Galaxybuilding

Today we’ll discuss worldbuilding universes and galaxies, giving you the phenomenal cosmic power to worldbuild your own itty bitty living space.

Hey everyone, my name is Matthew, at least it is when the stars align just right, and this post is part of a series where I will be going through a science-adjacent worldbuilding process step-by-step. Last time, we established the fundamental rules of the universe we’re going to be worldbuilding, looking at the laws physics and chemistry, how they work and how to work with them.

For today’s discussion, we’re going to be launching the universe into existence, starting with the big bang, establishing how galaxies have come to form, and making a unique galaxy of our own.

To begin with… behold, the big bang. The first moment among many for our fictional universe. But before we even move from this first picosecond of cosmic time, it may be worth discussing HOW the big bang may have occurred, and whether you’d like to establish it definitively for your worldbuilding process. A very popular scientific theory for why our real-world big bang occurred is called Cosmic Inflation. The premise of cosmic inflation is that gravity is not always attractive, but can sometimes be repulsive, at least according to Einstein’s theory of general relativity. This is especially true at extremely high levels of energy. The theory goes that the entire universe was contained within a single speck, and that the energy of the universe was so condensed, so concentrated, that it allowed for the extremely powerful repulsive force of gravity to inflate the universe outwards.

Or alternatively, perhaps your universe is part of a chain of many other universes, each creating new universes within themselves when certain conditions are met. Or maybe your universe was created by a sort of deity figure, operating from another realm separate from time and space. Of course, all these theories work on the basis that the universe was not created out of nothing, that it in fact started as something, which is reasonably well established within our real-world cosmological understanding. However, for a fictional universe, it doesn’t necessarily need to start from something, and can be made from nothing. If how the universe came to be isn’t particularly relevant to your worldbuilding, you can even leave the question unanswered.

For the universe I’ll be worldbuilding as we go along this series, I’m going to use the cosmic inflation theory of the big bang, but instead of the universe starting as a single speck, it’s going to start as two overlapping specks, representing the universe we’ll be working with, as well as its secondary adjacent universe. This secondary universe therefore is both pushed away and pulled towards the primary universe using a force similar to gravity. We know with reasonable certainty that our own universe is flat, and the universe we’ll worldbuild will be too. Therefore, the two universes we’ll be working with will lay flat up against each other, like two expanding disks.

To begin with, just like our real-life universe, this universe is going to be hot soup of atomic particles and wouldn’t have any light yet. In the first billion years, very basic elements such as hydrogen and helium would form and slowly accumulate into the first stellar objects like stars. Just like in real-life, particularly massive stars have very short life spans, and some would collapse to form the first black holes of the universe. A black hole is a region of spacetime where the gravity present is so strong that nothing, not even particles or energy, including light, can escape from it.

While black holes themselves are fascinating, it is their supermassive variants that are particularly crucial for galaxybuilding. A Supermassive black hole is a black hole that has a mass above 0.1 million solar masses, or 100,000 times as massive as our sun. However, the mass of supermassive black holes can be mind boggling larger than that, reaching billions of solar masses. There is even a new label given by some astronomers that is the ultramassive black hole, which refers to a black hole of at least 10 billion solar masses, and there is suggestion that the maximum mass a black hole can reach is somewhere in the order of around 50 billion solar masses.

Supermassive black holes that form in the early stages of the universe are likely to begin to form galaxies, slowly accumulating the matter from around the early universe. This has led to the umbrella term for all these types of stellar bodies, the Active Galactic Nucleus (AGN). If you’re worldbuilding a galaxy, it’s going to have an Active Galactic Nucleus.

In our fictional universe we also know of the existence of white holes; stellar bodies that are the opposite of black holes, releasing energy and matter rather than absorbing it. Because the energy and matter these white holes release comes accumulated from the secondary universe, they’re likely to form a little later in the universe’s chronology, somewhere between the first and second billion-year mark. It’s at this point that we can start to worldbuild the galaxy that our planet is going to reside in.

