And as I have been telling you, we don’t know if the universe is or isn’t eternal.
What the Big Bang cosmology explain is working backward the timeline using General Relativity on how stars and galaxies, then using a combination of nuclear/particle/quantum physics to explain how the earliest atoms formed (hydrogen, helium & lithium), and before that, how the smaller particles formed (protons and neutrons, then earlier quarks and leptons).
The further you go back in time, the more denser and more hotter was the universe, where the universe was more in plasma state.
The Big Bang theory only explain from the inflationary of the universe, starting with the Planck Epoch. It doesn’t explain the universe before this Planck Epoch, other than saying it was infinitely hot and infinitely dense.
So BB explain mostly the observable universe, and nothing much really beyond that.
You say you like history, then you should appreciate that the Big Bang theory is ongoing scientific theory, with 3 major contribution beginning with 1910s and 1920s, then the late 1940s to the mid-1960s, and then the late 1990s.
Before the 20th century, astronomers thought the whole universe was the Milky Way, because the telescopes was not very powerful before the building of the observatory with the Hooker Telescope was completed in 1919.
Before the Hooker Telescope, astronomers just assume the Andromeda and Triangulum, the most distant objects they could see with the naked eye (respectively about 2 and 3 million light years away) were nebulae within the Milky Way itself, not two separate galaxies.
Now, in 1906 and then in 1917, Albert Einstein respectively formulated Special Relativity and General Relativity.
Einstein had also worked on duality of light, having the properties of wave and particle - the Photoelectric Effect (1905) - and that, along with Max Planck (who was responsible for the black body radiation in 1894 and his famous Planck’s constant in1900), kickstarted the Quantum Theory.
It was Edwin Hubble, using the new Hooker Telescope in 1919, when he discovered that Andromeda and Triangulum were separate galaxies, not part of the Milky Way. And Hubble discovered a lot more galaxies even more distant than Triangulum Galaxy.
It was only then (1919) and from that time onward, that they discovered the universe was a lot larger than the Milky Way.
Not long after Hubble’s breakthrough discovery, 3 pioneers independently used Einstein’s General Relativity as their frameworks, came up with the expanding universe model or the inflationary universe model, before it was called the Big Bang theory in 1948 or 49. They were -
- the Russian astrophysicist Alexander Friedmann (1922),
- the American physicist Howard Percy Robertson (1924-25),
- and the Belgian physicist and Catholic priest, Georges Lemaître (1927).
Each one were Big Bang pioneers.
It was Robertson who predicted that the expansion can be explained by observing two galaxies moving apart, by measuring the redshift in the electromagnetic spectrum. His prediction was verified by Edwin Hubble in 1929. Robertson-Hubble redshift was the first piece of evidence favouring the B.B. cosmology.
Friedmann died in 1925, but he taught a young student, named George Gamow, who defected to the US and taught Friedmann’s model in university during the 1940s. He was mentor to his former student Ralph Alpher, whom he teamed up and wrote the papers on the Big Bang Nucleosynthesis in 1948. Alpher with Robert Herman wrote another paper in 1948, on the Cosmic Microwave Background Radiation (CMBR) that happened during the Recombination Epoch (which I have already spoke at length in my previous replies).
These 1948 predictions weren’t verified until 1964, by two physicists, Arno Penzias and Robert Wilson, who accidentally discovered CMBR when they were installing basic radio telescope at Bell Labs in New Jersey. That was 2nd and more important evidence.
1948 marked the 2nd most important contribution to the Big Bang theory.
The 3rd important important of the Big Bang theory, was to explain why the universe is not only still expanding, but also accelerating. So in come, the formulation and predictions of Dark Energy and Dark Matter, and in the late 1990s, was the Lambda-CDM model or the ΛCDM Big Bang.
The Greek letter lambda stands for Dark Energy, while CDM stands for Cold Dark Matter.
Georges Lemaître made be called the Father of the Big Bang theory, but he wasn’t the only contributor, and he certainly was the first.
So that the history of the Big Bang theory.