Photograph from

A few months ago, the Royal Swedish Academy of Sciences, conferred the 2019 Nobel Prize in Physics to Prof. James Peebles (Professor Emeritus, University of Princeton, USA) along with two others, for their contributions in the field of cosmology*.

A key architect of the field of physical cosmology, Prof. Peebles is widely recognized for his path breaking contributions that have helped in understanding the universe’s origin, structure and evolution.

In 2011, in an ICTS event titled ‘Unravelling the Universe’, organized in Mumbai, Prof. Peebles along with other eminent physicists- Prof. Kip Thorne and Prof. Richard Bond from Caltech, USA, and Prof. John Ellis from CERN, were invited to deliver public lectures on different topics in cosmology.

We bring to you from our archives, notes from Prof. Peebles lectures, during his visit to India in 2011.

Prof. James Peebles is the Albert Einstein Professor of Science, Emeritus, and Professor of Physics, Emeritus at Princeton University, USA.  Photograph © Princeton University

Prof. Peebles’s lecture for the occasion was titled ‘Discovering the Large-scale Nature of the Physical Universe’, where he summarised our current understanding of the universe, starting with planet Earth-its uniqueness and the similarities it shares with other planets in the solar system. “Earth is one-of-a kind planet”, said Prof. Peebles, at the beginning of his lecture. He went ahead and explained to the audience in simple words the different concepts and theories in the field of cosmology and how they came into being.

We are not alone” he said as he displayed to the audience, a map of stars closest to our solar system. The Milky Way galaxy, that our solar system is a part of comprises of millions of stars that are different in size and the energy or light they emit. Using simple illustrations, Prof. Peebles explained the differences between red stars, intermediate stars, massive stars and white dwarfs. He also explained how supernova explosions occur, giving birth to new stars.

Referring to the map of the Milky Way in his presentation, Prof. Peebles elucidated how, what is seen in the picture is merely one percent of the total number of stars and other cosmic objects that are actually present. He further pointed out to swirly dark bands that criss-crossed Milky Way’s central disc- where it appears like stars are absent- but described it as an illusion created by the presence of large amounts of cosmic dust. “A lot many more cosmological objects like planets and stars with wonderful traits are yet to be discovered”, he stated.

Today its common knowledge that our universe is fast expanding, but it was big puzzle for scientists when it was first discovered in the early 1900s. Prof. Peebles walked the audience through the story of how Hermann Weyl, a German theoretical physicist contemporary to Albert Einstein, was among the first to observe that the universe was expanding. But it was Alexander Friedman, a Russian physicist/mathematician, who used Einstein’s equation on theory of general relativity, to derive a mathematical solution to explain the expansion of the universe. However, the evidence for the expansion of the universe was gathered many years later by astronomers Percival Lowell, Vesto Slipher and Edwin Hubble.

Towards the end of his lecture, Prof. Peebles touched upon the topic of dark matter and dark energy. The presence of dark matter was first put forward by Fritz Zwicky, a Swiss astronomer, in the early 1930’s, after he noticed cluster of galaxies to be moving too quickly to understand its gravity and mass, and instead appeared to be influenced by an external mass that was not from the galaxies. This external mass was later termed ‘dark matter and dark energy’ and its presence has scientists puzzled even today. A large part of Prof. Peebles work has been focussed on understanding dark matter. Among his many contributions is the Standard Cosmic Model, through which he explained how matter known to us-such as stars, planets and living forms, only make up five percent of the universe, while t he other 95 percent is made up of unknown dark matter and dark energy. .

The complete lecture by the Noble laureate can be viewed here. His lecture at the International Conference on Gravitation and Cosmology 2011, held at Goa, can be accessed here.

*Prof. James Peebles shares the Nobel Prize in Physics for 2019 with Prof. Michel Mayor (University of Geneva, Switzerland) and Prof. Didier Queloz (University of Geneva, Switzerland and University of Cambridge, UK). Click here to access the official press release.

Ipsita Herlekar

is the Science Communication Coordinator at ICTS