Historical notes on the expanding universe

Livio and Riess reply: The letter by Michael Way, Ari Belenkiy, Harry Nussbaumer, and John Peacock describes our article as if its intention was to review “studies of the expanding universe from the 1920s to the present.” In fact, the intent was to describe current and future methods that are likely to yield values of the Hubble constant with errors not exceeding a few percent. We only provided a brief historical background to offer a context for the present work and for the recently discovered Lemaître letter. Accordingly, not only were many historical facts described very briefly or omitted altogether, but many past methods—planetary nebulae, novae, mass-loss rates from massive stars, and so on—that were not thought to have the potential to deliver the desired accuracy were not mentioned at all.

We are fully aware of the fascinating history of the subject. In fact, one of us reviewed the proceedings Origins of the Expanding Universe, Way and coauthors’ reference 5, for the Journal for the History of Astronomy. There are a few excellent reviews of the history of the discovery of cosmic expansion, including Way and company’s reference 4 by Nussbaumer and Lydia Bieri, which we encourage readers to seek out.