Nobel prize in Physics 2019

This year the The Royal Swedish Academy of Sciences awarded the Nobel prize in Physics “for contributions to our understanding of the evolution of the universe and Earth’s place in the cosmos” to three scientists: James Peebles from Princeton University, USA, Michel Mayor from University of Geneva, Switzerland, and Didier Queloz from University of Geneva, Switzerland, as well as University of Cambridge, UK.

Half of the prize goes to James Peebles for his work on the theoretical aspects of physical cosmology, essentially laying the foundation of modern cosmology. The other half is shared by Michel Mayor and Didier Queloz for their discovery in 1995 of a planet outside the solar system orbiting a Sun-like star.

James Peebles

The theoretical framework developed by James Peebles between 1960s and 1980s transformed cosmology from mere speculation to science. His gut feeling that the Cosmic Microwave Background holds the secrets of the early universe drove James Peebles to develop new theoretical tools for understanding what observed data can tell us about the birth of the Universe. In the Big Bang model, the universe is extremely hot and dense in the first fractions of time and undergoes a period of rapid expansion known as inflation. After 400.000 years, the primordial plasma cooled down to the point that neutral atoms could form (‘recombination’). Thermal photons no longer had the energy to ionize hydrogen (‘decoupling’) and as a consequence the universe became transparent to light. Peebles was part of the team that predicted that this primordial radiation could still be present, as an isotropic background radiation, and measurable, albeit cooled down by the expansion of the universe to a mere 2.7 K. This hypothesis was later confirmed by 1978 Nobel Laureates Allan Penzias and Robert Woodrow Wilson. Furthermore, it was realised that tiny variations in temperature of the primordial plasma played an important part in the formation of large-scale structure in the universe, i.e. formation of galaxies. These variations also left an imprint on this initial field of radiation. Peebles was one of the pioneering researchers that worked out what information might be hidden in the CMB and how it can be traced back to states of the primordial plasma at the time of Recombination. The results showed that all the known matter (stars, planets, humans etc.) represents only 5% of the universe with the rest shared by dark matter and dark energy. They are the challenge of the present day research in physical cosmology.

Cosmin Microwave Background

Michel Mayor and Didier Queloz

The discovery of the first planet outside our solar system, orbiting a solar-type star, was announced in October 1995 by Michel Mayor and Didier Queloz. 50 light years from us, the 51 Pegasi star surfs the universe with the planet 51 Pegasi b, officially named Dimidium, a gaseous giant the size of Jupiter. The discovery was made possible by using custom-made instruments at the Haute-Provence Observatory in southern France. They used the technique known as Doppler spectroscropy, where they measure “tiny” displacements (at cosmic scale) of the star position relative to us by analysing the light coming from that star. As a light source moves away/towards an observer the emitted radiation gets redshifted/blueshifted (its wavelength increases/decreases). The star-planet system moves around a common center of gravity which is not the center of the star. This means that the star moves slightly due to the presence of the planet. In the case of the Sun-Jupiter system the center of gravity is 1.07 solar radii away from the center of the Sun. By analysing the light spectrum over time of the 51 Pegasi star they realised that the redshifts and blueshift observed are caused by the presence of a large planet close to the star.

Doppler spectroscopy

Our ideas about the cosmos were profoundly transformed by this year Laureates. James Peebles gave us tools to explore periods of time beyond the measurable universe, while Michel Mayor and Didier Queloz explored the space around us in search of unknown objects.