Kungl. Vetenskapsakademien, the Royal Swedish Academy of Sciences, awarded the 2019 “Nobel Prize for Physics” to James Peebles, Michel Mayor and Didier Queloz, for “contributing to our understanding of the evolution of the Universe and of the place that the Earth occupies in the cosmos”.
PHYSICAL COSMOLOGY
The Canadian James Peebles, by now American in all respects, professor emeritus of Princeton University, has managed to move cosmology from pure theoretic to something measurable, so as to be able to talk about physical cosmology. Cosmology studies the origins, evolution and conformation of the universe, and to do so it uses fascinating tools, such as the theory of relativity.
Cosmic Microwave Background Radiation
The fundamental node is the study of Cosmic Microwave Background Radiation (CMBR). We start from the assumption that, obviously, we can not know what triggered the arcinote Big Bang. The hypothesis about the next moments concerns a sort of crucible of particles, held together by very high temperatures, what the Anglo-Saxons call “primordial soup”. After about 400,000 years, due to a progressive cooling, the photons, which are nothing but particles, managed to escape from the primordial soup, creating the first ray of light in the universe. Well, this ray is still present in some way and can be detected, being a radiation (not visible), with a wavelength of a few millimeters and a frequency in the microwave band.
Professor Peebles predicted everything on the analytical level. Later, this radiation was actually captured, albeit by pure chance. Peebles understood that the temperature of the CMBR could provide us with information on the process of releasing photons and other particles post Big Bang and therefore on the birth of the universe: cosmology was no longer something purely theoretical, it could use data to study and quantities to measure.
According to Albert Einstein‘s “Theory of General Relativity”, the geometry of the universe has to do with gravity. Let’s forget about the definition of gravity and look at how our universe is made. Is it a soccer ball? Is it shaped like a donut? According to Einstein, the more mass (and therefore energy) there is in the Universe, the more it tends to curve. However, there is a critical value of mass and energy for which the Universe can be considered flat. What does flat mean? Well, let’s put it this way: the famous definition of parallel lines, holds.
Flat Universe
The temperature measurements of the CMBR, of which Peebles spoke, gave us an extraordinary statement: since the CMBR has assumed a certain temperature over time, then the universe can only be flat. We expect, therefore, a total mass present in the Universe, such that the famous critical value foreseen by relativity is reached. Unfortunately, the accounts do not add up: there is a lack of mass. The baryonic matter (that is the “ordinary” one that emits radiations and therefore is identifiable) is equal only to 5% of the total computation that we would expect. There is then a good 26% constituted by the so-called “dark matter” (which does not emit ergo radiations is detectable only for the gravitational effects). It prevents, calculations to the hand, to the galaxies to implode. We are at 31%, it lacks about 69% of matter (that is of mass) useful to reach that critical value, which, we repeat, has been experimentally verified: this 69% must necessarily exist.
Dark energy
Peebles then introduced the concept of “dark energy”, or the energy of empty space. With it the accounts come to light. Remember that according to Einstein mass and energy are two sides of the same coin. Space has an intrinsic energy, determined by the fluctuations of vacuum. Eye, these fluctuations are detectable, we are not talking about hypotheses. Einstein’s famous “cosmological constant” (his “biggest mistake”), albeit with a different interpretation, could now be considered again in the equations, assuming the meaning of “energy of vacuum”. Professor Suzanne Staggs of Princeton summarized Jim Peebles’ work well: “[…] she always had the equivalent of the view of a satellite, but on the whole universe”.
THE SEARCH FOR EXOPLANETS
Michel Mayor and Didier Queloz, professors at the University of Geneva, discovered the first orbiting planet around a star similar to the Sun (main sequence stars also known as “dwarf stars”). It is 51 Pegasi b, 50 light years from Earth, a twin of our Jupiter but only 8 million kilometers from its star. Let’s dispel any doubt: exoplanet simply means “planet that does not belong to the solar system”. So, how can we find exoplanets because they don’t give off their own light? The work of Mayor and Queloz answers this question.
Planets and Doppler effect
The two scholars started from a non-trivial concept: just as a planet moves around a star, so too does the star move, albeit minimally, influenced by the planet that orbits it. This movement can be measured, obtaining a physical quantity known as “radial velocity”, that is the speed of variation of the star-planet distance. How is radial velocity measured?
Let’s take a wave (a radiation that propagates) and its source. If this source moves, the generic observer perceives the wave in a distorted way (that is variable with respect to the real value): it is the famous Doppler effect. Think of an ambulance speeding while you are on your sofa at home: the sound wave produced by the siren varies as it approaches you and then varies again when it moves away. The starlight behaves in the same way. Thanks to the Doppler effect we can study the radial speed, through which we can be sure that around the star orbits a planet, being the planet the cause of the effect detected.
Through the method just described, it is possible to determine the mass of the planet. To determine the dimensions, the “transit photometry” method is used instead. When a planet interposes itself between the star and our telescopes, the luminous intensity decreases and in proportion to it we can determine the size of the planet. There are also other methods for finding planets, but they are not part of the work of the two new Nobel laureates.
Thanks to Mayor and Queloz, we have discovered thousands and thousands of exoplanets. The farthest one is the beauty of 27,000 light years: wormholes are urgently needed for a possible holiday.