The reality of cosmology

In his recent book, The Whole Truth: A Cosmologist’s Reflections on the Search for Objective Reality, P. J. E. Peebles presents a tour de force on a rarely discussed subject: Is there objective reality in our theories of physical phenomena? Before delving into that subject in relation to cosmology, I now address the audience for the book. The inside flap of its dust jacket states that it is “essential reading for anyone interested in the practice of science.” That claim is misleading. Many people who have such an interest are not well versed in physics. Reading some parts of the book will be tough sledding indeed unless one has a good background in physics.

The book starts with a fairly broad dip into the philosophical underpinnings of science, which focuses mainly on 19th- and 20th-century ideas. Along the way, Peebles uses the example provided by physical cosmology to explore what we might mean by objective reality and how we might describe it. That choice is appropriate as he made major contributions to the model now accepted by astronomers and astrophysicists as our present best approximation to objective reality in that sphere of knowledge.

As Peebles notes, physics appears to be independent of cultural norms. He quotes the 19th-century philosopher Charles S. Peirce, who argued that scientists converge on the same reality despite starting with different assumptions. We assume that reality operates by rules we can hope to discover. Most scientists believe—not that I have taken a poll!—that the facts are out there to be uncovered, independent of the social norms of those who choose to look. The repeatability of measurements and observations is essential to that belief. The predictive power of science is what we expect if the world operates by rules, and scientists attempt to find good approximations to those rules.

Researchers have built up over decades knowledge relevant to the formulation of our present picture of the universe. Milestones include the first recognition of the expanding universe and its predominantly hydrogen composition in the 1920s, the development of the Big Bang theory in the 1940s, the discovery of the cosmic microwave background in the 1960s, the full recognition of the problem of dark matter starting in the 1970s, and the remarkable discovery of the acceleration of the expanding universe near the turn of the 21st century.

In large part due to Peebles’s contributions, those facts were incorporated into our present theory, which is usually said in shorthand to be the ΛCDM model, in which Λ denotes the cosmological constant and CDM denotes cold dark matter. The now-accelerating universe is assumed to be pushed by the cosmological constant, which was originally introduced into general relativity by Albert Einstein in the late 1910s to keep the universe static, as it was then believed to be by some astronomers. It has now been resurrected from the dustbin of history to account for the accelerating universe. Peebles does a truly admirable job of marshaling the evidence that supports the ΛCDM theory, which leaves the reader with the feeling that he is correct in concluding that the universe pretty much obeys it and that it is thus a good example of objective physical reality.

Peebles considers the heart of the theoretical underpinning to the ΛCDM to be Einstein’s theory of general relativity and spends substantial space discussing the establishment of that theory. He claims that there were no precision tests of it until the 1960s. If you believe, as I do, that a test with about 1% accuracy qualifies as precise, then that last characterization is incorrect. The very first test, which Einstein applied himself immediately upon completing his theory, was to compare its prediction of the perihelion position of the planet Mercury with the extra observed advance of 43 arcseconds per century that Isaac Newton’s theory of gravitation could not account for. Einstein’s theory passed that test with flying colors: Its prediction for the value was within about 1% of the observed advance, a limit set by the accuracy then achievable with the relevant measurements.

The book also discusses the possible future of the ΛCDM theory. Peebles expects the search for improvements in it to more likely end in exhaustion than in major changes. He makes the point that the search for a fundamental basis for more complicated observed phenomena will continue in the future as long as society continues to support such endeavors. He further predicts that all who investigate the cosmos will arrive at the same result—an assumption, he states, that has not been challenged by any contrary evidence in at least the last century. In science, he stresses—and almost all scientists agree—we cannot prove our theories of the universe; we can only disprove them when their predictions do not agree with our measurements or observations. Peebles also mentions several times that theories in our physical-science armamentarium are incomplete. He doesn’t, however, seem to discuss what would make a theory complete.

In concluding, I think that there are likely no more than a few educated people worldwide who wouldn’t learn from Peebles’s book. A physicist who wishes to learn the whole truth about our current knowledge of physical cosmology could accomplish that goal by reading this book and simultaneously learn a lot about related philosophy and sociology.