The origin of modern cosmology is Einstein's 1917 paper "Cosmological Reflections on the Theory of General Relativity", but the content is quite different from the introduction in the commentary. Substituting the entire universe structure equation into the relativity equation, the results showed that the universe is dynamically changing, but Einstein, who thought this was strange, did not find the universe in the original equation.
Unable to find a research job after graduating from university, Einstein worked for a while at the Swiss Patent Office, checking application documents. In the meantime, he wrote a physics paper, but was busy researching in the library. Without understanding the cutting-edge research of Lorentz and Poincaré, he wrote a paper on the special theory of relativity. Perhaps because of this profession, his papers were somewhat amateurish.
Professional physicists don't write superfluous stuff, just summarize the main points, but Einstein is interesting because it describes what ideas he was based on. In his cosmological treatises his cosmology was fully developed.
This paper is based on the time-dynamic theory that galaxies are the only celestial bodies that exist in outer space. I've studied various things about the geometric properties of space as I move infinitely far away from the galaxy, but I can't find a reasonable answer. He wrote that he came up with a spherical structure that would return to its original point when traveling straight, as a space that "cannot be further away from the galaxy."
However, even if the spherical space equation is substituted, the basic relativistic equations cannot be satisfied. Therefore, I chose to change the basic equation to a form that incorporates the cosmological constant. The solution to the spherical space that satisfies the modified equation is the so-called "Einstein's stationary universe". It's clear from these descriptions that what mattered to him was the geometry of the galaxy as he left it, and that he ultimately did not see change over time as a problem.
Following Einstein's idea of ??spherical space, Friedman discussed its dynamic behavior. His paper, published in 1922, is often described in history books of science as "considering solutions to equations without a cosmological constant," but this was not the case. The solution when adding the cosmological constant is calculated on a case-by-case basis.
There are some mathematical errors in Friedman's paper, but the reasons can be deduced by reading the original paper.
Since many of the basic equations in modern physics are in the form of partial differential equations, it is now commonplace to teach partial differential equations to science and engineering college students. However, at the beginning of the 20th century, even in Western Europe, there was no system for teaching partial differential equations systematically.
In Friedman’s Russia, the education system would be further delayed. Where he erred was by citing an old mathematical paper by Weierstrass from before 1866, but he probably did not have access to the latest literature on analytic studies etc.
Together with Friedmann, Lema?tre played an important role in cosmological research. He showed in 1927 that Einstein's universe at rest was unstable and shifted toward expansion with slight fluctuations. Sometimes it is said that Lema?tre discussed the general solutions of equations including the cosmological constant, unlike Friedman, but in fact it was Friedman who discussed the general solutions, while Lema?tre mainly dealt with the special solutions.
Although published in French in a little-known Belgian journal, Lema?tre's paper caught the attention of British astronomy tycoon Eddington and thrust it into the spotlight. Eddington suggested that the treatise be translated into English for dissemination, and Lema?tre's own English translation was published in 1931. Today, English translations are collected mainly in cosmological treatises.
This English translation is somewhat different from the original paper. You can now find the original French version online, so search for it if you're interested.
The original paper did not mention Friedman, but he did not steal his achievements, he simply did not know. In the English translation, newly added comments provide positive reviews.
Importantly, in the English translation, the section discussing the relationship between observational data and theoretical predictions has been completely removed. The reason for the deletion is unclear, but perhaps Hubble released his latest data on distant galaxies in 1929, so he saw no need to keep the old data.
However, if the missing piece remains, it would be more widely known that Sliver had observed the galaxy's decay rate before Hubble did.
In addition, Lema?tre predicted a scaling relationship between the redshifts found in distant galaxies and distance, and used previous Hubble data to verify this. Hubble insisted on this proportional relationship in his paper based on his own observational data, but the data shown there had a large error, and it was difficult to admit that the proportional relationship itself was true.
Hubble borrowed Lema?tre's ideas and argued for proportionality because both Lema?tre and Hubble attended an international conference in 1928, where they discussed the relationship between redshift and distance. Opinions were exchanged. That may be the case. Taking these circumstances into account, the 2018 International Astronomical Union Congress decided to rename the previously so-called "Hubble's Law" to "Hubble's Law."