Density big bang nucleosynthesis

When Ralph Alpher defended his PhD thesis in , over people came to watch.


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  • Big Bang Nucleosynthesis.

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. 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 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. There is no stable isotope of any element that has five nucleons. 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.

Shifting equilibria

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.

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 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 When He-3 comes across a neutron, they can fuse to form a full helium nucleus, He 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.

From fog to elementary particles

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. These primordial abundances can be tested, and, of course, have been. 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. Some of the less massive nuclei are also produced : deuterium 2 H , 3 He, 7 Li.

Big Bang Nucleosynthesis and the Density of Baryons in the Universe

The abundances of these elements can be used to constrain the baryonic density of the universe at the time of BBN. If the baryon density is high, the reactions are more efficient, more 4 He is created, and fewer intermediate nuclei like 2 H are left over. If the density is low, the reactions happen more slowly, and dont create as much 4 He before the temperatures drop too low. More intermediate nuclei are left over. The plot at the left shows the abundances of these light elements as a function of the present baryon density of the universe.

Big Bang Nucleosynthesis - Expii

Based on the observed abundances of these elements particularly deuterium -- 2 H , we infer that the baryonic density of the universe is only a few percent of the critical density. Dark matter cannot be normal baryonic material! Below 10 10 K, no new neutrons are formed, so that ratio is frozen in.

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