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Review: The Whole Truth

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The Whole Truth: A Cosmologist’s Reflections on the Search for Objective Reality
by P. J. E. Peebles
Princeton University Press, 2022
Hardcover, 264 pp.
ISBN 978-0-691-23135-8

Three years ago, cosmologist Jim Peebles won a share of the 2019 Nobel Prize in Physics for “theoretical discoveries in physical cosmology,” as the Royal Swedish Academy of Sciences described it. Peebles spent his career working on models to explain the formation of the universe, from the cosmic microwave background to the roles played by dark matter and dark energy. His work, the announcement of the prize stated, “laid a foundation for the transformation of cosmology over the last fifty years, from speculation to science.”

It was also, in his view, inevitable. “But given the state of technology in 1960, and the relative stability of the culture of science and society that went with it,” he writes in the book The Whole Truth, the “standard model” of the universe known as lambda cold dark matter, or ΛCDM, “seems sure to have been discovered.”

The book is partially an examination of some of those key topics in cosmology, from general relativity to the cosmic microwave background, and how understanding of them changed over the decades (including the roles he played in them.) The book, though, is also a more general examination of how science in general, and physics in particular, gets done.

There were multiple paths to getting to this standard cosmological model, he argues, with many Merton multiples along the way.

The book starts off with the latter, examining some philosophical questions about how science works. This includes theories that are accepted as “good enough approximations to reality” because “their applications yield successful predictions of situations beyond the evidence from which the theories were constructed.” He also notes that in science, including in cosmology, there is a tendency for discoveries or new models to be made independently by two or more people at the same time, something Peebles dubs “Merton multiples” after sociologist Robert Merton, who discussed it in an article more than 60 years ago.

Peebles then examines the development of cosmology, which evolved dramatically over the course of his career. For example, while Einstein’s theory of general relativity was “proven” by measurements of the movement of stars during a 1919 solar eclipse, the evidence supporting it was still weak for decades afterward. Only improved measurements of gravitational redshifts starting in the 1960s removed that uncertainty, providing ever-increasing precision to the predictions of general relativity.

He does the same with other aspects of cosmology, including the Big Bang and the development of the ΛCDM model. In the book’s final chapter, he explains his confidence that the model would exist today even if the specific steps that led to its development were not followed. For example, if Arno Penzias and Robert Wilson had not detected the cosmic microwave background, the signature of the Big Bang, at Bell Labs, a group at Princeton likely would have done so within a year. There were multiple paths to getting to this standard cosmological model, he argues, with many Merton multiples along the way.

That gives him confidence that the current model is correct, at least based on the knowledge we have today about the universe. “And let us note that this is what we would expect if our present physical cosmology is a good approximation to objective reality,” he says of the model, “not a social construction.”

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