Several people -- and the article at the top -- talk about a "static" universe. What this means is that the universe doesn't collapse into a point, even though

Several people -- and the article at the top -- talk about a "static" universe. What this means is that the universe doesn't collapse into a point, even though it is filled with galaxies. Something had to offset the long-term tendency of masses to gravitate towards one another, and he introduced the most simple mathematical object to represent a plausible "something". When it became apparent that distant quasars were moving away from our galaxy, he recognized that simple inertia from forces exerted on quasars in the distant past could push an eventual total collapse arbitrarily far into the future. This meant that the cosmological constant could be set to zero without observational conflict, and so he simply removed it as superfluous. He and a collaborator formulated the Einstein-de Sitter universe[1], which is a model of a universe that expands forever with zero cosmological constant. The model remains useful in teaching cosmology, as it is a good representation for the history of the universe back to the formation of the first galaxies to today. Observations of the Cosmic Microwave Background after his death raised difficulties for the Einstein-de Sitter model, but it wasn't until the accelerating expansion of the universe was detected in the late 1990s that it became necessary to add a non-zero cosmological constant back in. In essence, if an Einstein-de Sitter like universe had galaxy clusters flying apart at some threshold speed they could still fly apart for arbitrary zillions of years, however they would still exert a drag on each other, particularly early on when they are relatively close together. No such drag can be found, so observations do not require explanations more complicated than a small positive Cosmological Constant on an otherwise Einstein-de Sitter universe (and most such more complicated explanations predict things we should see but don't, or mispredict things we do see). I don't know why he called it a blunder. That seems overly dramatic. He thought evidence compelled an adjustment to his original field equations to avoid the universe collapsing; later observations made him think that adjustment was unnecessary, so he removed it. He died before even later observations made everyone else working in physical cosmology think that first adjustment was necessary after all, to avoid the universe's expansion slowing down at early times more than it did. That happens a lot to all sorts of physicists if they work in a particular area long enough, and Einstein himself proposed, modified, and abandoned numerous other models and theories over the course of his long career. - -- [1] encyclopaedic and reasonable, rather than overwhelmingly technical : https://en.wikipedia.org/wiki/Einstein%E2%80%93de_Sitter_universe https://en.wikipedia.org/wiki/Einstein%E2%80%93de_Sitter_uni...