The primeval atom et The accelerating universe

In 1929 Edwin Hubble formulated the empirical Redshift Distance Law of galaxies which is now known as Hubble's Law. After the observations of the cosmological red shift Einstein referred to his cosmological constant as the "biggest blunder" of his life.

In 1998 Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess discovered through observations of distant supernovae—used as so-called standard candles—that the universe is actually accelerating. This is where the cosmological constant actually becomes relevant again as the equations can be used with a positive value to calculate the acceleration. As it turns out, the universe could never be in perfect equilibrium due to its inherent instability: if the universe expands ever so slightly the expansion releases vacuum energy which causes yet more expansion. Likewise, a universe which contracts slightly will continue contracting. The observations of an accelerating universe were consistent with a cosmological solution which Alexander Friedmann derived from Einstein's original general relativity equations.

This brings me to the reason for writing this article. The way I learned it, Einstein and Hubble were major contributors of the cosmological solution which eventually became the Lambda-CDM model—what is frequently referred to as the standard model of Big Bang cosmology. On a side note, another explanation of the accelerating universe is quintessence, which is a hypothetical form of dark energy and a model which only differs from the cosmological constant in that it's a dynamic equation that changes over time. I would go further into this but I'm afraid it's beyond the scope of this article.

As is often the case in physics, several people are working on the same problems and so it can often be challenging to attribute theories verified by observations to the right people. In fact some of Einstein's ideas were (partly) conceived previously by people completely overlooked.

Although not completely overlooked, Georges Lemaître didn't really get all the credit he deserved until later. This Belgian priest, astronomer and professor of physics at Catholic University of Louvain was the first person to propose the theory of the expansion of the universe, which—as we now know—is widely misattributed to Edwin Hubble. He was also the first to derive what is now known as Hubble's law and made the first estimation of what is now called the Hubble constant. He published this in 1927, two years before Hubble's article. Moreover, Lemaître also proposed the hypothesis of the origin of the universe which became known as the Big Bang theory. This is the notion that the universe came from a single point. He called this the hypothesis of the primeval atom. Incidentally, Lemaître described this theory as "the Cosmic Egg exploding at the moment of the creation" and to his discontent was adapted by Pope John XXIII who took it as proof of the biblical story of Genesis and in complete accordance with the teachings of the Church. After all, in a universe which is infinite and timeless a God as the creator becomes irrelevant.