Cosmic Habitability Categories: Four Types of Universes

When we hear scientists talk about different types of universes, they are usually talking about the spatial geometry of the universe, and distinguish between a flat universe, an open universe and a closed universe. But there is another very interesting way to classify universes. We can classify universes based not on their geometry but on their habitability – how easy it is for life to develop in a universe. We can distinguish between universes that are abundantly habitable, moderately habitable, barely habitable and uninhabitable.

Before presenting such a classification, let's look at two types of things that can contribute to a universe's habitability. The first type of thing is what can be called habitability necessities – things that a universe must have in order for any life to exist in it. The second type of thing is what can be called habitability boosters. By “habitability boosters” I mean things that are not absolutely necessary for any type of life to exist in a universe, but things which will tend to make life more common and prevalent and long-lasting if they exist in a universe.

The Main Habitability Necessities

Let's look at some habitability necessities – things that absolutely must be present for any advanced life to exist in a universe.

One habitability necessity is stable atoms heavier than hydrogen, atoms such as carbon and oxygen. We take stable atoms for granted, but quite a few things have to go right in order for them to be possible. In order for you to have elements other than hydrogen, you need to have a strong nuclear force that binds protons together in the nucleus. Such a force must be very strong to overcome the electromagnetic repulsion between protons (all particles with the same charge repel each other). Another requirement of stable atoms other than hydrogen is an electromagnetic force, which keeps electrons orbiting around the nucleus of the atom. If you don't have such a force, or if electrons and protons have the same type of charge (both positive or both negative), or if the electron charge differs greatly from the proton charge, you cannot have atoms such as oxygen atoms and carbon atoms. Another requirement for stable atoms and stable molecules are certain quantum mechanical laws such as the Pauli exclusion principle. As mentioned here, if there were no Pauli exclusion principle there would be no chemistry.

Another habitability necessity is the existence of large bodies such as planets, with moderate levels of gravity on their surface. This requires a universal force such as gravitation. This force must be strong enough to hold planets together, but not so strong that organisms living on the planet have to endure a crushing force that always keeps them pinned in the same place on the ground. So a habitable universe needs not just a universal force of gravitation, but one that is neither too large nor small.

Another habitability necessity is a relatively empty vacuum. By this I simply mean a vacuum that has less mass-energy than solid steel. This is something we take for granted, but (as discussed here) there are physics considerations that imply such a thing should be incredibly unlikely. Strangely, quantum field theory predicts that there should be all kinds of quantum contributions to the vacuum that should cause it to be incredibly dense with mass-energy. This is an unresolved problem of physics known as the cosmological constant problem or the “vacuum catastrophe” problem.

Another habitability necessity is the existence of both carbon and oxygen in adequate amounts. As far as we know, life could not exist without either one. When physicists imagine universes with greatly different physics, particularly a strong nuclear force much stronger or weaker, it often means a universe with lots of carbon and no oxygen, or lots of oxygen and no carbon.

This is not a complete list of habitability necessities, but is enough to clarify the concept.

The Main Habitability Boosters

Now let's look at some habitability boosters – things that are not absolutely necessary for life to exist in a universe, but things that will tend to make life much more widespread and long-lasting if they do exist. The first habitability luxury I can think of is radiant stars – stars that produce light and heat.

It may surprise you that I have classified radiant stars as a habitability booster rather than a habitability necessity. How could life exist without stars? But the benefit that stars or suns supply is light and heat, and a planet might have light and heart without any radiant star near by. Volcanic activity, geological activity and tidal effects can produce light and heat, such as we see on Jupiter's moon Io. 

 

We can actually imagine intelligent life evolving on a small number of planets in a universe that had no radiant stars. It would require a rare type of planet which had volcanic activity or geological activity or tidal activity that consistently produced heat and light over many millions or billions of years. Even though there would be probably less than one planet in a thousand that would have such characteristics, there would still be quite a few such planets in a universe with trillions of planets.

But it could be argued that such planets could not exist, because carbon and oxygen are formed in radiant stars. However, even if there were no radiant stars there would probably be rare. freak events that would cause carbon and oxygen to be created in rare spots – events such as collisions of large astronomical bodies.

It has been estimated that the total number of stars in the observable universe is something like 10 to the twenty ninth power. Let's imagine an alternate universe in which heavy elements such as carbon and oxygen only form from freak events such as collisions of large astronomical bodies. That would still leave you with trillions of planets that might have heavy elements such as oxygen and carbon. If you then imagine no radiant stars, and life only forming on planets that gave off heat and light through geological activity, you would still have a few hundred, thousand or million such planets (possibly even billions).

So we must classify radiant stars as a habitability booster rather than a habitability necessity. There is a related thing that we can classify as a stronger and more specific habitability booster: the existence of sun-like stars. It is believed that intelligent life might evolve on planets revolving around small red stars. But it is believed that if a planet existed in the habitable zone of a red dwarf star, the planet would be so close to the star that it would keep the same side pointed towards the star, without rotation. This would probably limit life to existing in a ring-like area of the planet, between the side facing the star, and the side facing away from it. If a universe has not just red dwarf stars but also sun-like stars, it will tend to be more habitable, with more life (all other things being equal). Planets revolving around sun-like stars can have life existing all over them, not just in a narrow ring between the front and the back of the planet. 

