When he was working on what is now known as "Einstein's equation" (and no, it isn't E=mc^2), he found that it led to the suggestion that the universe might be changing its size. He asked his astrophysicist colleagues about it, they assured him that the universe was static, and he took their word for it, putting in his cosmologic constant. Of course, it was less than fifteen years later that Hubble published his findings that led to the inescapable conclusion that the universe is not static but is expanding, more or less as Einstein's field equations sans cosmologic constant predict.