Born at Winnipeg in Canada, Peebles was educated at the University of Manitoba and at Princeton, where he took his PhD in 1962. He has remained at Princeton ever since, becoming Einstein Professor of Science in 1984.

Peebles has made a number of contributions to modern cosmology. In 1965, in collaboration with Robert Dicke, he made the important prediction that a background radiation should be detectable as a remnant of the big bang. He also calculated that the amount of helium present in the universe as a consequence of the big bang should be about 25–30%, a figure that agrees with current observations.

In 1979, again in collaboration with Dicke, Peebles drew the attention of cosmologists to the so-called ‘flatness problem’ and asked how the standard model of the big-bang theory could deal with it. Cosmologists ask if the universe is ‘open’ or ‘closed’. If it is open it will continue to expand forever; if closed, the expansion will cease at some future point and it will begin to contract. To answer the question the value of omega (Ω) must be found.

Omega (Ω) is the ratio of the average density of mass in the universe to the critical mass density. This latter factor is the mass density needed just to halt the universe's expansion. If Ω is less than 1 the universe is open, and if Ω is greater than 1 the universe is closed. If Ω is equal to 1 the universe will be flat, that is, the universe will continue to expand, although at a decreasing rate.

The actual measured value of Ω is close to 1. A bit more matter in the universe and it would have collapsed long ago; a little less matter, and it would have expanded too quickly for galaxies to form. But if Ω is close to 1 now, it must have been close to 1 soon after the big bang. If it had differed significantly, Ω would either be approaching infinity and the universe would have the density of a black hole, or equal to 0 and the universe would be indistinguishable from a vacuum.

The question thus becomes why so early in the history of the universe was Ω so close to 1? Instead of merely laying this down as an arbitrary initial condition, there should be some way to see why, given the big bang, Ω should have this value. As Peebles's problem seemed to have no solution in the standard model of the big bang, it has been left to astronomers such as Alan Guth to propose alternative foundations based on an inflationary model.

Peebles is also the author of two important books, Physical Cosmology (Princeton, 1971) and The Large Scale Structure of the Universe (Princeton, 1980), which have between them done much to define the subject of cosmology for a generation of astronomers.

He was awarded the 2019 Nobel Prize in Physics in 2019 “for theoretical discoveries in physical cosmology”.