The Day We Found the Universe , Marcia Bartusiak , Pantheon Books, New York, 2009. $27.95 (337 pp.). ISBN 978-0-375-42429-8
Discovering the Expanding Universe , Harry Nussbaumer and Lydia Bieri , Cambridge U. Press, New York, 2009. $59.00 (226 pp.). ISBN 978-0-521-51484-2
In the 20th century, humans finally figured out that the universe is much bigger than the Milky Way galaxy. In The Day We Found the Universe, Marcia Bartusiak tells us that the revelation occurred on Thursday, 1 January 1925.
A paper presented that day at the annual American Astronomical Society meeting was dryly titled, “Cepheids in Spiral Nebulae.” It was written by 35-year-old Mount Wilson Observatory astronomer Edwin Hubble, who had been observing Cepheid variable stars to measure the distances of galaxies. Talks at conferences aren’t real papers. But Hubble’s work, when it was submitted to the Astrophysical Journal, was decisive in establishing that the fuzzy spiral nebulae dimly glimpsed in photographs were actually giant systems of stars millions of light-years away, far beyond the outer reaches of the Milky Way. That paper alone would have been enough to establish Hubble as a pioneer in extragalactic astronomy.
By 1929 Hubble had coupled his painstaking measurements of distances with the velocities of galaxies—mostly measured by Vesto Slipher at the Lowell Observatory in Flagstaff, Arizona—to establish the linear relationship between galaxies’ velocity and distance, a relationship we now call Hubble’s law.
The Day We Found the Universe is an energetic, thoughtful, and original guide to that pivotal point in the history of cosmology. Bartusiak has perfect pitch in telling the story of that scientific discovery. Her deft biographical sketches breathe life into a large cast of characters who are normally skipped over in textbook versions of the discovery. She patiently traces the deep roots of observatories and observing techniques that made it possible to see that we live in an expanding universe. She demonstrates what we all know from observing our colleagues: Science is not a solitary pursuit carried out by calculating automatons. It involves competition, cooperation, friendship, jealousy, loyalty, vanity, insight, missed opportunities, technical genius, and startling success, all jumbled together. Textbooks concentrate on the last step, but in this book you will be rewarded by meeting some people who are not Edwin Hubble.
Such a great story deserves to be told in more than one way. In Discovering the Expanding Universe, authors Harry Nussbaumer, a solar physicist at ETH Zürich, and Lydia Bieri, a recent PhD graduate from ETH now in the mathematics department at Harvard University, have written a very different book. Unlike Bartusiak, they are scientists. They are also theorists. They can read German and French and mathematics, too. So they can take you line-by-line through some of the key papers by Albert Einstein. It’s wonderful to have such expert guides. Though their book claims to be for the interested layman, it will prove most valuable to Physics Today readers. It is a much more formal book, maddeningly uneven, but not without its light touches, original points, and valuable insights. As the authors emphasize, the discussion of a possible relation between velocity and distance did not begin with Hubble. In 1917, for example, Willem de Sitter had already discovered an ostensibly static and empty solution to Einstein’s equations, in which a redshift was ascribed to distant light emitters.
Priest and scientist Georges Lemaître plays a pivotal role in Nussbaumer and Bieri’s narrative. Born in 1894, Lemaître traveled from Belgium to study with Arthur Eddington at the University of Cambridge and later with Harlow Shapley at Harvard before receiving his PhD from MIT. In 1930 Lemaître learned that de Sitter, in a January meeting of the Royal Astronomical Society in London, had wondered aloud what would happen if he could add matter to his empty cosmological solution. Upon hearing that, Lemaître wrote Eddington a longhand letter—which Nussbaumer and Bieri decipher—reminding his onetime instructor that he, Lemaître, had solved that problem in 1927 and had sent him the paper. Embarrassed, Eddington took the extraordinary step of having the article—originally written in French in the Annals of the Scientific Society of Brussels—translated and reprinted in the Monthly Notices of the Royal Astronomical Society in 1931. De Sitter was won over, saying, “Lemaître’s theory … gives a complete solution to the difficulties it was intended to solve, a solution of such simplicity as to make it appear self-evident.”
Not only had Lemaître devised an expanding-universe solution that contained matter and produced a linear relationship between velocity and distance, but he also worked out Hubble’s law and calculated the Hubble constant before Hubble did. Einstein and de Sitter subsequently jettisoned the cosmo-logical constant that had enabled them to enforce a static cosmology and instead opted for a spatially flat, ever-expanding universe, while Lemaître and Eddington continued to include the cosmological constant as a negative pressure term that could drive exponential cosmic expansion. In 1934, Lemaître associated a negative pressure with the energy density of the vacuum and said, “This is essentially the meaning of the cosmological constant.” That is exactly the way we talk about dark energy today. Looking back, Lemaître seems to have been severely under-rated—writing in French was bad enough, but publishing in Belgian journals was fatal.
Recent work revealing cosmic acceleration was also not done by automatons. How will historians evaluate the latest developments? Will they emphasize the early work and the biographies of the participants, as Bartusiak does, or will they parse the published papers like Nussbaumer and Bieri? Some of each, I imagine. Press releases, e-mail, preprints posted online, and blogs have come to replace letters and diaries, making the task harder. But the best way to establish the sequence of events is still to look at the refereed scientific publications. A flash of insight while working alone is good, but until you’ve made the case in a carefully prepared manuscript and answered objections from a referee, you haven’t got a real scientific result.
The two books are different: The Day We Found the Universe is the one I would send to a lively niece or nephew with a bent for science, but Discovering the Expanding Universe is the one you should read. The expanding (and, we now know, accelerating) universe is big enough to contain both.
Robert Kirshner is the Clowes Professor of Science at Harvard University and author of The Extravagant Universe: Exploding Stars, Dark Energy, and the Accelerating Cosmos (Princeton University Press, 2002).
