Originally posted by Stingray
Einstein, Friedmann, Robertson, Walker, and Lemaitre. I don't know the specific roles each of these people played, although I think at least two or three came up with identical results independently.
...
... This was originally hypothesized by Einstein 90 years ago, but he eventually rejected it when Hubble discovered that the universe is expanding. It turns out he was right all along though.
I wish we had an online source with some of the history.
IIRC both Friedmann and Lemaitre got results roughly around 1920.
If someone knows more exactly please correct the details.
So the story is first AE makes the general 1915 theory
and he notices that it is unstable and tends to have solutions
that are either expanding or contracting
and he doesn't like that because he thinks the U should stand still
so he puts a Lambda term into the equation to make it possible for the solution to be static----if Lambda is finetuned to be exactly right. this is a kind of fragile solution because if something jostles it the U will start expanding or contracting again. but
as a stopgap measure at least it gives a static solution
then observers like Hubble notice that the U is not static after all
but is expanding
so AE throws away the Lambda in disgust---he didnt have to sneak it into allow a static solution, because static was the wrong answer!
however the Lambda term also has the possibility that it can cause expansion to accelerate or decelerate depending on its size, and it can effect the overall curvature (as any form of energy can, since the curvature reflects average energy density in the U)
so 80-some years later people are re-invoking the Lambda term to explain apparent acceleration of expansion
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Friedmann lived in St. Petersburg and had the idea in the 1920s of making a SIMPLIFIED version of AE equations by assuming symmetry and uniformity. Then he could solve them. If you make simplifying assumptions like all the energy (material) is smeared out to just be a uniform average density, so things look the same from anywhere and in all directions, then AE's original complicated multiple-equations boil down to just two ordinary differential equations involving a simple number---the scale number that keeps track of the size or the distance between things as it changes due to expansion or contraction.
Maybe Lemaitre, who IIRC was a Belgian and maybe also a Catholic priest, did the same thing. and Lemaitre also proposed an expanding solution to AE's equations.
Whenever that was, sometime in the 1920s I think, cosmologists have been using the simple diff.equations of Friedmann ever since (instead of the Einstein equation in its full complexity) because in fact the U does look pretty uniform and the same in all directions. so why not take advantage of those simplifying assumptions?
Finally, Einstein's original Lamba term carries over into the Friedmann equations. And it is thru fitting the Friedmann model to the data that they determine what Lambda has to be.
And it turns out to be 0.6 joules per cubic kilometer, plus/minus some uncertainty.
So who gave us the sensational imagery of Bang and Crunch and Accelerated Expansion?
Those are just different solutions of the Friedmann equations if you run the equations putting in different parameters. So it was some guy in St.Petersburg (maybe it was called Leningrad) who gave us those images, which are now so etched into everybody's imagination.