[Einstein admitted that his model is untestable by observation because the observable universe is so small compared to the size in his calculations:]

I have erected but a lofty castle in the air... I compare space to a cloth floating (at rest) in the air, a certain part of which we can observe. This part is slightly curved similarly to a small section of a sphere's surface. We philosophize on how we must construe the continuation of the cloth so that an equilibrium is reached in its tangential tension, whether it is fastened in position at the edges, extends infinitely, or has a finite size and is a closed unit. Heine has provided the answer in a poem: "And a fool waits for an answer."... So let us be satisfied and not expect an answer, and rather see each other again as soon as possible in acceptable health in Leyden.
       —Einstein to de Sitter, 1916