Cosmic Coincidences

Image: Representation of a section
of DNA made by Richard Wheeler.

There are many things about the universe we live in that are just right for life as we know it to exist. Not only are there parameters that must have values that lie in a very narrow range, but there are also “coincidences” of seemingly unrelated phenomena that are crucial. One example is why some of the elements critical for life happen to be among the more abundant ones in the universe.

Our bodies, like most organic matter, are composed primarily of four elements: hydrogen, carbon, nitrogen and oxygen. The first, of course, is the simplest and most abundant element in the universe, but the remaining three are fairly special. All three are capable of forming multiple chemical bonds, and all are (unusually) capable of forming multiple bonds with the same partner atom. For example, elemental oxygen, O2 is a gas where two oxygen atoms share two chemical bonds. In contrast sulfur is a solid because, although it is in many ways chemically similar to oxygen and also forms two chemical bonds, it cannot form both of them with the same atom and therefore forms a solid network of atoms. The ability of these elements to form gaseous as well as liquid and solid compounds is critical for life, e.g. for respiration. Similarly, carbon dioxide is a gas because the carbon atom shares two of its four bonds with each oxygen atom, but silicon dioxide (a principle element of many rocks) cannot form double bonds. The unusually flexible atomic bonds of these elements, especially carbon, also allow them to a wide range of complex molecules that form the building blocks of life, such as sugars, proteins and DNA. The unusually flexible bonds of carbon, nitrogen and oxygen (C, N & O) give them special chemical properties such that it is difficult to conceive of life existing without them.

Fortunately for us, these three elements are among the more abundant in the universe. After hydrogen and helium (which composed the vast majority of the sun and the universe) they are the three most common types of atoms in our solar system and our galaxy. A remarkable thing is that C, N & O are far more abundant than elements before or after them on the periodic chart. Lithium, beryllium and boron before, and fluorine afterwards do not even make the top twenty five most common elements, and the only other element that comes close to C, N & O in abundance is chemically inert neon. The remaining elements in the top ten most common are iron, silicon, magnesium and sulfur, which are major components of Earth’s crust and core.

The reason for these elements abundance has to do with nuclear synthesis in stars. After the Big Bang, there was only hydrogen, a little helium, and trace amounts of lithium. Everything else has been produce in stars as they fuse hydrogen into helium other elements, making heavier elements and releasing energy in the process. In stars like our sun, this primarily happens by the direct route that is slowed by the weak nuclear force. However, in more massive stars a different path becomes possible that uses C, N & O as catalysts in what is known as the C-N-O cycle. This enables these massive stars to use up their hydrogen much faster, producing a lot of C, N & O in the process, and creating less stable stars that eventually explode in a supernova that blows much of the star’s matter into space and creates the heaviest elements, such as gold, lead and uranium. Thus our existence is in part the result of one or more stars exploding and seeding what became our solar system with their ashes.

Besides pointing out that not just anything will work as the building blocks for life and that one or more stars were sacrificed to enable our existence, I want to make the point out that the abundance of C, N & O is completely unrelated to their usefulness. The relative abundance of C, N & O has to do with their nuclear properties which in turn are based on the strong nuclear force and properties of neutrons and protons. The chemical properties, in turn, depend on the electrical force and the properties of electrons. There is no known fundamental physical reason why these two different processes should pick the same three elements.

I’ve gone on longer than I intended, so I’ll close with the observation that this is not the only one of seeming coincidences. Water, one of the most common compounds in our world, is also very unusual in its high ability to absorb heat, ability to dissolve ionic and non-ionic compounds, and that ice floats. For this reason, it is difficult to imagine life without the presence of water. These unique properties are a result of its unusual molecular structure resulting from electrical interactions and chemical bonding. Its high abundance on Earth and in the solar system stems that it is composed of the two most common chemically active elements in the solar system. Once again, nuclear processes give rise to the high abundance of something with unique chemical properties needed for life. This and the previous post may raise the question as to whether there is an explanation; I’d discuss several proposed ideas in my next post.