Another thing that must be classified as a habitability booster is the presence of large amounts of both carbon and oxygen. Having lots of carbon and oxygen around may be necessary for a universe with a huge amount of life. But for a universe to be barely habitable, with only a tiny amount of life, it needs only to have a few lucky spots where there is a decent amount of carbon and oxygen.

Another thing we can list as a habitability booster is low radioactivity. In our universe radioactivity has a negligible effect on habitability, because only rare very heavy elements such as uranium are radioactive. But we can imagine a universe in which the strong nuclear force was much weaker. In that case most elements would be radioactive. The effect would probably be that creatures such as us could not live longer than about 20 or 25 years before dying of cancer caused by radioactivity.

Another thing we can list as a habitability booster is low radiation from sources such as gamma ray blasts and supernova explosions. We can easily imagine a universe a little different in which there would be a high chance of any newly evolved intelligent species being wiped out by gamma ray blasts or supernova explosions, within a few million years of when it appeared.

Another thing we can list as a habitability booster is a low static electricity. This is something that we take for granted, but which is extremely unlikely in random universes. We have low amounts of static electricity because the number of protons in the solar system is roughly equal to the number of electrons, and the charge on the proton (1836 times more massive than the electron) is the exact opposite of the charge on the electron. But if such coincidences did not exist, we might live on a planet that was teeming with local charge imbalances -- lethal concentrations of static electricity. On such a planet it might be very common for organisms to be killed when a creature simply stepped on a rock with enough excess electrons to cause a lethal shock.

Four Types of Universes

Having listed these types of habitability necessities and habitability boosters, it is now easy to categorize four types of universes. They are as follows:

Uninhabitable universe: A universe in which intelligent life cannot evolve anywhere, because it is missing one or more of the habitability necessities.

Barely habitable universe: A universe that has all of the habitability necessities, but none or only one or two of the habitability boosters.

Moderately habitable universe: A universe that has all of the habitability necessities, and roughly half of the habitability boosters.

Abundantly habitable universe: A universe that has all of the habitability necessities, and all or almost all of the habitability boosters.

Let's imagine some universes, and then classify them as belonging to these four types.

Universe 1: There are no sun-like stars, but many red dwarf stars. In almost all solar systems in which a life-bearing planet revolves around a red dwarf star, the planet keeps the same side always pointing toward the star, and life exists in a narrow band between that side and the other side of the planet. There's also a lot more gamma ray bursts than in our universe.

What type of universe is this? It seems like a moderately habitable universe.

Universe 2: There are no radiant stars at all, and the night sky always looks dark. Carbon and oxygen are rare. But in certain rare places some freak events such as astronomical collisions led to local abundances of carbon and oxygen that eventually became planets. In a small number of these planets, special geological conditions created enough light and heat for intelligent life to evolve. But the future prospects are dim for such life forms, since a change in geological conditions might deprive them of the heat and light they need.

What type of universe is this? It seems like a barely habitable universe.

Universe 3: There are radiant stars, and countless stable planets. There are not just red stars, but also stars like the sun. There are no habitability factors that should limit the universe from having trillions of planets bearing intelligent life. Planets typically enjoy low amounts of harmful radiation, low radioactivity, and low amounts of static electricity. There are trillions of planets on which nothing physical prevents intelligent life from existing for billions of years.

Which type of universe is this? It's an abundantly habitable universe.

Which type of universe do we live in? Each of the habitability boosters I previously listed are conditions of our universe. And obviously our universe has all of the habitability necessities, or we wouldn't exist. So we must classify our universe as an abundantly habitable universe.

Recent news stories bolster this conclusion. A significant fraction of the universe's galaxies are rather spherical-shaped elliptical galaxies different from our own spiral galaxy. It used to be believed that elliptical galaxies must be lifeless, because of a lack of heavy elements. But recently scientists concluded that large elliptical galaxies can produce up to 10,000 times as many Earth-like planets as our galaxy. It seems, in fact, that all of the three major types of galaxies in our universe (spiral, irregular, and elliptical) can support planets with life. The Kepler telescope has helped confirm that there are planets all over the place in our galaxy. We know of very few or no habitability shortfalls in our universe, so it must be classified as abundantly habitable.

But what type of universe should we expect our universe to be? This is a fascinating question with some interesting philosophical implications. In my next post I will discuss this question, discussing the relative probabilities of getting the types of universes I have listed. I will show that conventional assumptions like those made by most scientists lead to shockingly erroneous predictions about the type of universe our universe should be. 

As I will show in my next post, without the cheat of assuming there must be an observer, such assumptions lead to the prediction that our universe should be uninhabitable. Even if one makes such a cheat, by assuming an observer, such assumptions lead to the prediction that we should live in a barely habitable universe much, much less favorable to life than the one we live in.