August 2, 2011 | 2
We take it for granted that there exists a periodic table with numerous elements (at last count, 118) from which we can construct the world around us. But when the universe began with a big bang, it started out with no elements at all. Many of the elements that make up Earth and the people on it had to be created in the nuclear furnaces inside stars and were only released once the star reached the end of its life. In fact, only light elements, like hydrogen and helium, were created at the start of the universe. We can use our knowledge of how particles react to work out how these elements formed just a few minutes after the big bang.
Alpher, Bethe, Gamow…
“It seemed unfair to the Greek alphabet to have the article signed by Alpher and Gamow only, and so the name of Dr. Hans A. Bethe (in absentia) was inserted in preparing the manuscript for print”
— George Gamow, The Creation of the Universe (1952)
When Ralph Alpher defended his PhD thesis in 1948, over 300 people came to watch. Thesis defences are not usually a source of so much excitement, at least not beyond the defender’s immediate family, but this one was different.
Before finishing his PhD, Alpher, along with his supervisor George Gamow, had written and published a paper arguing that the Big Bang would have created hydrogen, helium and other elements in certain abundances. Gamow, ever the humorist, felt it was inappropriate to publish a paper with author names so similar to “alpha” and “gamma” without including a “beta” — luckily, Gamow’s friend Hans Bethe was happy to oblige, and had his name added to the paper. Bethe did look over the manuscript and later worked on theories that made up for the shortcomings of the initial paper.
The paper was published in Physical Review on April 1st 1948. Titled “The Origin of Chemical Elements”, it described a process by which all of the known elements in the universe could have come into existence shortly after the big bang. It built on previous work by Gamow that suggested the elements originated “as a consequence of a continuous building-up process arrested by a rapid expansion and cooling of the primordial matter” — in other words, different atoms were made by adding one nucleon at a time to the nucleus, before the process was stopped when the universe became too cool.
Alpher and Gamow (with a little help from Bethe) set out a vision of the early universe in which all matter was a highly compressed “soup” of neutrons, some of which were able to escape and decay into protons and electrons as the universe expanded and became less dense. They believed that these new protons could then capture neutrons, together making deuterium nuclei — an isotope of hydrogen that has one proton and one neutron. They then extrapolated this idea and said that all that had to be done to create heavier nuclei was the capture of another nucleon.
But it’s a little more complicated than that. Their idea works for elements up to helium — and does produce hydrogen and helium, which together make up 99% of the matter in the universe, in the correct proportions to explain their abundances — but it fails when you try to put five nucleons together. There is no stable isotope of any element that has five nucleons. Alpher’s and Gamow’s theory relied on using each element as a stepping stone to the next, so it was stopped in its tracks by this piece of information.
Nevertheless, it was an important step in the right direction, and did describe most of the universe by virtue of the fact that hydrogen and helium make up such a large portion of it. The theory was recognised as significant at the time, too. Among the 300 people in the room at Alpher’s thesis defence, it seems, were the Washington Post. After his presentation, they ran an article with the headline “World Began in 5 Minutes, New Theory”.
Big Bang Nucleosynthesis
Since Alpher, Bethe and Gamow published their paper, cosmologists have done a lot more work on the formation of the light elements in the early universe. The process now has a name: big bang nucleosynthesis.
In the first few seconds after the big bang, the universe was very hot and dense, making it fully ionised — all of the protons, neutrons and electrons moved about freely and did not come together to make atoms. Only three minutes later, when the universe had cooled from 1032 to 109 °C, could light element formation begin.
At this point, electrons were still roaming free and only atomic nuclei could form. Protons were technically the first nuclei (when combined with an electron they make a hydrogen atom) and deuterons were the second. Deuterons are the nuclei of deuterium and are made when protons and neutrons fuse and emit photons.
Deuterons and neutrons can fuse to create a tritium nucleus with one proton and two neutrons. When the tritium nucleus comes across a proton the two can combine into a helium nucleus with two protons and two neutrons, known as He-4. Another path that leads to helium is the combination of a deuteron and a proton into a helium nucleus with two protons but only one neutron, He-3. When He-3 comes across a neutron, they can fuse to form a full helium nucleus, He-4. Each step in these reactions also emits a photon.
Photon emission can be a slow process, and there is a set of reactions that take deuterons and create helium nuclei faster because they bypass the emission of photons. They start by fusing two deuterons and the end result is a He-4 nucleus and either a proton or a neutron, depending on the specific path.
Lithium and beryllium were also made in very small amounts. This whole process was over 20 minutes after the big bang, when the universe became too cool and sparse for nuclei to form.
The abundance of the light elements can be predicted using just one quantity — the density of baryons at the time of nucleosynthesis. Baryons are particles made with three quarks, such as protons and neutrons. Using the baryon density predicted by big bang nucleosynthesis, the total mass of the universe would have been 25% helium, 0.01% deuterium and even less than that would have been lithium. These primordial abundances can be tested, and, of course, have been. Nowhere in the universe is helium seen with an abundance less than 23%. This is a major piece of evidence for the big bang.
The nuclei formed in big bang nucleosynthesis had to wait a long time before they could team up with electrons to make neutral atoms. When neutral hydrogen was finally made 380,000 years after the big bang, the cosmic microwave background (CMB) radiation was emitted.
Alpher and his colleague Robert Herman predicted the existence of the CMB in the late 1940s, when they realised that the relic radiation would be a side effect of the recombination of electrons with atomic nuclei. The CMB now provides us with a way to double check our working with an independent measurement on the baryon density. By looking at fluctuations in the CMB, we find a baryon density that would give the correct light element abundances — it seems we really do understand what went on only a few minutes after the universe began.
Alpher, R., Bethe, H., & Gamow, G. (1948). The Origin of Chemical Elements Physical Review, 73 (7), 803-804 DOI: 10.1103/PhysRev.73.803