The real reason why Einstein came to believe in an expanding universe

According to legend, Albert Einstein changed his mind about the static state of the universe in 1931 after astronomer Edwin Hubble showed him evidence that the universe was expanding. A recent paper shows this is a myth — and that Einstein needed a lot more to change his mind.

The traditional story is that Hubble's observations of redshift in the light emitted by far away nebulae (or galaxies as we now call them) was what contributed to Einstein's change of heart. But according to science historian Harry Nussbaumer from the Institute of Astronomy at ETH Zurich, Switzerland, Einstein stubbornly refused to reject the static interpretation of the universe until his model could be proven unstable. He wasn't swayed by the observational data alone.

Back in 1917, Einstein launched modern cosmology by describing the universe as a homogenous, static, and spatially curved universe. The theory, which was based on general relativity, introduced the idea of the cosmological constant — a necessary requirement to counteract gravitational contraction (without it, the universe would just collapse in on itself).

But four years later, the Soviet physicist Alexander Friedmann introduced the idea of an expanding universe that contained moving matter. He showed that Einstein's equations could still account for dynamical worlds. In 1927, the Belgian astronomer Georges Lemaître, using observational evidence, independently reached the same conclusion.

But Einstein was far from impressed. In the words of Nussbaumer, he "impetuously rejected Friedman's as well as Lemaître's findings...The immutability of the universe was obviously a very deep-rooted conviction of Einstein."

But in a gradual and tortuous process, he finally changed his mind. Much of it had to do with numerous correspondences he had between some of the most influential astrophysicists of his generation, including the Dutch theoretical physicist and astronomer, Willem de Sitter.

Nussbaumer provides a nice summary:

The letter to [Michele] Besso shows that during the second half of February 1931 Einstein must have had another look at some cosmological key-publications, in particular at Friedman's paper of 1922. On March 1, he seems inclined to accept a dynamic universe. However, there remained a last obstacle. The age of the universe, calculated from the redshifts of the nebulae and interpreted in the framework of the relativistic expansion theory, was much shorter than the lifetime of stars, as believed in those days. After his return to Berlin Einstein found a way out of that dilemma by invoking inhomogeneities in the distribution of matter; they might account for the "low age" resulting from the model based on a homogeneous density distribution. This loophole must have given him the courage to opt for Friedman's periodic universe with λ=0, thus freeing himself at the same time from the cosmological term, to which he had never warmed up.

Einstein's publication of 1931 was scientifically irrelevant, it contained nothing new. However, it is a historical landmark, as it announced Einstein's conversion to a dynamic, expanding universe without the action of a cosmological constant.

The following year, Einstein and de Sitter posited a model which proposed an eternally inflating universe — a standard cosmological model that was generally accepted until the mid 1990s.

Read the entire study at European Physical Journal H: "Einstein's conversion from his static to an expanding universe."