The Big Bang
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“At the big bang itself the universe is thought to have zero size, and so to have been infinitely hot. But as the universe expanded, the temperature of the radiation decreased. One second after the big bang, it would have fallen to about ten thousand million degrees. This is about a thousand times the temperature at the center of the Sun, but temperatures as high as this is reached in H-bomb explosions. At this time the universe would have contained mostly photons, electrons, and neutrinos (extremely light particles that are affected only by the weak force and gravity) and their antiparticles, together with some protons and neutrons. As the universe continued to expand and the temperature to drop, the rate at which electron/antielectron pairs were being produced in collisions would have fallen below the rate at which they were being destroyed by annihilation. So most of the electrons and antielectrons would have annihilated with each other to produce more photons, leaving only a few electrons left over. The neutrinos and antineutrinos, however, would not have annihilated with each other, because these particles interact with themselves and with other particles only very weakly. So they should still be around today. If we could observe them, it would provide a good test of this picture of a very hot universe. Unfortunately, their energies nowadays would be too low for us to observe them directly. However, if neutrinos are not massless, but have a small mass of their own, as suggested by recent experiments, we might be able to detect them indirectly: they could be a form of “dark matter,” like that mentioned earlier, with sufficient gravitational attraction to stop the expansion of the universe and cause it to collapse again.
one hundred seconds after the big bang, the temperature would have fallen to one
thousand million degrees, the temperature inside the hottest stars. At this
temperature protons and neutrons would no longer have sufficient energy to
escape the attraction of the strong nuclear force, and would have started to
combine together to produce nuclei of atoms of deuterium (heavy hydrogen), which
contain one proton and one neutron. The deuterium nuclei would then have
combined with more protons and neutrons to make helium nuclei, which contain two
protons and two neutrons, and also small amounts of a couple of heavier
elements, lithium and beryllium. One can calculate that in a hot big bang model
about a quarter of the protons and neutrons would have been converted into
helium nuclei, along with a small amount of heavy hydrogen and other elements.
The remaining neutrons would have decayed into protons, which are the nuclei of
ordinary hydrogen atoms”.
The information acquired in the quotes is from: A Brief History of Time, Stephen Hawking, pages 121-124.
© Copyright 1996, published by Bantam Books
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