In the 1930’s, following the discovery of the expanding universe, attempts began to extrapolate back to the beginning of the expansion. Running the expansion backward (in theory) led to the idea of a hot, dense early universe, too hot to be anything but a plasma, perhaps hot enough to cause fusion, and happening at a particular time. The idea appeared ridiculous to most scientists. How could there be a beginning? Did the laws of physics begin then too? At the time, most scientists thought that the universe was infinitely old. The Hubble law ended the static universe model, but it was hoped that some infinite age model would replace it.

The first logical, self-consistent calculation was done in the 1940’s, by George Gamow, Alpher and Herman. At first they started with an equal mix of protons and neutrons. They didn’t try to explain the origin of this mix, or claim it was realistic. They calculated the reactions that would happen at temperature of several million K, and hoped to discover the origin of all the elements, given some original mix of protons and neutrons.

The results were unexpected:

1. They couldn’t make any element but He, and a small amount of deuterium (H with a neutron bound to the proton in the nucleus.)
2. They soon realized the heat radiation in the early universe had no place to go! If it was hot, there would have to be radiation, and it would still be everywhere today.
3. They calculated the present temperature as around 5K, not exactly, but certainly greater than 1K.

At the same time, Fred Hoyle, Gold, Bondi and others developed the Steady State theory, consistent with General Relativity with an extra constant, which would be expanding but constantly creating matter and not changing density. Hoyle was partly motivated by religion – he was an atheist and didn’t like the idea of a creation event. Hoyle named the origin idea the “Big Bang”. It was derogatory, but not for long. (The word “Yankee” was derogatory too, but it outlasted its inventors and their empire.)

The modern theory is more developed and more interesting than the original.

1. The modern theory is developed partly from actual measurements of the cosmic radiation predicted by Alpher, Herman, and Gamow.
2. It makes no assumptions about the original mix. A hot enough environment will contain every elementary particle and anti-particle, in equal numbers. Photons are just part of the mix.
3. As things cool off (in a second or so!) particles and antiparticles annihilate until only photons and neutrinos are left. The photons become the 5K temperature radiation deduced above.
4. Um, well, there are a few particles too. These are our protons, neutrons, and electrons.
5. The proton-neutron ratio is not 50-50, and is not arbitrary. It follows from the calculations. The protons don’t last in isolation, but are found in He nuclei.
6. The exact ratio of H to He depends, to a few percent, on the number of families of neutrinos. By the time it became possible to measure that number in accelerators, the cosmic theory needed the number to be three, or just possibly 4. By then it was actually a test of the Big Bang theory. When measured, the number of neutrino families was indeed found to be three.
7. The deuterium fraction depends on the initial (and thus the present) density of protons & neutrons. This “baryonic” matter is about 4% of the critical density. Most of it (about 75%) is outside the galaxies.