What is the evolution of the universe?(2)
5 Recombination and the Cosmic Microwave Background (380,000 years after the Big Bang)
- When the universe cooled enough, electrons combined with nuclei to form neutral atoms, a process called recombination.
- This allowed light (photons) to travel freely for the first time, resulting in the Cosmic Microwave Background (CMB)—a faint glow that permeates the universe and provides a snapshot of the universe at that time.
6 Dark Ages (380,000 to 150 million years after the Big Bang)
- During the Dark Ages, there were no stars, and the universe was filled with neutral hydrogen and helium gas.
- There was no visible light, as stars had not yet formed, and gravity slowly pulled matter together, forming the first regions of higher density.
7 Formation of the First Stars and Galaxies (150 million to 1 billion years after the Big Bang)
- Gravitational attraction eventually caused gas to collapse into dense regions, igniting nuclear fusion and forming the first stars. These stars generated light and reionized the universe, ending the Dark Ages.
- Groups of these early stars merged to form the first galaxies. These galaxies evolved and merged into larger structures, eventually forming the galaxies we observe today.
8 Formation of Larger Galaxies and Galaxy Clusters (1–5 billion years after the Big Bang)
- Galaxies began to cluster due to gravitational attraction, forming galaxy clusters and superclusters, which are still evolving.
- Over time, galaxies merged and evolved, forming spiral, elliptical, and irregular shapes.
9 The Star Formation Peak and Element Enrichment (2–6 billion years after the Big Bang)
- During this period, star formation peaked as gas-rich galaxies produced large numbers of stars. This is sometimes called “cosmic noon.”
- As stars aged and died, they underwent supernova explosions, which spread heavier elements (like carbon, oxygen, and iron) into space, enriching the interstellar medium and allowing the formation of rocky planets.
(To be continued)