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Big Bang theory

Half of the 2019 Nobel Prize in Physics was awarded to James Peebles “for theoretical discoveries in physical cosmology” and the other half is shared by Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting a solar-type star”.

When we look at the night sky, we see not only the Moon but hundreds or thousands of stars — maybe even one of the brighter planets in our Solar System. It’s hard to grasp that this visible matter makes up a mere five per cent of our Universe. With the remainder thought to be dark matter and dark energy, we seem to know little about the Universe. However, to honour the knowledge that we have, this year’s Nobel Prize in Physics was awarded to James Peebles, Michel Mayor and Didier Queloz “for [their] contributions to our understanding of the evolution of the Universe and Earth’s place in the cosmos”.

An early indication for the origin of our Universe was the suggestion by Robert Dicke, James Peebles, Peter Roll and David Wilkinson that the cosmic microwave background could originate from a hot Big Bang1. But this would not remain Peebles’s only important contribution to modern cosmology. Throughout his career, many more publications ranging from the cosmic microwave background to galaxy formation followed.

One of his most notable works, carried out jointly with Jeremiah Ostriker, was the discovery that large amounts of dark matter must be present in the halo of spiral galaxies such as the Milky Way as otherwise the flat galactic disk would be unstable2. In 1982, Peebles’s studies of non-relativistic or cold dark matter (CDM; ref. 3) laid the foundations for the standard model of cosmology — the ΛCDM model. Apart from standard baryonic matter, this model incorporates CDM and dark energy associated with the cosmological constant Λ, which Peebles put back on the map after its famous dismissal by Albert Einstein.

Whereas Peebles shaped our current understanding of how galaxies and galaxy clusters form, Mayor’s and Queloz’s discovery influenced our knowledge of planet formation. When the two started their monitoring campaign, planets outside our Solar System (exoplanets) orbiting a pulsar had been discovered, but not in orbit around a solar-type star. Periodic variations in the radial velocity of the star 51 Pegasi revealed such a planetary companion, 51 Pegasi b (ref. 4). The exoplanet’s mass was estimated to be at least half of Jupiter’s — a puzzling observation in light of the short orbital period of around four days.

The discovery of 51 Pegasi b posed a riddle to planet formation as its separation from 51 Pegasi was too small for the planet to be Jupiter-like. The authors speculated that the exoplanet might have been formed from a stripped brown dwarf. Since Mayor’s and Queloz’s observations, thousands of exoplanets have been discovered and continue to inspire advances in planetary formation models.

Despite this huge leap in understanding of our Universe, plenty of discoveries are still waiting to be made — from the exact process of planet formation to figuring out what dark matter is made of. Whatever we’ll find along the way, we know that it all started with the Big Bang.


  1. Dicke, R. H., Peebles, P. J. E., Roll, P. G. & Wilkinson, D. T. Astrophys. J. 142, 414–419 (1965).

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  2. Ostriker, J. P. & Peebles, P. J. E. Astrophys. J. 186, 467–480 (1973).

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  3. Peebles, P. J. E. Astrophys. J. 263, L1–L5 (1982).

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  4. Mayor, M. & Queloz, D. Nature 378, 355–359 (1995).

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Big Bang theory. Nat. Phys. 15, 1103 (2019).

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