What was it like when the first atoms formed? | by Ethan Siegel | Starts With A Bang! | Dec, 2023


When free electrons recombine with atomic nuclei, the electrons cascade down the energy levels, emitting photons as they go. In order for stable, neutral atoms to form in the early Universe, they have to reach the ground state without producing a potentially ionizing, ultraviolet photon: an intricate process that causes neutral atoms to not form until hundreds of thousands of years after the start of the hot Big Bang. (Credit: Siarhei / Adobe Stock)

The first elements in the Universe formed just minutes after the Big Bang, but it took hundreds of thousands of years before atoms formed.

When it comes to our world, our Solar System, and all the planets, stars, and galaxies we can detect within our Universe, we find that they’re all made up of the same ingredients: atoms. And from the subatomic particles that bind together to make those atoms — electrons and atomic nuclei — we find that neutral atoms in turn interact and link up to form more complex structures: simple and complex molecules, some of which can eventually give rise to macroscopic structures and even life. It’s one of the most impressive facts about the Universe: that the laws and contents of the Universe exist in such a that they admit even the possibility of complex structure. When we look, that’s precisely what we find within it today.

But for hundreds of thousands of years, dating from the instant of the hot Big Bang, it was impossible to form even a single atom. Even though it only takes minutes to form atomic nuclei and leave the Universe in a state where electrons and those nuclei are steeped in a mere background bath of photons and neutrinos (and antineutrinos), it takes hundreds of thousands of years for atoms to stably form. It takes a huge amount of cosmic evolution, and a number of important evolutionary steps, in order to create them. Here’s the story of how the first atoms were formed.

The Universe doesn’t just expand uniformly, but has tiny density imperfections within it, which enable us to form stars, galaxies, and clusters of galaxies as time goes on. Adding density inhomogeneities on top of a homogeneous background is the starting point for understanding what the Universe looks like today. The density fluctuations imprinted in the CMB provide the seeds of structure, but the CMB itself cannot be seen until the Universe forms neutral atoms stably, rendering it transparent to the CMB’s light. (Credit: E.M. Huff, SDSS-III/South Pole Telescope, Zosia Rostomian)

By time the Universe is four minutes old, it’s already done fusing all the atomic nuclei it can fuse in this hot, dense, early state. There are no more free neutrons; they’ve all been incorporated into heavier nuclei. These include:

  • Helium-4 (two protons and two neutrons),
  • Deuterium (one proton and neutron),
  • Helium-3 (two protons and one neutron) and Tritium (one proton and two neutrons),
  • and Lithium-7 (three protons and four neutrons) and Beryllium-7 (four protons and three neutrons).


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