The Primordial Universe

The tale of how the primordial universe of hydrogen, helium, and a bit of lithium evolved to a universe harboring at least one world with intelligent life like us is a tale of many chapters.

The forces of nature had to be such that heavier elements—especially carbon—could be produced from the primordial elements, and remain stable for at least billions of years.

Those heavy elements were formed in stars, so the forces first had to allow stars and galaxies to form.

Those grew from the seeds of tiny inhomogeneities in the early universe, which was almost completely uniform but thankfully contained density variations of about 1 part in 100,000.

However, the existence of stars, and the existence inside those stars of the elements we are made of, is not enough.

The dynamics of the stars had to be such that some would eventually explode, and, moreover, explode precisely in a way that could disburse the heavier elements through space. In addition, the laws of nature had to dictate that those remnants could recondense into a new generation of stars, these surrounded by planets incorporating the newly formed heavy elements.

Just as certain events on early earth had to occur in order to allow us to develop, so too was each link of this chain necessary for our existence.

But in the case of the events resulting in the evolution of the universe, such developments were governed by the balance of the fundamental forces of nature, and it is those whose interplay had to be just right in order for us to exist.

One of the first to recognize that this might involve a good measure of serendipity was Fred Hoyle, in the 1950s. Hoyle believed that all chemical elements had originally been formed from hydrogen, which he felt was the true primordial substance.

Hydrogen has the simplest atomic nucleus, consisting of just one proton, either alone or in combination with one or two neutrons.

(Different forms of hydrogen, or any nucleus, having the same number of protons but different numbers of neutrons are called isotopes.)

Today we know that helium and lithium, atoms whose nuclei contain two and three protons, were also primordially synthesized, in much smaller amounts, when the universe was about 200 seconds old. Life, on the other hand, depends on more complex elements. Carbon is the most important of these, the basis for all organic chemistry.

Though one might imagine “living” organisms such as intelligent computers produced from other elements, such as silicon, it is doubtful that life could have spontaneously evolved in the absence of carbon. The reasons for that are technical but have to do with the unique manner in which carbon bonds with other elements.

Carbon dioxide, for example, is gaseous at room temperature, and biologically very useful. Since silicon is the element directly below carbon on the periodic table, it has similar chemical properties.

However, silicon dioxide, quartz, is far more useful in a rock collection than in an organism’s lungs. Still, perhaps life-forms could evolve that feast on silicon and rhythmically twirl their tails in pools of liquid ammonia. Even that type of exotic life could not evolve from just the primordial elements, for those elements can form only 2 stable compounds, lithium hydride, which is a colorless crystalline solid, and hydrogen gas, neither of them a compound likely to reproduce or even to fall in love.

Also, the fact remains that we are a carbon life-form, and that raises the issue of how carbon, whose nucleus contains six protons, and the other heavy elements in our bodies were created.

The first step occurs when older stars start to accumulate helium, which is produced when 2 hydrogen nuclei collide and fuse with each other.

This fusion is how stars create the energy that warms us. Two helium atoms can in turn collide to form beryllium, an atom whose nucleus contains four protons. Once beryllium is formed, it could in principle fuse with a third helium nucleus to form carbon. But that doesn’t happen, because the isotope of beryllium that is formed decays almost immediately back into helium nuclei.

The situation changes when a star starts to run out of hydrogen. When that happens the star’s core collapses until its central temperature rises to about 100 million degrees Kelvin.

Under those conditions, nuclei encounter each other so often that some beryllium nuclei collide with a helium nucleus before they have had a chance to decay.

Beryllium can then fuse with helium to form an isotope of carbon that is stable. But that carbon is still a long way from forming ordered aggregates of chemical compounds of the type that can enjoy a glass of Bordeaux, juggle flaming bowling pins, or ask questions about the universe.

For beings such as humans to exist, the carbon must be moved from inside the star to friendlier neighborhoods. That, as we’ve said, is accomplished when the star, at the end of its life cycle, explodes as a supernova, expelling carbon and other heavy elements that later condense into a planet.

This process of carbon creation is called the triple alpha process because “alpha particle” is another name for the nucleus of the isotope of helium involved, and because the process requires that three of them (eventually) fuse together.

The usual physics predicts that the rate of carbon production via the triple alpha process ought to be quite small. Noting this, in 1952 Hoyle predicted that the sum of the energies of a beryllium nucleus and a helium nucleus must be almost exactly the energy of a certain quantum state of the isotope of carbon formed, a situation called a resonance, which greatly increases the rate of a nuclear reaction.

At the time, no such energy level was known, but based on Hoyle’s suggestion, William Fowler at Caltech sought and found it, providing important support for Hoyle’s views on how complex nuclei were created.

Hoyle wrote, “I do not believe that any scientist who examined the evidence would fail to draw the inference that the laws of nuclear physics have been deliberately designed with regard to the consequences they produce inside the stars.” At the time no one knew enough nuclear physics to understand the magnitude of the serendipity that resulted in these exact physical laws.

But in investigating the validity of the strong anthropic principle, in recent years physicists began asking themselves what the universe would have been like if the laws of nature were different.

Today we can create computer models that tell us how the rate of the triple alpha reaction depends upon the strength of the fundamental forces of nature. Such calculations show that a change of as little as 0.5 percent in the strength of the strong nuclear force, or 4% in the electric force, would destroy either nearly all carbon or all oxygen in every star, and hence the possibility of life as we know it.

Change those rules of our universe just a bit, and the conditions for our existence disappear!