Dr. Peebles’s work helped place cosmology on a more solid, mathematical foundation.
“No one has done more to establish our current paradigm than Jim,” Michael Turner of the University of Chicago and the Kavli Foundation, a philanthropy that supports science, wrote in an email.
In 1964, two radio astronomers, Arno Penzias and Robert Wilson, discovered by accident a background hiss of microwaves pervading the universe. Dr. Penzias and Dr. Wilson, who won the Nobel Prize in Physics in 1978, were perplexed until they came across theoretical calculations by other scientists, including Dr. Peebles.
Dr. Peebles and his colleagues had predicted this background radiation, a residue from about 400,000 years after the Big Bang, when the universe had cooled enough for hydrogen and helium atoms to form.
The microwave background was almost uniform in all directions, reflecting a temperature of only a few degrees above absolute zero, but it was not perfectly smooth. Dr. Peebles calculated that there should be faint fluctuations, and that the fluctuations would reveal regions where matter had begun to clump together — the structure that would eventually be revealed as stars, galaxies and clusters of galaxies.
Like sound waves produced by a musical instrument, the fluctuations had characteristic wavelengths from vibrations bouncing back and forth.
In the early 1980s, Dr. Peebles proposed the idea that the universe was filled with unseen “cold dark matter” — particles that did not interact with ordinary matter but whose gravitational pull formed galaxies and clusters of galaxies. A couple of years later, he added to his model a term that Albert Einstein had originally proposed but later discarded as his “biggest blunder.”
Einstein had invented this idea, called the cosmological constant, to balance gravity and keep the universe static and unchanging. But astronomers established that the universe is actually expanding. Dr. Peebles utilized the cosmological constant, now known as dark energy, for a different reason: He aimed to show that the universe contained considerably less mass than was thought at the time.