The Evolution of the Early Universe: From Atomic Formation to Galactic Development.

Soumendra Nath Thakur

December 10,2024

Stable atoms began to form in the universe approximately 380,000 years after the Big Bang. While the first elements—primarily hydrogen, helium, and trace amounts of lithium—were created within minutes of the Big Bang during a process known as Big Bang nucleosynthesis, it took hundreds of thousands of years for the universe to cool and expand enough for electrons to be captured by nuclei. This critical phase, known as recombination, allowed neutral atoms to form over a span of about 100,000 years cantered around the 380,000-year mark. The newly formed atoms entered their lowest energy states, releasing excess energy as photons. This released light persists as the cosmic microwave background (CMB), a faint glow that provides a snapshot of the universe in its infancy.

Galaxies began to form roughly one billion years after the Big Bang, which itself is estimated to have occurred 13.8 billion years ago. The first stars emerged a few hundred million years after the Big Bang during a period called the cosmic dawn. These stars coalesced into protogalaxies during the cosmic dark ages, a time lasting around 100 million years when hydrogen gas cooled and collected into dark matter halos. Early galaxies were smaller and more irregular than modern ones, and their continued evolution has been shaped by collisions and mergers. For instance, the Andromeda galaxy is currently on a collision course with the Milky Way, with the two expected to merge in the distant future.

The Big Bang primarily created hydrogen and helium, with trace amounts of lithium, but it left the universe devoid of heavier elements. These elements, including carbon, oxygen, and iron, were synthesized later in the cores of massive stars through nuclear fusion. About 150–200 million years after the Big Bang, the first stars formed from primordial gas clouds. These stars exhausted their hydrogen and helium fuel and forged heavier elements in their cores. The most massive stars ended their lives as supernovae, dispersing these heavier elements into the cosmos and enriching the interstellar medium. This process paved the way for the formation of subsequent generations of stars, planets, and eventually, life as we know it.

References:

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This textbook provides a comprehensive overview of cosmological phenomena, including the formation of atoms, galaxies, and elements in the early universe.

[2] Planck Collaboration (2020). Planck 2018 Results: Cosmological Parameters. Astronomy & Astrophysics, 641, A6.

The Planck mission's findings give precise measurements of the cosmic microwave background (CMB) and the timeline of early universe events.

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This resource explores stellar evolution, nucleosynthesis, and the development of cosmic structures like galaxies and stars.