The origin of the universe and humanity’s place within it shared the spotlight during today’s announcement of the Nobel Prize in Physics. James Peebles, a physicist at Princeton University, won half of the prize for his contributions to physical cosmology, while Michel Mayor, a physicist at the University of Geneva, and Didier Queloz, an astronomer at Geneva and at the Cavendish Laboratory in Cambridge, shared the other half for the 1995 discovery of an exoplanet orbiting a sunlike star.

When Peebles began his career at Princeton in the early 1960s, there was little evidence for the Big Bang theory, the idea that the expanding universe could be extrapolated back in time to a small, hot, dense state. Then in 1965, Arno Penzias and Robert Wilson discovered the cosmic microwave background (CMB) — ancient light emitted when the universe was 380,000 years old. The CMB provides a snapshot of the young universe, revealing a smooth, hot, simple place. Peebles later recalled his mentor, Robert Dicke, who “told me with a wave of his hand, ‘Why don’t you go think about the theory,’” said Peebles. “And I’ve been doing it ever since.”

Peebles teased information about the origin and contents of the universe from the CMB, helping to lead theoretical cosmology into a new, rigorous age. In 1965, shortly after Penzias and Wilson’s discovery, Peebles, Dicke and two colleagues laid out the basic explanation of what the CMB is and how it relates to the Big Bang. They argued that the light had propagated through space almost since the beginning, growing fainter and less energetic over time as the expansion of space stretched it out. From the energy of these photons today, they could infer an early-universe temperature of more than 10 billion degrees Celsius.

In that hot early epoch, pairs of electrons and pairs of neutrinos would have spontaneously materialized, leading to the synthesis of protons and neutrons and — when these protons and neutrons came together — to the creation of atomic nuclei. In 1966, Peebles made detailed calculations of the abundances of different isotopes that would have been produced in this process, known as Big Bang nucleosynthesis.

He calculated the relative amounts of deuterium, helium-3 and helium-4 by inferring the primordial density of neutrons and protons from the temperature of the CMB. However, this CMB-based estimate differed from what astronomers have observed in the present-day universe. The discrepancy indicated that crucial ingredients might have been missing. As both theory and observation of the CMB improved, Peebles and other theorists grew confident that the early density of protons and neutrons paled next to that of a different kind of matter, now known as dark matter, that did not readily interact except through gravity.

Later, in the 1970s, Peebles pioneered the theory of cosmic structure formation, which describes how the subtle hot spots and cold spots seen in the CMB evolved into the galaxies and voids in the present-day universe. He went on to make major contributions to the theory of cosmic inflation, a posited period of exponential expansion at the start of the Big Bang, as well as increasing our understanding of dark energy — repulsive energy that is thought to infuse space itself.

“Jim is one of the true giants in the field,” wrote Paul Steinhardt, Peebles’ colleague at Princeton, in an email. “His work transformed our understanding of the hot, expanding universe from qualitative to precise, revealed the existence of dark matter, and pointed out the puzzles that remain.”

While one half of this year’s physics prize honored Peebles’ work on large-scale questions about the universe, the other half went to researchers who transformed our understanding of our own place within the cosmos and our sense of the uniqueness — or not, as the case may be — of our planetary home.