Nucleosynthesis in the Big Bang

Hydrogen, Helium, Lithium, & Beryllium Made in the Early Universe

Aug 15, 2007

The lightest four elements on the periodic table were made in nuclear fusion reactions shortly after the big bang.

Cosmic Nucleosynthesis

During the range from about 100 to 1000 seconds after the big bang, the cosmic nucleosynthesis era, nuclear reactions made the elements manufactured during the big bang. At the beginning of this nucleosynthesis era the temperature of the universe was a few billion degrees Kelvin.

Hydrogen

A hydrogen nucleus is simply a proton, so hydrogen existed as the basic raw material to make other elements. Some of these protons combined with neutrons to form an isotope of hydrogen, deuterium, which contains one proton and one neutron in the nucleus. Prior to the start of the nucleosynthesis era, the universe was too hot and energetic for the deuterium to form and remain stable. The abundant high energy gamma rays energized any deuterium nuclei that formed and tore the proton and neutron apart.

Helium, Lithium, and Beryllium

Why does the deuterium matter? Helium forms from the fusion of four hydrogen atoms. But it does not occur all in one step. In both the reactions powering the Sun and the reactions that manufactured helium during the big bang, making deuterium is a necessary intermediate step in fusing hydrogen into helium. The nuclear reactions to make helium could not begin until the temperature of the universe dropped below 3 billion Kelvins and deuterium could remain stable.

Hydrogen fusion reactions like those that power the Sun made helium during this nucleosynthesis stage. At the same time trace amounts of lithium and beryllium, the next two elements in the periodic table, were also made. Before heavier elements, such as the carbon in our bodies, could be manufactured by nuclear fusion reactions, there had to be enough of the raw material for these reactions: helium.

Why No Heavier Elements

But while the necessary helium is being made the universe isn't sitting around doing nothing. It is still expanding and cooling. After about 1000 seconds the temperature of the universe had dropped from a few billion Kelvins to a few hundred million Kelvins, a tenfold decrease.

At these relatively cool temperatures the helium nuclei do not smash into each other with enough force to overcome the repulsive forces of the two protons in each helium nucleus. So nuclear reactions fusing helium into heavier elements can not ignite. The universe is too cool to ignite the helium fires. Soon it becomes too cool to even maintain the hydrogen fusion reactions.

After a mere 15 minutes, the cosmic nucleosynthesis era ended. Gamow's wonderful idea that all the elements were made from the ylem during the big bang simply does not work. During the initial big bang, the universe could only manufacture the very lightest elements: hydrogen, helium, lithium, and beryllium.

We Are Recycled Stardust

The other 88 elements were not made during the big bang because the universe was too cool. Nuclear fusion reactions can only take place at very high temperatures, densities, and pressures. Fusion reactions involving heavier elements require higher temperatures, densities, and pressures. However, as the universe expands, it behaves as a gas. The temperature, density, and pressure decrease.

To form a carbon atom the raw materials are helium atoms. But by the time helium had formed in the big bang, the universe had expanded to the point where its temperature, density, and pressure were all too low for helium to fuse into carbon. Fusion reactions using carbon as the raw material to make even heavier elements of course don't even have a chance if the carbon never formed. The 88 elements too heavy to have been during the initial big bang, including those needed for life, formed later in stars. So we are recycled stardust.

Origin of Matter in the Universe