Obsolete Universe

"...our knowledge of the universe is changing all the time. My universe is beginning to look hopelessly obsolete. For instance, in a Flandry tale I gave Betelgeuse planets, on some of which were life; in another, I gave Jupiter solid land masses. Both these assumptions were reasonable when I was composing, but are so no longer. I've written rationalizations into later stories. Yet this can be done to just a limited extent before it becomes ridiculous."

-Poul Anderson, "Concerning Future Histories" IN SFWA Bulletin, Fall 1979, p. 13.

Solid Jovian land masses seem dated. Maybe Betelgeusean planets do as well to the better informed. But an author does not need to rationalize beyond a certain point. Any future history series assumes past history up to the time of writing and scientific knowledge as at the time of writing. Beyond that point, the series is set in an alternative history and might even have different laws of physics. Robert Heinlein's Future History has a habitable Venus. Larry Niven's Known Space future history series has an outmoded Mercury with one side permanently facing the Sun in its very first instalment and Niven made the decision not to rewrite the texts on the assumption that fans wanted to read the series as it had originally been written.

In sf written before 1922, our galaxy would have been the entire universe.