Looking back on a modern scientific revolution

The 20th century saw a revolution in our observational and theoretical understanding of the universe. P. J. E. Peebles’s latest book, Cosmology’s Century: An Inside History of Our Modern Understanding of the Universe, offers an inside look at how the pillars of modern cosmology were built. Using detailed historical research, quotes from key scientists, and his own recollection, the author presents an engaging story of the twists and turns taken on the long road toward our modern understanding of cosmology.

Peebles is uniquely positioned to recount that tale, as he is the key player in the story’s genesis. After making monumental contributions to essentially every aspect of our current cosmological model, he received the 2019 Nobel Prize in Physics. As both a fan of history and a practicing cosmologist, I found his book to be a captivating read that immediately put my understanding of the universe into a new historical perspective. It should be a mandatory read for cosmology graduate students and seasoned cosmologists alike.

Peebles begins his book by discussing Albert Einstein’s key ideas: that a philosophically sensible universe was homogeneous and isotropic, and that in general relativity such a universe must expand or contract. He then reviews the discovery of the expansion of the universe, evidenced by the redshift of distant galaxies, and the establishment of Big Bang cosmology. Throughout the book, Peebles carefully highlights alternative research directions that appeared promising at first but eventually fell out of favor due to their conflicts with observations. He emphasizes the historical importance of those false starts, such as the once-popular steady-state cosmology, and argues that they were catalysts for the development of innovative ideas that tested what eventually became the dominant paradigm.

Peebles then outlines how the abundance of light elements and, most importantly, the discovery of the cosmic microwave background (CMB) tilted the balance in favor of the hot Big Bang model. It is fascinating to see how George Gamow had the amazing intuition to establish that picture decades before it came into focus observationally. A description of early ideas about the formation of structure and galaxies in our universe follows. The author’s depiction of early hypotheses about the formation of galaxies is somewhat reassuring: It shows that scientists first tried the most intuitive approach—which failed—and only arrived at today’s more complex models after decades of refinement.

Cosmology’s Century contains an illuminating and riveting discussion of the growing evidence for subluminal matter in astronomy during the 20th century. It gradually became apparent that the mysterious subluminal matter was identical to the nonbaryonic dark matter proposed by particle physicists. The text also tracks the numerous theoretical and experimental dead ends that eventually led to the community’s acceptance of the cold dark matter (CDM) paradigm. Peebles himself was instrumental in that process: In 1982 he wrote a paper showing that CDM provides a simple explanation for both the highly inhomogeneous state of the local universe and the smoothness of the CMB. Showing characteristic humility, he recounts his surprise at how quickly the model became a convincing picture of the early universe. Yet again, Peebles’s insightful speculation led the community toward a fuller cosmological picture.

The book ends with an account of the cosmological “revolution” of 1998–2003, which firmly established the validity of the Lambda–Cold Dark Matter (ΛCDM) cosmological model. That such a simple model can explain both the initial seconds after the Big Bang and the distribution of galaxies today is a testament to a century of cosmology. Those of us who entered the field after that revolutionary era can find it easy to take our current cosmological paradigm for granted and forget that it took decades to get there.

Cosmology’s Century serves as a reminder that we stand on the shoulders of giants. It also holds many lessons for today’s researchers. At one point, Peebles emphasizes the importance of speculation to scientific progress, humorously saying that “one of the arts of science is to probe the boundaries between empty and productive speculation.” He also draws a historical parallel to Lord Kelvin’s turn-of-the-century pronouncement that “clouds” were hanging over physics. Those “clouds” led to the discovery of special relativity and quantum mechanics. Peebles similarly sees two “clouds” hanging over the ΛCDM model of the universe: the unknown physics of the very early universe and the enigmatic simplicity of dark energy and dark matter. Will those “clouds” lead to genuinely new ideas in physics?

Peebles accurately points out that large failures akin to the breakdown of classical physics on the atomic scale have yet to occur in cosmology. He hopes that such failures will lead to a much deeper understanding of our universe. Will the next century teach us as much about cosmology as the last?

Francis-Yan Cyr-Racine is an assistant professor of physics and astronomy at the University of New Mexico in Albuquerque. His research focuses on particle cosmology, dark matter, and the early universe.