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The Big Bang theory is famously attributed to Lemaître .

In a moment of rare sentience, I decided to consult Georges Lemaître's works to find out what the original Big Bang theory looked like. After a few hours of aggressively searching the internet, I finally found a full digital pdf of The Primeval atom - an essay on cosmogony translated by Betty H., and at first glance thought surely this is a metaphor for the singularity. I began reading, for a while wondered why he wasn't getting to the point before looking at the contents page and jumping straight to the last chapter where he wrote:

...The two ideas which have been formulated, thus far, are: First, the origin of matter as a single atom which disintegrated; second, the equilibrium of clusters of nebulae. These ideas can easily be combined into a single theory. At the time of the atom’s disintegration, matter was strongly condensed. The fragments of the atom were separated from one another with great speeds, speeds which were progressively slowed down by the force of gravitation which, for these large densities, amply overcame the cosmic repulsion
[ The Primeval Atom - an essay on cosmogony p. 126 ]

and he also described the CMBR as the emissions of the original atom. So did I read the wrong book or something? it was just floating around on the internet after all. Or did Georges later modify his theory from primeval atom -> singularity.

Because the primeval atom idea hardly bares any resemblance to the Big Bang model at all. The principle of the modern Big Bang theory is the singularity, this in effect is the only part of the theory that tells us about the origins of the universe, the rest of the theory devoted to universal evolution is just deductions based on the singularity model and current knowledge of atomic, photon theory and astronomical observations. So if Georges' theory completely contradicts the most fundamental tenet of the Big Bang theory, how can the theory even be ascribed to him?

I've seen people make arguments like: "Well, you see. It furnished the foundations of the Big Bang theory"

But the foundations of the big bang theory are universal expansion and CMBR. In fact anyone could suppose an expanding universe to mean the universe was smaller in the past so perhaps the universe originated as a tiny dot, so even in forming the foundations of the Big Bang theory, I'm not convinced Lemaître has a place. Although I read somewhere that he proved universal expansion, if so then fair enough.

So this leads to my actual question. Who really invented the Big Bang theory? and this question is equivalent to "Who invented the idea the that the universe started from a singularity?" Since in my understanding that is the defining and most fundamental characteristic of the theory.

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    $\begingroup$ Actually, the credit is usually shared between Lemaitre and Friedmann, who derived expanding solutions to the field equations 5 years earlier, in 1922, including solutions with initial singularity, see Science News. However, he died in 1925, before the idea gained popularity, so it was left to Lemaitre to promote it and link to observational evidence. Btw, I am not sure why initial singularity is particularly essential, it is considered a mathematical artifact anyway, the feature was expansion. $\endgroup$
    – Conifold
    Jun 1, 2023 at 4:22
  • $\begingroup$ "the principle of the modern big bang theory is the singularity" is not true, as Conifold said. The concept of a primeval atom is vague enough that it might be said to cover modern ideas about the origin of the universe (but those ideas aren't part of big bang cosmology proper, since it isn't about the origin). Also, cosmologists don't call it "the big bang theory"; that's just a TV show. They call it big bang cosmology or FLRW cosmology among other names. $\endgroup$
    – benrg
    Jun 1, 2023 at 7:37

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