That’s right, our galaxy is going to have a supermassive white hole as its Active Galactic Nucleus. Specifically, let’s say the supermassive white hole at the galactic core formed 1.6 billion years after the big bang, and that it’s going to be spewing hydrogen into the surrounding space, creating an extremely dense cluster of stars in the centre of the galaxy that would result in frequent, catastrophic supernovas. These hyper-massive stars, continuously fed by the supermassive white hole, are likely to eventually form a belt of black holes surrounding the white hole. Given that for most of the galaxy’s history matter has been spewed outwards rather than being sucked inwards, the galaxy is likely to be elliptical in shape.

Elliptical galaxies are, unsurprisingly, approximately ellipsoidal in shape, usually with a smooth featureless façade. They are characterised by very little interstellar matter, which results in less star formation occurring than in other types of galaxies, meaning that most stars are old stars. Movement in an elliptical galaxy is radial, and stars tend to move outwards or inwards in relation to the Active Galactic Nucleus.

However, our supermassive white hole as the Active Galactic Nucleus changes many of those stereotypical traits, and while it’s movement would still be radial, it would be much more densely packed, with gases and dusts, making it much more visually similar to a spiral galaxy.

Spiral galaxies, such as our own milky way galaxy, appear as a rotating disk that is usually flat, consisting of gas, dust, and most importantly stars that cluster heavily towards the centre, which is called the bulge. These central stars are often then surrounded by other clusters of stars, which orbit in a rotational movement around the Active Galactic Nucleus, giving the galaxy its distinctive spiral shape.

Given the nature of the supermassive white hole, our radial galaxy will also have a central cluster of stars like a spiral galaxy, with gases and dusts radiating outwards from the Active Galactic Nucleus.

For completion’s sake, the final of galactic morphologies are Irregular galaxies, which are often chaotic in appearance, lacking a bulge or a spiral structure. An elliptical or spiral galaxy that comes into contact with a massive external gravitational force, like another supermassive black hole for example, can lose its original structure, becoming an irregular galaxy. Most commonly, irregular galaxies are far smaller than elliptical or spiral galaxies.

So, now that we’ve determined our galaxy’s Active Galactic Nucleus and its morphology, we need to determine its habitable zone. The Galactic Habitable Zone is the region within a galaxy that is most likely to be suitable for life to develop, assuming all other micro factors required for life are met. The most important two factors for determining the Galactic Habitable Zone are metallicity, and the presence of supernovae.

Metallicity is the abundance of all elements that are heavier than hydrogen and helium. Even though chemists have a very different definition of what a ‘metal’ is, astronomers simply use the word ‘metal’ to refer to any element heavier than helium. Metallicity increases as new stars are born, and stars born from the remnants of a star, that were also born from the remnants of another star, and so on, will have much higher metallicity than stars that were born in the early stages of the universe.

However, for this process to happen, stars have to go supernova. A supernova is a stellar explosion, occurring during the last stage of life of massive stars. Supernovae release astronomical amounts of energy, which as we mentioned earlier is involved in the process of creating elements heavier than hydrogen and helium.

While this sounds great, this process releases a colossal amount of radiation, and any life caught in the process would be utterly obliterated. So, life can only form in areas where supernovae have occurred, but not in areas where supernovae are really common. Whilst these constraints on paper sound like there would only be tiny pockets of this Galactic Habitable Zone, in reality, an estimated 50% of the Milky Way is actually within the Galactic Habitable Zone. For worldbuilders, the short answer for where this zone will be is not too close to the Active Galactic Nucleus where supernovae are common, but not so far out that not enough supernovae would have occurred. When galaxies are young, the habitable zone is smaller, but it will expand as galaxies age.

So, to recap, our universe is going to be born from cosmic inflation. We’ve defined what an Active Galactic Nucleus is and chosen a supermassive white hole to be the centre of the galaxy we’re worldbuilding together, which we have established is an elliptical galaxy. Finally, we looked at the Galactic Habitable Zone, making sure that any life we worldbuild isn’t going to be set up to fail before it even starts.

Join me next time where we’ll make the actual planet that we’ll be using for all of our worldbuilding moving forward. And until next time… stay awesome!