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Welcome to our Physics lesson on Amount of Primordial Helium in the Universe, this is the fifth lesson of our suite of physics lessons covering the topic of Chronology of the Universe, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
The period of helium nuclei formation represents an important keystone in the history of the Universe. Helium obtained during this stage is known as primordial helium; it is different from helium produced in the stars core. Let's explain how to calculate the ratio of this primordial helium to hydrogen in the Universe.
Based on the standard model, and after taking into consideration the conditions for temperature and density provided by the Big Bang theory, scientists have calculated that by the end of the nuclear era the proton-neutron ratio in the Universe was 7:1 (i.e. 7 protons for every neutron). The number of protons was much greater than that of neutrons because each reaction of neutrons fission turns one neutron into one proton and one electron. In the subsequent era (that of nuclei formation) two neutrons are combined with two protons to form one helium nucleus He-4, which is a very stable structure. Thus, based in the above ratio, in every 16 primordial particles prior to helium formation, there were 14 protons (or hydrogen nuclei if we want) and 2 neutrons. Hence, since two protons and two neutrons form a helium nucleus, there was one helium nucleus and 12 protons (or hydrogen nuclei) at the end of this era. This means that after the formation of hydrogen and helium atoms (i.e. after the combination of protons, neutrons and electrons in a single structure), we have 4 nucleons (2 protons and 2 neutrons) out of 16 in helium atoms and 12 others (all of them protons) in hydrogen atoms. Hence, by the end of nuclear era the Universe contained 25% helium and 75% hydrogen (helium: hydrogen = 4:16 = 1:4).
The above ratio is confirmed from astronomic observations made in very ancient celestial objects, such as in spherical groups of stars at centre of galaxies, where their spectral lines have been analysed to reach the above conclusion.
You have reached the end of Physics lesson 22.11.5 Amount of Primordial Helium in the Universe. There are 6 lessons in this physics tutorial covering Chronology of the Universe, you can access all the lessons from this tutorial below.
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