Origin of Matter in the Universe
How Protons, Electrons, and Neutrons Formed in the Big Bang
In the big bang theory, matter formed in the early universe as elementary particles including protons, electrons, and neutrons.
Formation of Matter in the Universe
After the inflationary period matter began to form in the early universe. Initially heavy particles such as protons formed. Next lighter particles such as electrons formed. After these basic particles formed, nucleosynthesis could begin to form the few elements made during the big bang. How did these particles form?
In his special theory of relativity, Einstein found that matter and energy are interchangeable. Matter can be converted into energy and vice versa. His famous formula, E=mc squared, gives the conversion for how much energy corresponds to a given amount of matter.
Matter and Antimatter
One way in which this conversion can take place involves antimatter. For each type of elementary particle, there is a corresponding antimatter particle that has the same properties but an opposite electric charge. For example a positron, the antimatter particle corresponding to an electron, has the same properties as an electron, with the exception of a positive rather than a negative electric charge.
When matter and antimatter come into contact, they mutually annihilate and convert into energy. The amount of energy generated is determined by Einstein's formula.
The reverse process also occurs. When two high energy gamma rays, possessing sufficient energy, interact in the right way, they can produce a particle-antiparticle pair. Protons and antiprotons are about 2000 times more massive than electrons and positrons. For this reason, much more energy is needed to produce a proton-antiproton pair than an electron-positron pair.
Heavy Particles
In this early stage of the universe most of the mass-energy of the universe was in the form of energy. The elementary particles formed when gamma rays of the proper energy interacted to form the particle-antiparticle pairs. Protons and other relatively massive particles were formed by this process first because the universe was so hot that the gamma rays had enough energy to make the massive particles.
Light Particles
As the universe aged, expanded, and cooled, the gamma rays did not have enough energy to form the massive particles. The interactions made less massive particles such as electron-positron pairs. Neutrons then formed, just as they do in neutron stars, when protons and electrons merged.
Where Has all the Antimatter Gone?
When astronomers first deduced this history, they wondered what had happened to all the antimatter. We are still not sure. However as we learned more about particle physics at very high energies, some of our theories predict that the process is not quite perfectly symmetric. The theories still need to be completely tested. We do know from observing the universe that more matter than antimatter is made. For every billion or so proton-antiproton pairs an extra proton was made. This small asymmetry is enough that the universe is matter rather than either antimatter or a mixture of the two.
By the time the universe was about a second or so old, it had expanded and cooled to the point that the gamma rays no longer had enough energy to make even light particles such as electrons. The fundamental particles such as protons, electrons, and neutrons had been made. Now they needed to combine to form the various types of atomic nuclei.
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GUTS and Inflation in the Big Bang
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Nucleosynthesis in the Big Bang
Further Reading
Barrow, J.H. and Silk, J., The Left Hand of Creation, Oxford, 1983.
Silk, J., The Big Bang, Times Books, 2000.
Harrison, E.R., Cosmology The Science Of the Universe, Cambridge, 1981.
Friedman, H., The Astronomer's Universe, Norton, 1998.
