) Formation

There are numerous theories as to how they could have formed. Remember that, according to the first law of thermodynamics, energy cannot be created or destroyed, which means that since the Big Bang, the amount of energy in the universe has remained constant. Taking into account that the universe was much smaller in its first 100 million years than now, we can say that the universe had a much higher energy density. Since mass and energy are interchangeable (and were especially so in the early eras of the universe because temperatures were ridiculously high back then), there was much more mass available for black holes than there is now. In simple terms, the universe was super packed with matter and energy, which allowed black holes to have a 5-course meal every second and devour everything around it. This is one of the theories of SMBH formation: growth fueled by high energy density.

There's another theory that's my favorite, and it's called the direct collapse theory. Advancements on the theory have been made, especially in recent years. On January 26, 2024, Live Science (a science news site) posted an article (Live Science: Scientists may finally know where the biggest, oldest black holes in the universe came from) about a new groundbreaking theory regarding... well, the formation of SMBHs. A team of astronomers, collectively from UCLA and the University of Tokyo, thought about hydrogen clouds, primordial black holes, and Hawking radiation, and what would occur with all 3 together. Here is what they came up with:

In the early universe, hydrogen existed as ions, atoms, or diatomic molecules; and as I mentioned earlier, the early universe's ludicrous energy density made it easy for even a relatively small amount of mass to collapse down to its Schwarzschild radius and become a black hole. Because of how tiny these quantum black holes were, their Hawking radiation tore them apart rather quickly, and they exploded. The overall philosophy is that the high energy density accelerated collapsing gas clouds past the stage of forming a massive star, leading it straight into forming a black hole. These medium black holes would then rapidly merge and form a supermassive black hole. The problem is that hydrogen loves to form diatomic molecules, which is an exothermic reaction: it releases energy. That dissipation takes away the energy needed for a gas cloud to even condense and collapse into a star, so black holes were out of the question. The catch here is that the team of astronomers proposed that those exploding minuscule primordial black holes released enough energy to keep hydrogen atoms from forming diatomic molecules. That way, the hydrogen atoms in the gas cloud would retain enough energy to keep collapsing and form a black hole. The resultant black holes would then rapidly merge, fueled by the high concentration of mass and energy around them, and thus, supermassive black holes would emerge.

I'll gently remind you that the theory I just presented to you is a theory, as in a hypothesis or a supposition, and that it has not been proven to be true. The article itself states that the team of astronomers had not yet presented their findings for peer review and they would run simulations to test their theory. I will make it clear that I am not trying to spread misinformation by saying that the theory in the article is correct because as of now that is not the case. I found their research paper after it got peer-reviewed, and you are more than welcome to take a look: https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.133.091001

Conclusion

I'll be honest and admit cosmology is a bit of a step away from my current realm of knowledge, but I'm definitely enthusiastic when it comes to learning more about it. That's not to say I'm completely clueless about it; it's just that my knowledge concerning cosmology is disproportionately low compared to my knowledge of other topics in astrophysics. SMBHs tie in with galaxy evolution, a topic so advanced that many astrophysics PhD dissertations have been written based on just that topic alone. For that reason, I won't write an article dedicated to galaxies and their evolution because of how surface-level and limited my knowledge on that topic is.

The next article will cover an introduction to both astrodynamics (a fancy word for orbital mechanics) and observational astrophysical measurements.

As always, thank you to those who read my articles.

- Nikhil