Quarterly research: “light and matter” – Sciences et Avenir

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Let’s follow the fate of a photon, a particle of light, born in the heart of the Sun, during a nuclear fusion reaction. The density is initially so high that it has difficulty moving forward, permanently colliding with the other particles. This random walk of the photon lasts on the order of a million years. It ends when it reaches the convective zone of our star, where it only takes about ten days to ascend to the solar surface.

No wonder the night is dark …

An 8 minute journey between the Sun and the Earth, and here it is absorbed by our eye or by a detector. The analysis of such particles of the Sun and the stars has made it possible to understand the vibrations that occur within them and to deduce their internal structure (see p. 72). During billions of years of cosmic history, all stars in the observable Universe are estimated to have created 4 × 1084 photons (1). Although they are very numerous, today they represent only a small fraction of the energy density of the cosmos. Furthermore, in our vast Universe, the brightness of all the stars in all the galaxies, if we subtract the light from the Milky Way and the Sun, is equivalent to that of a 60-watt light bulb seen from a distance of 4 kilometers. No wonder the night is dark …

“The age of photons”

However, there is a moment when light dominated: “the era of photons”, described by Steven Weinberg, Nobel Prize in Physics 1979 – he died in 2021 -, in his book The first three minutes of the universe (Seuil, 1978 ). It begins 10 seconds after the Big Bang, this colossal explosion of energy that took place nearly 14 billion years ago. The Universe is then a hot fluid of photons, protons, neutrons and electrons. The temperature is so high there that everything collides violently, to the point where nuclear fusion reactions cannot occur!

After a minute, the protons and neutrons combine to form the first atomic nuclei, essentially hydrogen and helium, but also lithium and beryllium. This primordial nucleosynthesis creates the first nuclei of matter. However, the whole is still very dense and the photons are trapped. Everything changes 380,000 years later: the expanding Universe has cooled enough for the electrons and nuclei of nucleosynthesis to combine. It is the creation of the first neutral atoms. By interacting less frequently with matter, light can eventually escape from this opaque matrix, giving rise to radiation. It is the first visible light in the cosmos! Called the fuzzy cosmic background, it keeps track of the Universe’s past. From the behavior of stars and galaxies to the history of our world, photons have a lot to teach us.

(1) Fermi-Lat Collaboration, Science, 362, 1031, 2018.

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